Chemistry, Geochemistry, and Geology of Chromium and ...baholmen/docs/ENVE290W/National Chromium … · 2 Chemistry, Geochemistry, and Geology of Chromium and Chromium Compounds William

23

1-5667-0608-4/01/$0.00+$1.50© 2004 by CRC Press LLC

2

Chemistry, Geochemistry, and Geology

of Chromium and Chromium Compounds

William E. Motzer and Todd Engineers

CONTENTS

2.1 Chromium Chemistry .................................................................................242.1.1 Background ......................................................................................242.1.2 Elemental/Metallic Chromium Characteristics .........................252.1.3 Ionic Radii ........................................................................................292.1.4 Oxidation States...............................................................................302.1.5 Stable and Radioactive Isotopes ...................................................312.1.6 Characteristics of Chromium Compounds .................................34

2.2 Natural Chromium Concentrations..........................................................342.2.1 Mantle ...............................................................................................462.2.2 Chromium Minerals........................................................................462.2.3 Chromium Ore Deposits ................................................................46

2.2.3.1 Stratiform Mafic-Ultramafic Chromite Deposits .........622.2.3.2 Podiform- or Alpine-Type Chromite Deposits ............63

2.2.4 Crude Oil, Tars and Pitch, Asphalts, and Coal ..........................632.2.5 Rock ...................................................................................................642.2.6 Soil .....................................................................................................662.2.7 Precipitation (Rain Water) and Surface Water ...........................672.2.8 Groundwater....................................................................................672.2.9 Sea Water ..........................................................................................672.2.10 Air ......................................................................................................672.2.11 Biogeochemical Cycling .................................................................68

2.3 Chromium Geochemistry ...........................................................................702.3.1 Cr(III) Geochemistry.......................................................................702.3.2 Cr(VI) Geochemistry.......................................................................712.3.3 Chromium Reaction Rates (Kinetics)...........................................73

2.4 Chromium Distribution in Primary Environments ...............................742.4.1 Possible Sources of Natural Cr(VI) in Rocks..............................742.4.2 Known Sources of Natural Cr(VI) in Rocks ...............................77

L1608_C02.fm Page 23 Thursday, July 15, 2004 6:57 PM

24

Chromium(VI) Handbook

2.5 Chromium Distribution In Secondary Environments ...........................782.5.1 Known Natural Cr(VI) Occurrences in Surface

Water and Groundwater ................................................................782.6 Forensic Geochemistry................................................................................80

2.6.1 Soil .....................................................................................................802.6.2 Groundwater....................................................................................802.6.3 Air ......................................................................................................81

2.7 Acknowledgments.......................................................................................81Bibliography ........................................................................................... 82

2.1 Chromium Chemistry

2.1.1 Background

In 1797, the French chemist Nicholas-Louis Vauquelin hypothesized thatchromium (Cr) was a separate and distinct element. He had isolated theoxide of this element from a Siberian mineral known as crocoite (PbCrO

4

).In 1798, Vauquelin successfully isolated metallic chromium by heating(reducing) chromic oxide (Cr

2

O

3

) with charcoal. He then named the newelement after the Greek word

χρωµα

(chro

^

ma), pronounced khrma, for colorbecause it produced chemical compounds with distinct and unique colors.Vauquelin also analyzed a Peruvian emerald, determining that its green colorwas due to the presence of chromium. About two years after chromium’sdiscovery, Tassaert, a German chemist, determined that chromium waspresent in an ore that we now know as chromite (Greenwood and Earnshaw,1998; ChemGlobe, 2000; Papp, 2000; Winter, 2002).

Since its discovery, chromium has become a very important industrialmetal because of its many applications in ferrous (cast iron and stainlesssteel) and in nonferrous (aluminum, copper, and nickel) alloy metal fabrica-tion, and in the chemical industry (metal finishing, plating, corrosion control,pigments and tanning compounds, and wood preservatives) (Papp, 2000).Chromium and chromium compounds are used in a wide variety of indus-trial and manufacturing applications including steel alloy fabrication, wherethey enhance corrosion and heat resistance in other metals, and in platedproduct fabrication where they are used for metal decoration or increasedwear resistance. They are also used in nonferrous alloy metal fabrication toimpart special qualities to the alloys; in production and processing of insol-uble salts, as chemical intermediates; in the textile industry for dyeing, silktreating, printing, and moth proofing wool; in the leather industry for tan-ning; in the manufacture of green varnishes, inks, paints, and glazes; ascatalysts for halogenation, alkylation, and catalytic cracking of hydrocar-bons; as fuel and propellant additives; and in ceramics (Spectrum Labora-tories, 1998).


Chemistry, Geochemistry, and Geology of Chromium

25

While chromium in its Cr(III) form is not considered a toxic element andis a required diet nutrient with recommended daily adult dosages rangingfrom 0.5 to 2 mg per day (required for glucose metabolism), in its Cr(VI)form, it does have toxic effects (see Guertin, Section 6, this volume). Acuteexposure to Cr(VI)-laden dust results in skin rashes, ulcers, sores, andeczema in occupational workers. In humans, Cr(VI) exposure causedmarked irritation of the respiratory tract and ulceration and perforation ofthe nasal septum in workers in the chromate producing and -using indus-tries. Ingestion of 1.0 to 5.0 g of Cr(VI) as chromate results in severe acutegastrointestinal disorders, hemorrhagic diathesis, and convulsions. Deathmay occur following cardiovascular shock. Doses in animals of Cr(VI)greater than 10 mg/kg mainly affect the gastrointestinal tract, kidneys, andhematopic system (IPCS, 1988). Cr(VI) causes cancerous tumors in mice byinhalation and is considered a possible human carcinogen by this routebecause workers engaged in the production of chromate salts and chromatepigments experience an increased risk of developing bronchial carcinomas.However, ingestion of Cr(VI) has not been observed to cause cancerbecause it is believed that Cr(VI) is reduced to Cr(III) in the gastrointestinaltract (IPCS, 1988; WHO, 1988 and 1996; Smith and Huyck, 1999; CDHS,2003).

The understanding of chromium chemistry and geochemistry is thereforeimportant in developing remediation systems that can deal with industrial-caused pollution (see Chapter 8). This chapter is a review of the character-istics of chromium in the natural environment; its concentration within theearth’s crust, atmosphere, and biosphere; and its geochemistry.

2.1.2 Elemental/Metallic Chromium Characteristics

Chromium (atomic number 24) is a transition metal occurring in Group VIBof the periodic table. General elemental chromium characteristics are sum-marized in Tables 2.1a to 2.1d. Chromium has a ground state electron con-figuration of 1s

2

2s

2

2p

6

3s

2

3p

6

4s

1

3d

5

(Table 2.2). In the periodic table, transitionmetals (Groups IB to XIIIB) occur between the main group elements (GroupsIA to IIA and Groups IIIA to VIIA and the inert gases—Group VIIIA) (Drew,1972; Timberlake, 2003). The atoms of transition elements have electronsfilling d subshells consisting of 5d orbitals.

The transition metals are noteworthy because they:

1. Form alloys with one another and the main group metals. 2. Commonly are white lustrous metals with high melting and boiling

points. The transition metals vary in abundance in the continentalcrust from iron, which is common at 5.63% to scandium which israre at 22 (parts per million) ppm (Ronov and Yaroshevsky, 1972).

3. Have high melting points and densities because the electrons inthe d orbitals, bind atoms together in the crystal lattice.


26


TAB

LE 2

.1A

Ele

men

tal C

hrom

ium

Pro

pert

ies

Per

iod

ic T

able

Ato

mic

no.

2

4A

tom

ic m

ass

51

.996

1G

roup

no.

6

B

Mas

s N

um

ber

52

Gro

up n

ame

Tr

ansi

tion

met

als

Sym

bol

Cr

Peri

od n

o.

4B

lock

d

Ato

mic

No.

(Z)

24

Ch

emic

al R

egis

try

CA

S no

.74

40-4

7-3

(mos

t co

mm

on is

otop

e: 8

3.78

9%)

Pro

per

ties

Ph

ysic

alE

lect

rica

lT

her

mal

Ato

mic

rad

ius

(nm

)0.

185

e

–

in s

hell

1,2,

3,4

2,8,

13,1

Boi

ling

poin

t29

44 K

; 263

1 K

Ato

mic

vol

ume

(cm

3

/m

ol)

0.13

9e

–

con

figu

rati

onA

r4s

1

3d

5

Mel

ting

poi

nt

2180

°

C; 1

907

°

CB

ond

leng

th: C

r-C

r (n

m)

0.25

0

1

/

2

Fill

s of

sub

shel

l3d

5

Hea

t of

fus

ion

(kJ/

mol

)0.

325

Cov

alen

t ra

diu

s (n

m)

0.11

8E

lect

ron

bind

ing

ener

gies

Tabl

e 2.

1bH

eat

of v

apor

izat

ion

(kJ/

g)6.

622

Cry

stal

str

uctu

reC

ubic

bod

yce

nter

edE

ffec

tive

nuc

lear

cha

rges

Tabl

e 2.

1cSp

ecifi

c he

at c

apac

ity

(J/

g

⋅

K)

0.45

1

Den

sity

(g/

cm

3

at

293

K)

7.19

Ele

ctri

cal c

ond

ucti

vity

(cm

Ω

)0.

0774

×

10

6

The

rmal

con

duc

tivi

ty (

W/c

m

⋅

K)

0.93

7

Elas

tic

Prop

erti

es:

Elec

tron

egat

ivit

y:

Youn

g’s

mod

ulus

(G

Pa)

279

Paul

ing

1.66

Rig

idit

y m

odul

us (

GPa

)11

5A

bsol

ute

(eV

)3.

72B

ulk

mod

ulus

(G

Pa)

160

Ele

ctri

cal r

esis

tivi

ty (

10

−

8

Ω

)12

.7



27

Har

dnes

s:Io

niza

tion

Pot

enti

al (

eV):

Min

eral

: Moh

s (n

o un

its)

8.

5Fi

rst

6.76

66B

rine

ll (M

N/

m

2

)11

20Se

cond

16.5

0V

icke

rs (

MN

/m

2

)10

60T

hird

30.9

6Io

nic

rad

ius

(nm

)0.

62O

xid

atio

n st

ates

(Ta

ble

3.4)

+

2 to

+

6Su

cces

sive

ioni

zati

on E

nerg

ies

Tabl

e 2.

1d

Sour

ces:

Che

mG

lobe

(20

00);

Che

mPr

os (

2000

); W

inte

r: W

ebE

lem

ents

(20

01).


28


4. Form compounds that are commonly brightly colored [e.g., Cr(III)chloride is violet]. This occurs because lower energy electrons movefrom a lower energy electrons move formula lower energy d orbitalto higher energy d orbitals resulting in energy being taken in. Whenthese electrons return to their original position, they release specificenergies producing light of specific colors.

5. Like the main group metals, they form salts. However, where themain group salts will have cations that balance anions [e.g., halite

TABLE 2.1B

Chromium Electron Binding Energies

Label Orbital

eV

K 1s 5989L

I

2s 696L

II

2p

1

/

2

583.8L

III

2p

1

/

2

574.1M

I

3s 74.1M

II

3p

1

/

2

42.2M

III

3p

3

/

2

42.2

TABLE 2.1C

Chromium Effective Nuclear Charges

Orbital Z

eff

Orbital Z

eff

Orbital Z

eff

Orbital

1s 23.41 — — — —2s 16.98 2p 20.08 — — —3s 12.37 3p 11.47 3d 9.76 —4s 5.13 4p — 4d — 4f5s — 5p — 5d — —6s — 6p — — — —7s — — — — — —

TABLE 2.1D

Chromium Ionization Energies

Ionization State kJ/molIonization

State kJ/mol

Cr

0

–Cr

+

652.7 Cr

5

+

–Cr

6

+

8,738Cr

+

–Cr

2

+

1,592 Cr

6

+

–Cr

7

+

15,550Cr

2

+

–Cr

3

+

2,987 Cr

7

+

–Cr

8

+

17,830Cr

3

+

–Cr

4

+

4,740 Cr

8

+

–Cr

9

+

20,220Cr

4

+

–Cr

5

+

6,640 Cr

9

+

–Cr

10

+

23,580

Note:

Values in bold involve the removal of innershell electrons; for references see Table 2.1a.



29

or sodium chloride (NaCl) when dissolved in water forms an ionicsolution of Na

+

+

Cl

−

], transition metals are more likely to formcomplex ions with varying predominantly negative valences [e.g.,Cd(OH)

42

−

and CrO

42

−

] (Royal Society of Chemistry, 2000).

At ambient temperatures (20 to 25

°

C), metallic chromium is very hard,brittle,* blue-white to steel-gray, corrosion resistant, and capable of taking ahigh polish (ChemGlobe, 2000). Chromium can also be considered as botha heavy metal and trace element; heavy metals are those with densitiesgreater than 5 g/cm

3

at ambient temperature (de Haan and Bolt, 1979). Inmany cases, in the natural environment (i.e., in soils, rocks, etc.), chromiumcan also be considered as a trace element in that a trace element is definedas any chemical element that has a solid phase mass concentration less than100 ppm (Sposito, 1989).

2.1.3 Ionic Radii

The radius of the neutral chromium atom is 0.130 nm (Chang, 1994); chro-mium ionic radii vary from 0.04 nm in Cr(VI) to 0.094 nm in Cr(II). Radiivariations depend on coordination type, species, and spin (Winter, 2001;Table 2.3). Ionic radii are important in determining ionic substitution forvarious chromium species.

TABLE 2.2

Electronic Configuration of Elements in Period 4

AtomicNo.

Element Name

Subshell

1s 2s 2p 3s 3p 3d 4s 4p 4d 4f

21 Sc 2 2 6 2 6 1 2 — — —22 Ti 2 2 6 2 6 2 2 — — —23 V 2 2 6 2 6 3 2 — — —24 Cr 2 2 6 2 6 5 1 — — —25 Mn 2 2 6 2 6 5 2 — — —26 Fe 2 2 6 2 6 6 2 — — —27 Co 2 2 6 2 6 7 2 — — —28 Ni 2 2 6 2 6 8 2 — — —29 Cu 2 2 6 2 6 10 1 — — —30 Zn 2 2 6 2 6 10 2 — — —

Note:

By energy 4s fills before the 3d as in Cr = 1s

2

2s

2

2p

6

3s

2

3p

6

4s

1

3d

5

; for references see Table 2.1a.

* Chromium metal’s reported brittleness in most of the literature may be caused by oxidizedimpurities. Pure chromium metal is extremely susceptible to extracting and combining withatmospheric oxygen. Therefore, it is almost impossible to have pure chromium in an oxygen-richatmosphere (see Kohl, 1967).


30


2.1.4 Oxidation States

Oxidation states in the transition metals are important in that transition metalions that have charges greater than

+

3 cannot exist in aqueous solution.Chromium oxidation states range from

−

4 to

+

6 (Table 2.4). The differentoxidation states are important in determining what chromium compoundsform in the environment (Smith, 1972). Oxidation states

−

2,

−

1, 0, and

+

1primarily occur in synthetic organic-chromium compounds such as the chro-mium carbonyls, chromium bipyridine, carbonyl nitrosyls, and organome-tallic complexes (Kotz et al., 2000; Luis, 2001).

Only three oxidation states are readily found in nature; these are:

1. Cr(0) which occurs in metallic or native chromium2. Cr(III) which occurs in chromic compounds3. Cr(VI) which occurs in chromate and dichromate compounds

Cr(0) is rarely found in the natural environment, although many referencesindicate that it does not occur. However, native chromium occurs as metallicinclusions in cryptocrystalline diamonds (carbonado) from kimberlite pipesin the Siberian Yakutia diamond deposits of Russia (Gorshkov et al., 1996).Native chromium also has been found in vein deposits from Sichuan, China(Guisewite, 2001), in meteorites such as the Agpalilik meteorite fragmentfrom Cape York, Greenland, and as metal alloys in placer deposits (seeTable 2.8 in Section 2.2.2).

Cr(III) occurs as insoluble chromium oxide (Cr

2

O

3

) and chromium hydrox-ide [Cr(OH)

3

]; it also occurs as soluble chromium hydroxide cations: CrOH

2

+

and Cr(OH)

2

+

. Cr(VI) generally occurs as soluble dichromate (Cr

2

O

72

−

) andchromate (CrO

42

−

) anions.

TABLE 2.3

Chromium Ionic Radii

OxidationState

CoordinationTypea Species Spin

Radius (nm)

Cr(IV) 4 Tetrahedral 0.055Cr(V) 4 Tetrahedral 0.0485Cr(VI) 4 Tetrahedral 0.040Cr(II) 6 Octahedral Low 0.087Cr(II) 6 Octahedral High 0.094Cr(III) 6 Octahedral Low 0.0755Cr(IV) 6 Octahedral Low 0.069Cr(V) 6 Octahedral Low 0.063Cr(VI) 8 Octahedral Low 0.058Cr(V) 0.071

a Coordination type refers to covalent bonding.

Source: Winter: WebElements (2001).


Chemistry, Geochemistry, and Geology of Chromium 31

2.1.5 Stable and Radioactive Isotopes

Currently, there are 26 known chromium isotopes (Table 2.5), of which fourare stable (nonradioactive), naturally occurring isotopes (ChemGlobe, 2000;Winter, 2001; LBNL, 2002). These include chromium-50 (50Cr), 52Cr, 53Cr, and54Cr; their naturally occurring abundances are 4.345%, 83.789%, 9.509%, and2.465%, respectively (Winter, 2001).

Stable chromium isotopes are known to fractionate (Table 2.5), that is, whenone isotope is preferentially enriched over another relative to a known stan-dard, which represents its natural abundance (see footnote in Section 3.6).There are several environmental and geologic controlling processes in sta-ble isotopic fractionation; these may include (1) isotopic exchange reactions,(2) evaporation and condensation, (3) melting and crystallization of rocks,(4) adsorption and desorption, (5) mass dependent diffusion, (6) tempera-ture, (7) ultrafiltration in water-rock reactions, and (8) the preference for somebiological organisms in concentrating lighter over heavier isotopes (Hurst,1991). By experimental methods, Ottonello (2002) has identified chromiumfractionation under various conditions.

TABLE 2.4

Chromium Oxidation States

Oxidation State

Example Compound

Name Formula

−2 Sodium chromium (−II) carbonyl Na2[Cr(CO)5]−1 Sodium chromium (−I) carbonyl Na2[Cr2(CO)10]

0Chromium (0) (metal)Chromium (0) carbonyl

Cr0

Cr(CO)6

+1 Chromium bipyrydil (=L) [Cr(L)3]

+2

Chromium oxideChromium difluorideChromium dichlorideChromium sulfide

CrOCrF2

CrCl2

CrS

+3

Chromium oxideChromium trifluorideChromium trichlorideChromium hydroxide

Cr2O3

CrF3

CrCl3

Cr(OH)3

+4Chromium dioxide Chromium tetrafluoride

CrO2

CrF4

+5Barium chromate (V)Chromium pentafluoride

Ba3(CrO4)2

CrF5

+6

Barium chromate (VI)Chromate anionSodium dichromateDichromate anion

BaCrO4

Cr2O7−

CrO42−

Cr2O72−

Note: Oxidation states in bold are those commonly found in min-erals and compounds in the natural environment.

Source: Modified from USEPA (1984); Marques et al. (1999).

Au: Pleasecheck thechange.


32 Chromium(VI) Handbook

TAB

LE 2

.5

Chr

omiu

m N

uclid

e (I

soto

pe)

Prop

erti

es

Isot

ope

Ato

mic

Mas

s (m

a/u

)

Nat

ura

l A

bu

nd

ance

(a

tom

%)

Nu

clea

r S

pin

(I)

Nu

clea

r M

agn

etic

M

omen

t(µ

n/µ

N)

Hal

f L

ife

(t1/

2)M

ode

of

Dec

ayD

ecay

s to

Dec

ay E

ner

gy

(MeV

)

42C

r—

Art

ifici

al—

——

——

—43

Cr

43.9

8556

Art

ifici

al(3

/2+

)—

0.

21 s

ecε

43V

15.8

9043

Cr:

met

a st

ate

Art

ifici

al(3

/2+

)—

0.

21 s

ecε

+ α

39Sc

15.7

0043

Cr:

met

a st

ate:

0.0

00 M

eVA

rtifi

cial

——

0.

21 s

ecε

+ p

41Sc

11.9

30

44C

r43

.985

56A

rtifi

cial

0+—

0.

53 s

ecε

+ p

43Ti

8.50

044

V10

.310

45C

r44

.979

11A

rtifi

cial

7/2–

—

0.05

sec

ε +

β+

44Ti

10.8

50ε

45V

12.4

6046

Cr

45.9

6836

Art

ifici

al0+

—

0.26

sec

ε +

β+

46V

7.60

347

Cr

46.9

6290

5A

rtifi

cial

3/2–

—

0.51

sec

ε +

β+

47V

7.45

148

Cr

47.9

5403

3A

rtifi

cial

0+—

21.5

6 h

ε48

V1.

659

49C

r48

.951

338

Art

ifici

al5/

2–0.

476

42.3

min

ε49

V2.

631

50C

r49

.946

0464

4.34

50+

—R

el. S

tabl

e:1.

8 ×

1017

yr

ε50

Ti—

51C

r50

.944

768

7/2–

−0.9

3427

.702

5 d

ε51

V0.

753

52C

r51

.940

5098

83.7

890+

—St

able

NA

NA

NA

53C

r52

.940

6513

9.50

93/

2––0

.474

54St

able

NA

NA

NA

54C

r53

.938

8825

2.46

50+

—St

able

NA

NA

NA

55C

r54

.940

842

Art

ifici

al3/

2–—

3.

497

min

β−55

Mn

2.60

356

Cr

55.9

4064

3A

rtifi

cial

0+—

5.

94 m

inβ−

56M

n1.

617

57C

r56

.943

44A

rtifi

cial

3/2–

, 5/

2–, 7

/2–

—21

.1 s

ecβ−

57M

n5.

090

58C

r57

.944

12A

rtifi

cial

0+0.

0834

7.

0 se

cβ−

58M

n3.

970

59C

r58

.949

Art

ifici

al—

—

0.74

sec

β− 59

Mn

7.70

0



60C

r59

.95

Art

ifici

al0+

—

0.57

sec

β− 60

Mn

5.90

061

Cr

60.9

54A

rtifi

cial

(5/

2–)

—

0.27

0 se

cβ

−61

Mn

8.80

062

Cr

—A

rtifi

cial

0+—

0.

190

sec

β−—

—63

Cr

—A

rtifi

cial

(1/

2–)

—

0.19

0 se

cβ−

——

64C

r—

Art

ifici

al0+

—

0.11

0 se

cβ−

——

65C

r64

.97

Art

ifici

al(1

/2−

)—

—

β−

(?)

——

66C

r—

Art

ifici

al0+

——

β−—

—67

Cr

—A

rtifi

cial

(1/

2–)

——

β−—

—

Not

e:Is

otop

es i

n bo

ld a

re s

tabl

e no

n-ra

dio

acti

ve i

soto

pes

. ε =

ele

ctro

n ca

ptu

re; α

= a

lpha

em

issi

on β

− =

beta

em

issi

on; β

+ =

pos

itro

n em

issi

on;

p =

pro

ton

emis

sion

. µn/

µ N =

mag

neti

c m

omen

t in

nu

clea

r m

agne

tron

s. N

A =

not

ap

pli

cabl

e. s

ec =

sec

ond

s, m

in =

min

ute

s, h

= h

ours

, d =

day

s,

y =

year

s. D

ash

(—)

ind

icat

es n

o av

aila

ble

dat

a. V

alu

es i

n p

aren

thes

is a

re t

enta

tive

.

Sour

ces:

Win

ter:

Web

Ele

men

ts (

2001

); M

arqu

es e

t al

. (19

99);

Bar

bala

ce e

t al

. (20

01);

Law

renc

e B

erke

ley

Nat

iona

l Lab

orat

orie

s (2

002)

.



Radioactive isotopes of chromium have been artificially produced. Mosthave very short half-lives (t1/2). For example, Table 2.5 shows that the t1/2 forisotopes from 42Cr to 47Cr and from 57Cr to 64Cr are much less than 1 sec.

Chromium isotope studies have been important in determining the ageof solids (planetesimals) first formed in the solar nebula (Carlson andLugmair, 2000) and in investigations of the solar wind (Kitts et al., 2002).Stable isotope fractionation may be important for forensic geochemicalinvestigations (see Section 2.6).

2.1.6 Characteristics of Chromium Compounds

Chromium can be combined with various nonmetals (oxygen, fluorine, chlo-rine, etc.) and polyatomic anions (nitrate, sulfate, etc.), forming relativelystable, soluble and insoluble compounds (Table 2.6). More common are Cr(III)compounds such as chromium tribromide (insoluble), chromium nitrate (sol-uble), chromic hydroxide (insoluble), and chromic oxide (insoluble). In thechemical production industry, most chromium chemicals are produced fromsodium dichromate, which is the principal feedstock. Chemicals made fromsodium dichromate include chromic acid, chromic oxide, and potassiumdichromate (Papp, 2000).

Most chromium compounds are brightly colored and these colors arereflected in synonyms for their respective compounds. For example, basicchromium sulfate is known as chrome tan, chromic oxide is known as chromegreen, barium chromate is known as baryta yellow or lemon chrome, basiclead chromate is known as chrome orange and chrome red, calcium chromateis known as calcium chrome yellow, and lead chromate is also known aschrome green. All chromium compounds are considerably denser than waterwith specific gravities ranging from 1.77 (for hydrated chromium sulfate) to6.10 [for chromium(II) selenide] (Dean, 1992; ChemIDplus, 2001; Chem-finder, 2001). Therefore, saturated and very concentrated chromium com-pound solutions would tend to sink through the groundwater column.

2.2 Natural Chromium Concentrations

As with other elements in the periodic table, chromium concentrations innatural substances are quite variable. Chromium preferentially concentratesin various rocks throughout the earth’s crust with concentrations dependenton the rock’s origin and source (Table 2.7). Chromium concentrations arealso quite variable in secondary geochemical environments, particularly insoils, sediments, and stream and lake water. Concentrations may signifi-cantly vary because of anthropogenic influences and inputs, largely fromsmelting of chromium ore and the burning of fossil fuels such as coal andpetroleum products.



TAB

LE 2

.6

Phys

icoc

hem

ical

Pro

pert

ies

of S

ome

Chr

omiu

m C

ompo

und

s

Com

pou

nd

Nam

esan

dS

ynon

yms

Form

ula

CA

S

Nu

mb

er

Mol

ar

Mas

s (g

/mol

)D

ensi

ty(w

ater

= 1

)M

elti

ng

Poi

nt

(°°°°C

)B

oili

ng

Poi

nt

(°°°°C

)

Aq

ueo

us

Sol

ub

ilit

y at

20°°°° C

(m

g/l)

Ph

ysic

alD

escr

ipti

on

Chr

omiu

m:

(II)

acet

ate

Chr

omiu

m a

ceta

teC

r(C

2H3O

2)2

1759

3-70

-317

0.10

1.79

Solu

ble

(II)

ace

tate

hy

dra

teC

hrom

ium

ace

tate

; ch

rom

ium

ace

tate

m

onoh

ydra

te

C4H

8CrO

5;C

r(C

2H3O

2)2

· H2O

628-

52-4

188.

101

1.79

Red

dis

h- b

row

npo

wd

er; r

edm

onoc

linic

cry

stal

s(I

II)a

ceta

teC

hrom

ium

ace

tate

; ch

rom

ic a

ceta

te;

chro

miu

m t

riac

etat

e

Cr(

C2H

3O2)

3 ·

12H

2O10

66-3

0-4

229.

1295

Slig

htly

So

lubl

eG

rayi

sh--

gree

n po

wd

eror

vio

let

plat

es

(III

)ace

tate

he

xahy

dra

teC

hrom

ium

(II

I) a

ceta

tehe

xahy

dra

teC

r(C

2H3O

2)3

1066

-30-

428

5.22

6So

lubl

eB

lue

need

les

acet

ylac

eton

ate

Chr

omiu

m

acet

ylac

eton

ate

[CH

3CO

CH

C

(CH

3)O

] 3C

r21

634

0In

solu

ble

Purp

le p

owd

er o

rre

dd

ish-

viol

et c

ryst

als

amm

oniu

m

sulf

ate

Chr

omiu

m a

mm

oniu

m

sulf

ate

·12

hyd

rate

CrN

H4(

SO4)

·12H

2O1.

72

94So

lubl

eG

reen

pow

der

or

dee

p vi

olet

cry

stal

san

tim

onid

eC

hrom

ium

ant

imon

ide

CrS

b21

679-

31-2

349.

324

7.11

1110

–122

0H

exag

onal

cry

stal

sar

seni

de

Chr

omiu

m a

rsen

ide

Cr 2

As

1225

4-85

-217

8.91

47.

04Te

trah

edra

l cry

stal

sbr

omid

eC

hrom

ium

bro

mid

eC

rBr 3

1003

1-25

-129

1.70

84.

6811

30So

lubl

e in

hot

H

2OD

ark

gree

n he

xago

nal

crys

tals

(III

) br

omid

e he

xahy

dra

teC

hrom

ium

bro

mid

e he

xahy

dra

teC

r(H

2O) 6

Br 3

1003

1-25

-139

9.79

9So

lubl

eV

iole

t hy

dro

scop

iccr

ysta

lsbo

rid

eC

hrom

ium

bor

ide

CrB

1200

6-79

-062

.807

6.1

2100

Ref

ract

ory,

or

thor

hom

bic

crys

tals

(IV

)bor

ide

Chr

omiu

m b

orid

eC

rB2

1200

7-16

-873

.618

5.22

2200

Ref

ract

ory

solid

; he

xago

nal c

ryst

als

bori

de

Chr

omiu

m b

orid

eC

r 5B

312

007-

38-4

292.

414

6.10

1900

(II)

brom

ide

Chr

omiu

m b

rom

ide;

ch

rom

ium

dib

rom

ide

CrB

r 210

049-

25-9

211.

804

4.23

684

2So

lubl

eW

hite

mon

oclin

ic

crys

tals

; for

ms

blue

aq

ueou

s so

luti

on

(Con

tinu

ed)



TAB

LE 2

.6

(Con

tinu

ed)

Com

pou

nd

Nam

es a

nd

S

ynon

yms

Form

ula

CA

S

Nu

mb

er

Mol

ar

Mas

s(g

/mol

)D

ensi

ty

(wat

er =

1)

Mel

tin

g P

oin

t (°

C)

Boi

lin

g P

oin

t (°

C)

Aq

ueo

us

Sol

ub

ilit

y at

20

°C

(mg/

l)P

hys

ical

D

escr

ipti

on

(III

)bro

mid

eC

hrom

ium

tri

brom

ide

CrB

r 310

031-

25-1

291.

708

4.68

081

2; 1

130

(?)

Inso

lubl

e;

Solu

ble

in h

ot

H2O

Oliv

e gr

een

or d

ark

gree

n so

lid

(IV

)bro

mid

eC

hrom

ium

te

trab

rom

ide

CrB

r 423

098-

84-2

371.

612

——

——

Gas

(III

)bro

mid

e he

xahy

dra

teC

hrom

ium

bro

mid

e he

xahy

dra

teC

rBr 3

·6H

2O10

060-

12-5

266.

445

Solu

ble

Vio

let

crys

tals

; hy

dro

scop

icca

rbid

eC

hrom

ium

car

bid

eC

r 3C

212

012−

35−0

180.

010

6.65

1890

–189

538

00In

solu

ble

Gra

y, o

rtho

rhom

bic

crys

tals

carb

ide

chro

miu

m c

arbi

de

Cr 2

3 C

612

105-

81-6

carb

onyl

Chr

omiu

m c

arbo

nyl;

chro

miu

m

hexa

carb

onyl

Cr(

CO

) 613

007-

92-6

220.

058

1.77

dec

: 11

0–13

012

0In

solu

ble

<1,

000

Whi

te c

ryst

allin

e so

lid

carb

onat

eC

hrom

ium

car

bona

te;

chro

mus

car

bona

teC

rCO

32.

75Sl

ight

ly

solu

ble

Gra

yish

-blu

e am

orph

ous

pow

der

(II)

chlo

rid

eC

hrom

ium

dic

hlor

ide;

ch

rom

us c

hlor

ide

CrC

l 210

049-

05-5

122.

902

2.87

881

5–82

411

20V

ery

solu

ble

Lus

trou

s w

hite

nee

dle

s or

fus

ed fi

brou

s m

ass;

hy

dro

scop

ic(I

II)c

hlor

ide

Chr

omiu

m t

rich

lori

de

(a)

CrC

l 3 (b

) C

rCl 3

·6H

2O10

025-

73-7

158.

355

(a)

2.76

(b)

2.87

011

5013

00In

solu

ble

in

cold

H2O

; sl

ight

ly

solu

ble

in h

ot

H2O

Red

dis

h-vi

olet

cr

ysta

lline

sol

id;

hyd

rosc

opic

(IV

)chl

orid

eC

hrom

ium

te

trac

hlor

ide

CrC

l 415

597-

88-3

193.

807

0.00

85 (

gas)

dec

: >60

0St

able

at

high

te

mpe

ratu

res

(II)

chlo

rid

e te

trah

ydra

teC

hrom

ium

chl

orid

e te

trah

ydra

teC

r(H

2O) 4

Cl 2⋅

4H

2O13

931-

94-7

267.

023

dec

: 51

Solu

ble

(II)

coba

ltC

obal

t ch

rom

ite

CoC

r 2O

413

455-

25-9

226.

923

5.14

Inso

lubl

eB

luis

h-gr

een

cubi

c cr

ysta

ls



(II)

chro

mit

eC

oppe

r ch

rom

ite

CuC

r 2O

412

018-

10-9

231.

536

5.4

Inso

lubl

eG

rayi

sh-b

lack

te

trah

edra

l cry

stal

s(I

I)fl

uori

de

Chr

omiu

m fl

uori

de;

ch

rom

ium

difl

uori

de;

ch

rom

us fl

uori

de

CrF

210

049-

10-2

89.9

928

3.79

089

413

00Sl

ight

ly

solu

ble

Blu

e-gr

een

mon

oclin

ic

crys

tals

; anh

ydro

us

(III

)fluo

rid

eC

hrom

ium

tri

fluo

rid

eC

rF3

7788

-97-

810

8.99

13.

814

04In

solu

ble

Gre

en c

ryst

als

(III

)fluo

rid

e tr

ihyd

rate

Chr

omiu

m fl

uori

de

trih

ydra

teC

rF3·3

H2O

1667

1-27

-516

3.03

72.

2So

lubl

eG

reen

hex

agon

al

crys

tals

(IV

)fluo

rid

eC

hrom

ium

tet

rafl

uori

de

CrF

410

049-

11-3

127.

990

2.9

277

400

Gre

en t

o vi

olet

cr

ysta

lline

sol

id(V

)fluo

rid

eC

hrom

ium

pe

ntafl

uori

de

CrF

514

884-

42-5

146.

988

3411

7R

ed o

rtho

rhom

bic

to

crim

son

crys

talli

ne

solid

(VI)

fluo

rid

eC

hrom

ium

hex

afluo

rid

eC

rF6

1384

3-28

-216

5.98

7d

ec: −

100

−100

Solu

ble

Yello

w c

ryst

allin

e so

lid;

stab

le a

t lo

w

tem

pera

ture

s(I

II)f

orm

ate

6-w

ater

Hyd

rate

d c

hrom

ium

fo

rmat

eC

r(C

HO

2)3

·6H

2O29

5.15

Solu

ble

(III

)hyd

roxi

de

Chr

omic

hyd

roxi

de;

ch

rom

ium

tr

ihyd

roxi

de

Cr(

OH

) 313

08-1

4-1

103.

0179

dec

: >10

0In

solu

ble

(III

)hyd

roxi

de

trih

ydra

teC

hrom

ium

hyd

roxi

de

trih

ydra

teC

r(O

H) 3

·3H

2O13

08-1

4-1

157.

063

Inso

lubl

eB

luis

h-gr

een

pow

der

(II)

iod

ide

Chr

omiu

m io

did

eC

rI2

1347

8-28

-930

5.80

55.

100-

5.32

868

1100

Solu

ble

Red

dis

h-br

own

crys

talli

ne s

olid

(III

)iod

ide

Chr

omiu

m t

riio

did

eC

rI3

1356

9-75

-043

2.71

5.30

0d

ec: 5

0050

0Sl

ight

ly

solu

ble

Dar

k gr

een

crys

talli

ne

solid

(IV

)iod

ide

Chr

omiu

m t

etra

iod

ide

CrI

423

518-

77-6

559.

614

(II)

iron

Iron

chr

omit

eFe

Cr 2

O4

1308

-31-

222

3.83

55.

0B

lack

cub

ic c

ryst

als

(III

)nit

rate

Chr

omic

nit

rate

Cr(

NO

3)3

1354

8-38

-423

8.01

0760

Ver

y so

lubl

eG

reen

, hyd

rosc

opic

po

wd

er(I

II)n

itra

te

9-w

ater

Hyd

rate

d c

hrom

ium

ni

trat

e; c

hrom

ium

ni

trat

e no

nahy

dra

te

Cr(

NO

3)3·9

H2O

7789

-02-

840

0.14

81.

8066

.3 (

dec

: 10

0)2,

080,

000

Gre

enis

h bl

ack

to

purp

le r

hom

bic

(mon

oclin

ic)

crys

tals

(Con

tinu

ed) Au:

Deletion OK?



TAB

LE 2

.6

(Con

tinu

ed)

Com

pou

nd

Nam

es a

nd

S

ynon

yms

Form

ula

CA

S

Nu

mb

er

Mol

ar

Mas

s(g

/mol

)D

ensi

ty

(wat

er =

1)

Mel

tin

g P

oin

t (°

C)

Boi

lin

g P

oin

t (°

C)

Aq

ueo

us

Sol

ub

ilit

y at

20°C

(m

g/l)

Ph

ysic

al

Des

crip

tion

(III

)nit

rid

eC

hrom

ium

nit

rid

e;

chro

miu

m

mon

onit

rid

e

CrN

2409

4-93

-766

.003

5.9

dec

: 108

0G

ray

crys

talli

ne s

olid

nitr

ide

Chr

omiu

m n

itri

de

Cr 2

N12

053-

27-9

117.

999

6.8

1650

Hex

agon

al c

ryst

als

oxal

ate

Chr

omus

oxa

late

m

onoh

ydra

te;

chro

miu

m(I

I) o

xala

te

mon

ohyd

rate

CrC

2O4⋅H

2O81

4-90

-415

8.03

12.

468

Solu

ble

Yello

wis

h-gr

een

crys

talli

ne p

owd

er

(III

)oxi

de

Chr

omia

; chr

omic

ox

ide;

dic

hrom

ium

tr

ioxi

de;

chr

omiu

m

sesq

uiox

ide;

gre

en

cinn

abar

Cr 2

O3

1308

-38-

915

1.99

025.

2123

30; 2

435;

24

50

~30

00; 4

000

Inso

lubl

eD

ark

gree

n, a

mor

phou

s po

wd

er f

orm

ing

hexa

gona

l cry

stal

s up

on h

eati

ng;

hyd

rosc

opic

(II)

(III

)oxi

de

Chr

omiu

m o

xid

eC

r 3O

412

018-

34-7

219.

968

6.1

Cub

ic c

ryst

als

(IV

)oxi

de

Chr

omiu

m o

xid

e;

chro

miu

m d

ioxi

de

CrO

212

018-

01-8

83.9

948

4.89

dec

: 400

(app

roxi

ma

te; l

oses

O2)

Inso

lubl

eB

row

nish

bla

ck a

cicu

lar

crys

talli

ne (t

etra

gona

l)

solid

(VI)

oxid

eC

hrom

ic t

riox

ide;

ch

rom

ium

anh

ydri

de

CrO

313

33-8

2-0

99.9

942

2.70

019

0; 1

95;

197

dec

: ~

250

617,

000

Dar

k re

d o

rtho

rhom

bic

crys

talli

ne (

flak

es o

r po

wd

er)

solid

; hy

dro

scop

icpe

rchl

orat

eC

hrom

ic p

erch

lora

te;

chro

miu

m p

erch

lora

teC

r(C

lO4)

313

537-

21-8

(III

)pho

spha

teC

hrom

ic p

hosp

hate

CrP

O4

7789

-04-

014

6.96

74.

6>

1800

Inso

lubl

eB

lue

orth

orho

mbi

c cr

ysta

ls(I

II)p

hosp

hate

he

mih

epta

-hy

dra

te

chro

miu

m p

hosp

hate

he

mih

epta

hyd

rate

CrP

O4

⋅ 3.5

H2O

8435

9-31

-921

0.02

12.

15In

solu

ble

Blu

ish-

gree

n po

wd

er

(III

)pho

spha

te

hyd

rate

Chr

omiu

m p

hosp

hate

hy

dra

teC

rPO

4⋅4

H2O

7789

-04-

02.

12@

14°C

Inso

lubl

eG

reen

cry

stal

s



(III

)pho

spha

te

hexa

hyd

rate

Chr

omiu

m p

hosp

hate

he

xahy

dra

teC

rPO

4 ⋅6

H2O

8435

9-31

-925

5.05

92.

121

dec

: >50

0In

solu

ble

Vio

let

crys

tals

phos

phid

eC

hrom

ium

pho

sphi

de

CrP

2634

2-61

-082

.970

5.25

Ort

horh

ombi

c cr

ysta

lspo

tass

ium

su

lfat

ePo

tass

ium

chr

omic

su

lfat

eC

rK(S

O4)

210

141-

00-1

1.81

389

Solu

ble

Dar

k vi

olet

-red

cry

stal

s

(III

)pot

assi

umsu

lfat

ed

odec

ahyd

rate

Chr

ome

alum

; chr

ome

alum

(do

deca

hydr

ate)

;po

tass

ium

bis

ulfa

te

12-w

ater

CrK

(SO

4)2

⋅12H

2O77

88-9

9-0

499.

405

1.82

622

0,00

0Pu

rple

to

viol

et-b

lack

cu

bic

crys

tals

(II)

sele

nid

ech

rom

ium

sel

enid

eC

rSe

1205

3-13

-313

0.95

66.

100

~15

00H

exag

onal

cry

stal

ssi

licid

ech

rom

ium

sili

cid

eC

r 3Si

1201

8-36

-918

4.07

46.

417

70C

ubic

cry

stal

ssi

licid

ech

rom

ium

sili

cid

eC

rSi 2

1201

8-09

-610

8.16

74.

9114

90G

ray

hexa

gona

l cry

stal

s(I

II)s

ulfa

tech

rom

ium

sul

fate

Cr 2

(SO

4)3

1010

1-53

-839

2.18

33.

1In

solu

ble

Red

dis

h-br

own

hexa

gona

l cry

stal

ssu

lfat

e pe

ntah

ydra

tech

rom

ium

(II)

sul

fate

pe

ntah

ydra

teC

rSO

4 ⋅5

H2O

1382

5-66

-023

8.13

6So

lubl

eB

lue

crys

tals

sulf

ate

7-w

ater

Hyd

rate

d c

hrom

ium

su

lfat

eC

rSO

4 ⋅7

H2O

274.

1722

9,00

0

sulf

ate

12-w

ater

Dic

hrom

ium

sul

fate

, 12

-hyd

rate

; d

ichr

omiu

m t

risu

lfat

e

CrS

O4

⋅12H

2O10

101-

53-8

608.

3472

Inso

lubl

ePe

ach-

colo

red

sol

id

(III

)sul

fide

Chr

omiu

m s

ulfid

e;

dic

hrom

ium

tri

sulfi

de

Cr 2

S 312

018-

22-3

200.

190

3.8

Bro

wn

to b

lack

cr

ysta

lline

sol

idst

eara

teC

hrom

ium

ste

arat

eC

r(C

18H

35O

2)3

95–1

00D

ark

gree

n po

wd

er(I

II)t

ellu

rid

eC

hrom

ium

tel

luri

de

Cr 2

Te3

1205

3-39

-348

6.79

7.0

~13

00H

exag

onal

cry

stal

szi

ncZ

inc

chro

mit

eZ

nCr 2

O4

1201

8-19

-823

3.38

5.29

Gre

en c

ubic

cry

stal

s

Chr

omat

e:

amm

oniu

mA

mm

oniu

m c

hrom

ium

ox

ide;

dia

mm

oniu

m

chro

mat

e

(NH

4)2C

r 2O

477

88-9

8-9

152.

0702

185

Solu

ble

Yello

w c

ryst

als

bari

um(V

)B

ariu

m c

hrom

ate

Ba 3

(CrO

4)2

1234

5-14

-164

3.96

85.

25So

lubl

eG

reen

ish-

blac

k he

xago

nal c

ryst

als

(Con

tinu

ed)



TAB

LE 2

.6

(Con

tinu

ed)

Com

pou

nd

Nam

es a

nd

S

ynon

yms

Form

ula

CA

S

Nu

mb

er

Mol

ar

Mas

s (g

/mol

)D

ensi

ty

(wat

er =

1)

Mel

tin

g P

oin

t (°

C)

Boi

lin

g P

oin

t (°

C)

Aq

ueo

us

Sol

ub

ilit

y at

20

°C

(mg/

l)P

hys

ical

D

escr

ipti

on

bari

um(V

I)B

ariu

m c

hrom

ate

BaC

rO4

1042

94-4

0-3

253.

3236

4.50

Inso

lubl

eYe

llow

, ort

horh

ombi

c cr

ysta

lsca

dm

ium

Cad

miu

m c

hrom

ate

Cd

CrO

414

312-

00-0

622

8.40

54.

5In

solu

ble

Yello

w, o

rtho

rhom

bic

crys

tals

calc

ium

Cal

cium

chr

omiu

m

oxid

e ca

lciu

m

chro

mat

e

CaC

rO4

1376

5-19

-015

6.07

362.

89Sl

ight

ly

solu

ble:

<10

0 @

22

°C

Bri

ght

yello

w p

owd

er

calc

ium

d

ihyd

rate

Cal

cium

chr

omat

e d

ihyd

rate

CaC

rO4 ⋅

2H2O

1376

5-19

-019

2.10

22.

50Sl

ight

ly

solu

ble

Yello

w, o

rtho

rhom

bic

crys

tals

chro

mic

Chr

omic

aci

d;

chro

miu

m c

hrom

ate

Cr 2

(CrO

4)3

2461

3-89

-6

coba

ltC

obal

tous

chr

omat

e:ba

sic

coba

lt c

hrom

ate

CoC

rO4

2461

3-38

-517

4.92

7~

4.0

Solu

ble

Bla

ck t

o br

own

crys

tals

or

yel

low

pow

der

copp

erC

oppe

r(II

) ch

rom

ate

CuC

rO4

1354

8-42

-017

9.54

0So

lubl

eR

edd

ish-

brow

n cr

ysta

lsco

pper

Cup

ric

chro

mat

e ba

sic

CuC

rO4⋅2

CuO

⋅ 2H

2OL

ight

cho

cola

te b

row

n po

wd

erir

onIr

on(I

II)

chro

mat

eFe

2(C

rO4)

310

294-

52-7

459.

671

Inso

lubl

eye

llow

pow

der

lead

Lea

d c

hrom

ate;

chr

ome

yello

wPb

CrO

477

58-9

7-6

323.

1936

6.12

384

4In

solu

ble:

<10

0Ye

llow

ish-

oran

ge

mon

oclin

ic c

ryst

als

or

oran

ge-b

row

n po

wde

r(I

I)ch

rom

ate

(VI)

oxid

eL

ead

chr

omat

e ox

ide

PbC

rO4 ⋅

PbO

1845

4-12

-154

6.4

Inso

lubl

eR

ed p

owd

er

lithi

umL

ithi

um c

hrom

ate

Li 2C

rO4

1430

7-35

-8So

lubl

eYe

llow

cry

stal

line,

d

eliq

uesc

ent

pow

der

lithi

um

dih

ydra

teL

ithi

um c

hrom

ate

dih

ydra

teL

i 2CrO

4 ⋅2H

2O77

89-0

1-7

165.

906

2.15

dec

: 75

Ver

y so

lubl

eYe

llow

ort

horh

ombi

c cr

ysta

lsm

ercu

ryM

ercu

ry c

hrom

ate

Hg 2

CrO

4d

ec o

n he

atin

gIn

solu

ble

Bri

ck r

ed p

owd

er



(II)

mer

cury

Mer

cury

chr

omat

eH

gCrO

413

444-

75-2

316.

586.

06So

lubl

eR

ed m

onoc

linic

cry

stal

ssi

lver

Silv

er c

hrom

ate

Ag 2

CrO

477

84-0

1-2

331.

730

5.62

5In

solu

ble

Bro

wni

sh-r

ed

mon

oclin

ic c

ryst

als

sod

ium

Chr

omat

e of

sod

a;

sod

ium

chr

omat

eN

a 2C

rO4

7775

-11-

316

1.97

314

2.72

792

Slig

htly

so

lubl

eYe

llow

ort

horh

ombi

c cr

ysta

lsst

ront

ium

Stro

ntiu

m c

hrom

ate

SrC

rO4

7789

-06-

220

3.61

363.

9d

ecSo

lubl

eYe

llow

mon

oclin

ic

crys

tals

pota

ssiu

mPo

tass

ium

chr

omat

eK

2CrO

477

89-0

0-6

2.73

997

1–97

5V

ery

solu

ble

Yello

w o

rtho

rhom

bic

crys

tals

zinc

Bas

ic z

inc

chro

mat

eZ

nCrO

413

530-

65-9

Solid

zinc

hyd

roxi

de

Zin

c hy

dro

xid

e ch

rom

ate

1593

0-94

-6

zinc

pot

assi

umPo

tass

ium

zin

c ch

rom

ate

hyd

roxi

de

1110

3-86

-9<

1,00

0G

reen

ish-

yello

w s

olid

Chr

omyl

:

chlo

rid

eC

hrom

ium

oxy

chlo

rid

eC

rO2C

l 214

977-

61-8

154.

901.

9145

–96.

511

6–11

7re

acts

wit

h H

2Od

ark

red

, tox

ic, f

umin

g liq

uid

fluo

rid

eC

hrom

ium

oxy

fluo

rid

eC

rF2O

3su

b at

29.

6bl

ack

crys

tals

; po

lym

eriz

es o

n ex

posu

re t

o lig

ht

Dic

hrom

ates

/Bic

hrom

ates

:

amm

oniu

mA

mm

oniu

m

dic

hrom

ate;

dic

hrom

ic

acid

(NH

4)2C

r 2O

777

88-0

9-5

252.

0644

2.15

170

310,

000

Bri

ght

oran

ge c

ryst

als

bari

um

dih

ydra

teB

ariu

m d

ichr

omat

e d

ihyd

rate

BaC

r 2O

7⋅2

H2O

1003

1-16

-038

9.34

6d

ecR

eact

s w

ith

wat

erB

row

nish

-red

nee

dle

s

calc

ium

Cal

cium

bic

hrom

ate

CaC

r 2O

714

307-

33-6

256.

0678

2.13

6So

lubl

eB

row

nish

-red

cry

stal

s

calc

ium

tr

ihyd

rate

Cal

cium

dic

hrom

ate

trih

ydra

teC

aCr 2

O7⋅3

H2O

1430

7-33

-631

0.11

22.

37V

ery

solu

ble

Red

dis

h or

ange

cry

stal

s

(III

)iro

nIr

on d

ichr

omat

eFe

2(C

r 2O

7)3

1029

4-53

-875

9.65

4So

lubl

eR

edd

ish-

brow

n so

lid

(Con

tinu

ed)



TAB

LE 2

.6

(Con

tinu

ed)

Com

pou

nd

Nam

es a

nd

S

ynon

yms

Form

ula

CA

S

Nu

mb

er

Mol

ar

Mas

s (g

/mol

)D

ensi

ty

(wat

er =

1)

Mel

tin

g P

oin

t (°

C)

Boi

lin

g P

oin

t (°

C)

Aq

ueo

us

Sol

ub

ilit

y at

20

°C(m

g/L

)P

hys

ical

D

escr

ipti

on

(II)

dic

hrom

ate

dih

ydra

teC

oppe

r d

ichr

omat

e d

ihyd

rate

CuC

r 2O

7σ⋅

2H

2O13

675-

47-3

315.

565

2.28

6V

ery

solu

ble

Red

dis

h-br

own

tric

linic

cr

ysta

lslit

hium

Lit

hium

dic

hrom

ate

Li 2C

r 2O

713

843-

81-7

2.34

@ 3

0°C

130

Solu

ble

Yello

wis

h-re

d

crys

talli

ne p

owd

erlit

hium

d

ihyd

rate

Lit

hium

dic

hrom

ate

dih

ydra

teL

i 2Cr 2

O7 ⋅

2H2O

1002

2-48

-726

5.90

12.

34d

ec: 1

30V

ery

solu

ble

Yello

wis

h-re

d

hyd

rosc

opic

cry

stal

s H

eavy

red

cry

stal

line

pow

der

mer

cury

Mer

cury

dic

hrom

ate;

m

ercu

ric

dic

hrom

ate;

m

ercu

ric

bich

rom

ate

HgC

r 2O

777

89-1

9-8

416.

58In

solu

ble

pota

ssiu

mPo

tass

ium

dic

hrom

ate;

bi

chro

mat

e of

pot

ash

K2C

r 2O

777

78-5

0-9

294.

1678

2.67

639

850

049

,000

Bri

ght

oran

gish

-red

tr

iclin

ic c

ryst

als

silv

erSi

lver

dic

hrom

ate

Ag 2

Cr 2

O7

7784

-02-

343

1.72

44.

770

Solu

ble

Red

dis

h cr

ysta

lsso

diu

mD

isod

ium

dic

hrom

ate;

+

sod

ium

dic

hrom

ate

Na 2

Cr 2

O7

1058

8-01

-926

1.96

734

3.57

356.

740

0V

ery

solu

ble

Red

hyd

rosc

opic

cr

ysta

lsso

diu

m

tetr

ahyd

rate

Sod

ium

chr

omat

e te

trah

ydra

teN

a 2C

r 2O

7 ⋅ 4H

2O10

034-

82-9

234.

035

dec

Ver

y so

lubl

eYe

llow

hyd

rosc

opic

cr

ysta

lszi

ncZ

inc

bich

rom

ate

ZnC

r 2O

714

018-

95-2

281.

3778

Not

e:g/

mol

= g

ram

s pe

r m

ole;

mg/

l = m

illig

ram

s pe

r lit

er; °

C =

deg

rees

Cel

sius

; dec

= d

ecom

pose

s; s

ub =

sub

limes

.

Sour

ce:

Dea

n (1

992)

; Che

mID

plus

(20

01);

Che

mfi

nder

(20

01).



TABLE 2.7

Total Chromium Concentration in Natural Substances

ParameterReported

Units Average RangeEnrichmentFactor (EF) References

Universe ppm 15 — — (21)Solar system Relative to

H = 1 × 1012

5.13 × 105 — — (17)

Sun ppm 20 — — (21)Carbonaceous meteorites mg/kg — — — (21)Mantle mass percent 0.41–0.43 — (3)Crust mg/kg 100 — — (10) (22)Igneous rocks: mg/kg 117 — 1.17 (4) (14)Ultramafic mg/kg 2000 — 20.0 (4) (14)Mafic (basalts) mg/kg 220 40–600 2.20 (4) (14)Felsic (granites) mg/kg 20 2–100 0.20 (4) (14)

Sedimentary rocks:Limestone mg/kg 10 <1–120 0.10 (4) (14)Sandstone mg/kg 35 34–90 0.35 (4) (14)Shale mg/kg 90, 120 30–590 0.9–1.20 (14)Black shale mg/kg 90, 100 26–1000 0.9–1.0 (4) (13) (14)Sediments: mg/kg 72 — — (5)Deep sea clay mg/kg 90 — 0.90 (5)Shallow water mg/kg 60 — 0.60 (5)River suspended mg/kg 100 — 1.00 (5)Stream bed (U.S.A.) mg/kg 64a <1.0–700 0.64 (17)

Soil:World mg/kg 200 — 2.00 (1)U.S.A. mg/kg 54 1–2000 0.54 (6) (14)California mg/kg 15.4 4–32 0.15 (7)Coal ppm 15 250 (max) 0.15 (11)Petroleum ppm 0.09–3.15 10.7 (max) (13)Asphalt ppm 6.0 (max) — (2)Plant (ash) ppm 9 (1)

Water:Ground µg/L — 0.04–20 — (14)Ocean µg/L 0.05–0.3 0.156–0.26 — (8) (15) (19)

(22)North Sea µg/L — 0.7 — (16)Atlantic (surface) ppb 0.18 — — (15) (17)Atlantic (deep) ppb 0.23 — — (15) (17)Pacific (surface) ppb 0.15 — — (15) (17)Pacific (deep) ppb 0.25 — — (15) (17)River (total)b µg/L 0.5b — — (5)River (dissolved) µg/L — 0.02–0.3 — (16)Great Lakes (U.S.A/Canada)

µg/L 1.0 0.2–19 (8)

Central Canada(surfacewater)

µg/L — 0.2–44 — (8) (16)

(Continued)



TABLE 2.7

(Continued)

ParameterReported

Units Average RangeEnrichmentFactor (EF) References

Atlantic Region-Canada µg/L — 0.2–24 (8)Antarctic Lakesc µg/L — <0.6–30 — (16)Precipitation µg/L — 0.2–1.0 — (16)

Air:Arctic pg/m3 — 50–70 — (16)U.S.A. ng/m3 <300 10–50 — (8) (16)California (2000) ng/m3 4.9 1–40 — (17)

ng/m3 0.13 0.1–1.2 — (20)Canada (5 remote areas) ng/m3 0.25–0.32 (8)Netherlands ng/m3 — 2–5 — (16)Indoor (tobacco smoke) ng/m3 ~1000 — — (16)

Human tissue:mg/kg 30 — —

Blood mg/dL — 0.006–0.11 — (17)Bone ppm — 0.1–0.33 — (17)Liver ppm — 0.02–3.3 — (17)Muscle ppm — 0.024–0.84 — (17)

Food:µg/kg <10–1300 — (16)

Milk and dairy products mg/kg 0.06 — — (8)Meat mg/kg 0.07 — — (8)Cereal mg/kg 0.17 — — (8)Potatoes mg/kg 0.05 — — (8)Fruits mg/kg 0.06 — — (8)Sugars mg/kg 0.34 — — (8)Intake from food and water

µg/day 52–943 — — (16)

Total intake from air,water, and food in U.K.

µg/day 76–106 — (16)

Total intake from air, water, and ford in

Netherlands

µg/day 100 50–200 — (16)

a Median concentration.b Background concentration.c Concentration increases with depth.

Note: EF = average materials concentration/average crustal concentration.

Sources: 1. Hawkes and Webb (1962); 2. Krejci-Graf (1972); 3. Henderson (1982); 4. Thornton(1983); 5. Salomons and Förstner (1984); 6. Shacklette and Boerngen (1984); 7. Pettygrove andAsano (1985); 8. Canadian Environmental Health Directorate (1986); 9. Hem (1989); 10. Sposito(1989); 11. Finkelman (1993); 12. Manning and Gize (1993); 13. Leventhall (1993); 14. Allard(1995); 15. Donat and Bruland (1995); 16. WHO (1996); 17. Emsley (1999); 18. Rice (1999);19. California Air Resources Board (2001a); 20. California Air Resources Board (2001b); 21.Winter: WebElements (2001); 22. Firestone (2002).



Efforts have been made to define what is meant by the terms “natural,”“normal,” and “background” and why such distinctions are important.Fergusson (1990) notes the following:

An important reason for knowing the natural concentrations of the heavyelements, is that they provide a true reference point for estimating theextent of pollution from the elements. This is of particular importancewhen assessing the toxic effects of the elements. Natural levels allowcontemporary levels to be seen in perspective, i.e., whether they areexcessive or not.

Element concentrations currently found in the biosphere are often callednormal, which may be misinterpreted for natural. Fergusson notes that the term“typical” is more applicable to contemporary trace element concentrations. Asfor natural element concentrations, distinctions must be made between:

1. Ancient concentrations as determined in prehistoric human remainssuch as bone, teeth, and hair recovered from skeletal and mummifiedremains. True prehuman background concentrations in naturalmaterials may also be found in deep ice, sediment and peat cores,and in deep ocean waters as opposed to surface sea water which hasbeen contaminated by human inputs (e.g., streams, rivers, and theatmosphere).

2. Remote places that include the Arctic, Antarctic, Greenland, mid-ocean water, and in mountainous or alpine areas such as the Hima-laya Mountains of Tibet. These areas have had little human impact,although with increasing atmospheric pollution even remote areascan be significantly affected.

3. Mineralized areas where element concentrations are higher than thesurrounding region. Such element concentrations often surroundknown ore deposits or areas having undergone element enrichmentfrom hydrothermal (hot spring) activity. The difference in mineral-ized areas from natural background is often called “threshold con-centrations” (Hawkes and Webb, 1962). The determination of naturalbackground or threshold concentrations of metals in surface waterin mineralized areas has become a significant problem because manyformerly mined areas have been designated as Superfund sitesresulting from metals contamination produced by waste rock dumpsand tailings at mines and smelter sites. Therefore, understandingbackground concentrations derived from mineralized areas becomesimportant in distinguishing natural threshold background concen-trations from mined areas (Runnels et al., 1992).

4. Rural settings, that is, those areas away from urban environments.However, some rural areas such as farms may have element con-centrations that have been influenced by the use of fertilizers andpesticides.

Au: Figure citations in text were missing, we have sup-plied them. Please check.



5. Urban areas, such as those within cities, which generally have ele-ment concentrations higher than natural background. This is partic-ularly true of surface soils in parks, greenbelts, and alongside roadsand highways that are impacted by vehicles using and burningpetroleum products and fuels (lubricating oil, gasoline, and diesel).

6. Industrial areas and zones where element concentrations may be con-siderably above those found in the other settings but still belowregulatory agency levels which might be considered toxic to humans.

For a detailed explanation of the above, the reader is referred to Fergusson(1990, Chap 6, pp. 166–176).

2.2.1 Mantle

In the earth’s mantle, chromium occurring as Cr2O3 may range in concentra-tions from 0.41 to 0.55% depending on the model used (Henderson, 1982).That chromium originated in the upper mantle has been fairly well establishedin the study of podiform chromites, which are believed to have almost exclu-sively originated in the upper mantle or in crust-mantle transition that occursin suprasubduction zones (Li et al., 2002).

2.2.2 Chromium Minerals

The International Minerals Association (IMA) recognized, as of mid-2002,the existence of 82 chromium minerals occurring in the natural environment(Table 2.8a); most are rather rare with some minerals unique to one minerallocality or deposit. Several chromium minerals are unique to meteorites. Themajor chromium ore mineral is chromite, a magnesium–iron–chromium–aluminum oxide, [(Mg,Fe)(Al,Cr,Fe)2O4] in which the chromic oxide contentvaries from approximately 15 to 65% due to isomorphous substitution ofchromium for iron or aluminum. Chromium concentrations in chromitegenerally average 46.46% (Sposito, 1989; Darrie, 2001; Barthelmy, 2002).

Of the 82 known chromium minerals, 23 (approximately 30%) are Cr(VI)-bearing minerals; these are in the Dana mineral classes of anhydrous chro-mates; compound chromates; compound phosphates; compound borates;compound iodates, hydroxides, and oxides; and multiple oxides.

Many chromium minerals are quite colorful (Table 2.8b); for example,uvarovite (a chromium-bearing garnet) is bright green and crocoite (an anhy-drous chromate) forms bright-reddish orange acicular crystals. Mineral col-lectors covet such chromium minerals.

2.2.3 Chromium Ore Deposits

Chromite ore is not currently commercially mined in the United States, Can-ada, or Mexico. In the Western Hemisphere it is mined only in Brazil andCuba. U.S. mining ceased in 1961, when the United States Defense Production

Au: Ok?



TAB

LE 2

.8A

Chr

omiu

m M

iner

als:

Che

mic

al F

orm

ula,

Cla

ssifi

cati

on, a

nd L

ocal

itie

s

Min

eral

Nam

eFo

rmu

laC

r C

onc.

(%

)

Dan

a C

lass

ifica

tion

Typ

e L

ocal

ity/

Loc

alit

ies

En

viro

nm

ent/

Rem

ark

sM

iner

al C

lass

No.

Ank

angi

teB

a(Ti

, V+

3 , C

+3 )

8O16

1.29

Mul

tipl

e ox

ides

7.9.

4.2

Ank

ang

Co.

, Sha

anxi

pr

ovin

ce, C

hina

Bar

ber

ton

ite

Mg 6

Cr 2

(CO

3)(O

H) 1

6⋅4(

H2O

)15

.90

Car

bona

tes—

hy

dro

xyl o

r ha

loge

n

16b.

6.1.

2K

aaps

che

Hoo

p, B

arbe

rton

, Tr

ansv

aal,

Rep

ublic

of S

outh

A

fric

aB

ento

rite

Ca 6

(Cr,

Al)

2(SO

4)3

(OH

) 12⋅2

6 (H

2O)

6

.03

Hyd

rate

d s

ulfa

tes

31.1

0.2.

2H

atru

rim

For

mat

ion,

Dea

d

Sea,

Isr

ael

Bra

cew

ellit

eC

r+3 O

(OH

)61

.17

Hyd

roxi

des

and

ox

ides

con

tain

ing

hyd

roxy

l

6.1.

1.5

Mer

ume

Riv

er, K

amak

usa,

M

arar

uni d

istr

ict,

Guy

ana

Bre

zina

ite

Cr 3

S 454

.88

Sulfi

des

— in

clud

ing

sele

nid

es a

nd

tellu

rid

es

2.10

.2.2

Tucs

on M

eteo

rite

, Pim

a C

o.,

AZ

Cal

sber

gite

CrN

78.7

8N

ativ

e el

emen

ts1.

1.20

.1A

gpal

ilik

frag

men

t, C

ape

York

met

eori

te, G

reen

land

Car

mic

hael

ite

(Ti,

Cr,

Fe)[

O2x

(OH

) x]

15.8

1M

ulti

ple

oxid

esw

ith

Nb,

Ta,

and

Ti

8.7.

13.1

Gar

net

Rid

ge U

ltra

mafi

cD

iatr

eme,

Col

orad

oPl

atea

u, A

Z

Mic

rosc

opic

incl

usio

ns in

m

antl

e-d

eriv

ed p

yrob

e ga

rnet

.C

asse

nd

ann

eite

Pb5(

VO

4)2(

CrO

4)2

(H2O

)

6.8

6C

ompo

und

ph

osph

ates

43.3

.2.2

Ber

esov

, Sve

rdlo

vsk

(Eka

teri

nbur

g), U

ral

Mou

ntai

ns, R

ussi

a

Ass

ocia

ted

wit

h em

brey

ite.

Cas

wel

lsilv

erit

eN

aCrS

237

.38

Sulfi

des

—in

clud

ing

sele

nid

es a

nd

tellu

rid

es

2.9.

17.1

Nor

ton

Cou

nty

met

eori

te, K

S

(Con

tinu

ed)



TAB

LE 2

.8A

(Con

tinu

ed)

Min

eral

Nam

eFo

rmu

laC

r C

onc.

(%

)

Dan

a C

lass

ifica

tion

Typ

e L

ocal

ity/

Loc

alit

ies

En

viro

nm

ent/

Rem

ark

sM

iner

al C

lass

No.

Ch

rom

atit

eC

aCrO

433

.32

Anh

ydro

us

chro

mat

es35

.3.2

.1Je

rusa

lem

-Jer

icho

Hig

hway

, Jo

rdan

Chr

ombi

smit

eB

i 16C

rO27

1.

36O

xid

e m

iner

als

4.0.

0.0

Jial

u m

ine,

Luo

nan,

Sha

anxu

pr

ovin

ce, C

hina

Chr

omce

lad

onit

eK

CrM

g[Si

4O10

](O

H) 2

12.3

3M

ica

grou

p71

.2.2

a.15

Sred

nyay

a Pa

dm

a U

-V

dep

osit

, Sou

ther

n K

arel

ia,

Rus

sia

Foun

d in

met

asom

atiz

ed-

hyd

roth

erm

ally

alt

ered

U-V

dep

osit

s. C

r-d

omin

ant

anal

og 1

of

alum

inoc

elad

onit

e.

Form

erly

IM

A 1

999-

024.

Chr

omd

ravi

teN

aMg 3

(Cr,

Fe+

3 )6(

BO

) 33S

i 6O18

(OH

) 420

.99

Cyc

osili

cate

s:

tour

mal

ines

61.3

e.1.

11O

nega

dep

ress

ion,

Kar

elia

, R

ussi

aC

hrom

feri

de

Fe3C

r 1−x

(x

= 0

.6)

11.0

4N

ativ

e el

emen

ts1.

1.12

.2K

umak

reg

ion,

Ura

l M

ount

ains

, Rus

sia

Chr

omit

e (c

hrom

e ir

on o

re; c

hrom

ic

iron

)

Fe+

2 Cr 2

O4

46.4

6M

ulti

ple

oxid

es7.

2.3.

3St

illw

ater

, MT;

Bas

tid

e d

e la

C

arra

de,

Gas

sin,

Var

, Fra

nce

Prim

ary

ore

of c

hrom

ium

; oc

curs

in g

rani

tes

and

ot

her

igne

ous

rock

s;

occu

rs in

pla

cer d

epos

its.

Chr

omiu

mC

r10

0.00

Nat

ive

elem

ents

1.1.

12.1

Sich

uan

(Sze

chua

n), C

hina

Chr

omph

yllit

e(K

, Ba)

(Cr,

Al)

2[A

lSi 3

O10

](O

H, F

) 2Tr

ace

Mic

as—

mus

covi

te

subg

roup

71.2

.21.

11Pe

reva

l mar

ble

quar

ry; K

aper

pi

t Pok

habi

kha

Riv

er V

alle

y,

Sout

hern

Lak

e Ba

ikal

regi

on,

Sibe

ria,

Rus

sia

Slyu

dyan

ka c

ompl

ex,

Sibe

ria,

Rus

sia:

Cr-

enri

ched

laye

rs in

qua

rtz-

diop

side

-bea

ring

roc

ks.

Coc

hrom

ite

(Co,

Ni,

Fe+

2 )(C

r, A

l)2O

4

trac

eM

ulti

ple

oxid

es—

sp

inel

gro

up7.

2.3.

5B

on A

ccor

d n

icke

l dep

osit

, B

arbe

rton

, Tra

nsva

al,

Rep

ublic

of

Sout

h A

fric

a



Coc

roit

ePb

CrO

416

.09

Anh

ydro

us

chro

mat

es35

.3.1

.1A

del

aid

e M

ine,

Dun

das

, Ta

sman

iaSe

cond

ary

Cr

min

eral

inox

idiz

ed P

b ve

ins

infi

ltra

ted

by

Cr-

bear

ing

flui

ds

Dau

bree

lite

Fe+

2 Ce 2

S 436

.10

Sulfi

des

— in

clud

ing

sele

nid

es a

nd

tellu

rid

es

2.10

.1.1

1B

olso

nde

Map

imi m

eteo

rite

, M

exic

o

Dea

nes

mit

hit

eH

g+2H

g+2 3C

r+6 O

5S2

4.

34A

nhyd

rous

ch

rom

ates

35.4

.3.1

Cle

ar C

reek

mer

cury

min

e,

New

Id

ria

dis

tric

t, Sa

n B

enit

o C

o., C

AD

ietz

eite

Ca 2

(IO

3)2(

CrO

4)9.

52C

ompo

und

iod

ates

23.1

.1.1

Lau

taro

, Ata

cam

a D

eser

t, A

ntof

agas

ta, C

hile

Don

athi

te(F

e+2 ,

Mg)

(Cr,

Fe+

3 )2O

423

.65

Oxi

de

min

eral

s4.

0.0.

0H

estm

and

ö Is

land

, Nor

way

Mix

ture

of

two

spin

els:

on

e cu

bic

and

one

te

trag

onal

Du

kei

teB

i 24C

r+6 8O

57(O

H) 6

(H2O

) 3

6.40

Hyd

roxi

des

and

ox

ides

con

tain

ing

hyd

roxy

l

6.4.

12.1

Lav

ra d

a Po

sse,

San

Jos

e d

eB

reja

uba,

Con

ceic

ao d

oM

ato

Den

tro

Cou

nty,

Bra

zil

Foun

d o

n a

mus

eum

sp

ecim

en o

f puc

heri

te a

t D

uke

Uni

vers

ity,

D

urha

m, N

CE

doy

leri

teH

g+2 3C

r+6 O

4S2

6.

65A

nhyd

rous

ch

rom

ates

35.4

.4.1

Cle

ar C

reek

mer

cury

min

e,

New

Id

ria

dis

tric

t, Sa

n B

enit

o C

ount

y, C

AE

mb

reyi

tePb

5(C

rO4)

2(PO

4)2 ⋅

H2O

7.

05C

ompo

und

ph

osph

ates

43.4

.3.1

Cum

bria

, Eng

land

; Ber

ezov

, E

kate

rinb

urg

(Sve

rdlo

vsk)

, U

ral M

ount

ains

., R

ussi

aE

skol

aite

Cr 2

O3

68.4

2Si

mpl

e ox

ides

4.3.

1.3

Out

okum

pu, K

arel

ia, F

inla

ndFe

rchr

omid

eC

r 3Fe

1−x

(x =

0.6

)87

.47

Nat

ive

elem

ents

1.1.

12.3

Kum

ak r

egio

n, U

ral

Mou

ntai

ns, R

ussi

aFl

oren

sovi

teC

u(C

r 1.5Sb

0.5)

S 423

.59

Sulfi

des

—in

clud

ing

sele

nid

es a

nd

tellu

rid

es

2.10

.1.1

4Sl

yud

yank

a co

mpl

ex, L

ake

Bai

kal r

egio

n, Z

abai

kaly

e (T

rans

baik

al) S

iber

ia, R

ussi

a

(Con

tinu

ed)



TAB

LE 2

.8A

(Con

tinu

ed)

Min

eral

Nam

eFo

rmu

laC

r C

onc.

(%

)

Dan

a C

lass

ifica

tion

Typ

e L

ocal

ity/

Loc

alit

ies

En

viro

nm

ent/

Rem

ark

sM

iner

al C

lass

No.

Forn

acit

e(P

b, C

u)3[

(Cr,

As)

O4]

2

(OH

)6.

93C

ompo

und

ph

osph

ates

43.4

.3.2

El K

hun

min

e, A

nara

k, I

ran

Geo

rgee

rick

sen

ite

Na 6

CaM

g(IO

3)6

(CrO

4)2⋅

12(H

2O)

6.

12C

ompo

und

iod

ates

23.1

.2.3

Ofic

ina

Cha

cabu

co, C

hile

Ari

d c

limat

e m

iner

al

asso

ciat

ed w

ith

Chi

lean

ni

trat

e ca

liche

dep

osit

s.

Coe

xist

s w

ith

halit

e,

nitr

atin

e, a

nd n

iter

Gri

mal

diit

eC

r+3 O

(OH

)61

.17

Hyd

roxi

des

and

ox

ides

con

tain

ing

hyd

roxy

l

6.1.

5.1

Mer

ume

Riv

er, K

amak

usa,

M

arar

uni d

istr

ict,

Guy

ana

Trig

onal

—he

xago

nal

scal

enoh

edra

l

Guy

anai

teC

r+3 O

(OH

)61

.17

Hyd

roxi

des

and

ox

ides

con

tain

ing

hyd

roxy

l

6.1.

2.3

Mer

ume

Riv

er, K

amak

usa,

M

arar

uni d

istr

ict,

Guy

ana

Ort

horh

ombi

c—

dip

yram

idal

Has

hem

ite

Ba(

Cr,

S)O

415

.70

Anh

ydro

us

chro

mat

es35

.3.3

.1L

isd

an-S

iwag

a Fa

ult,

Has

hem

reg

ion,

Am

man

, Jo

rdan

Haw

thor

init

eB

a[Ti

3Cr 4

Fe4M

g]O

1919

.99

Mul

tipl

e ox

ides

7.4.

1.3

Bul

tfon

tein

, Kim

berl

ey,

Rep

ublic

of

Sout

h A

fric

aH

eid

eite

(Fe,

Cr)

1+x(

Ti, F

e)2S

43.

55Su

lfid

es —

incl

udin

g se

leni

des

and

te

lluri

des

2.10

.2.3

Bus

tee

met

eori

te, G

orak

hpur

, B

asti

dis

tric

t, U

ttar

Pra

des

h,

Ind

ia

Hem

ihed

rite

Pb10

Zn(

CrO

4)6

(SiO

4)2F

2

10.2

1C

ompo

und

ch

rom

ates

36.1

.1.2

Flor

ence

Pb-

Ag

min

e,

Whi

cken

burg

, Mar

icop

a C

o., A

Z



IMA

199

8-02

9(C

e, R

EE

,Ca)

4

(Mg,

Fe2+

)(M

g, F

e3+) 2

(Ti,

Nb)

2 Si

4O22

6.26

Soro

silic

ates

56.2

.8.7

Cr a

nalo

g of

che

vkin

ite

—

(Ce)

str

uctu

re

IMA

199

9-01

8C

a 0.2(H

2O) 2

CrS

232

.46

Sulfi

des

— in

clud

ing

sele

nid

es a

nd

tellu

rid

es

32.4

6C

lose

to

scho

llhor

nite

.

IMA

1999

-034

PbC

r+3 2(

CO

3)2(

OH

) 4H

2O20

.10

Car

bona

tes

—

hyd

roxy

l or

halo

gen

16b.

2.1.

4C

r an

alog

of

dun

das

ite

IMA

200

0-04

2M

g 6C

r 2(O

H) 1

6C12

⋅ 4H

2O15

.64

Car

bona

tes

—

hyd

roxy

l or

halo

gen

16b.

6.2.

5

Iqu

iqu

eite

K3N

a 4M

g(C

r+6 O

4)B

24

O39

(OH

) ⋅ 12

(H2O

)

3.55

Com

poun

d b

orat

es27

.1.8

.1Z

apig

a, I

quiq

ue, T

arap

acá

prov

ince

, Chi

leIr

anit

ePb

10C

u(C

rO4)

6(Si

O4)

2

(F, O

H) 2

10.2

2C

ompo

und

ch

rom

ates

36.1

.1.1

Seba

rz m

ine,

Ana

rak,

Ira

n

Isov

ite

(Cr,

Fe) 2

3C6

67.4

1N

ativ

e el

emen

ts1.

1.16

.3Is

Riv

er, I

sovk

y D

istr

ict,

mid

dle

Ura

ls, R

ussi

aC

r an

alog

of

haxo

nite

; fo

und

in A

u-Pt

pla

cers

al

ong

the

Is R

iver

.K

alin

init

eZ

nCr 2

S 434

.94

Sulfi

des

— in

clud

ing

sele

nid

es a

nd

tellu

rid

es

2.10

.1.1

3Sl

yud

yank

a d

epos

it, S

outh

B

aika

l, Z

abai

kaly

e (T

rans

baik

al),

Sibe

ria,

R

ussi

aK

norr

ingi

teM

g 3C

r 2(S

iO4)

322

.95

Nes

osili

cate

s —

ga

rnet

gro

up65

.1.3

c.4

Kao

kim

berl

ite

pipe

, Les

otho

Kos

moc

hlor

NaC

r+3 S

i 2O6

22.8

9In

osili

cate

s (c

linop

yrox

enes

)51

.4.3

a.4

Tolu

ca m

eteo

rite

, Xiq

uipi

lco,

M

exic

oK

rino

vite

NaM

g 2C

rSi 3O

1014

.14

Inos

ilica

tes

(aen

igm

atit

es)

69.2

.1a.

4C

anyo

n D

iabl

o m

eteo

rite

, M

eteo

r C

rate

r, C

ocon

ino

Co.

, AZ

Subh

edra

l gra

ins

in

grap

hite

nod

ules

in

octa

hed

rite

s

(Con

tinu

ed)



TAB

LE 2

.8A

(Con

tinu

ed)

Min

eral

Nam

eFo

rmu

laC

r C

onc.

(%

)

Dan

a C

lass

ifica

tion

Typ

e L

ocal

ity/

Loc

alit

ies

En

viro

nm

ent/

Rem

ark

sM

iner

al C

lass

No.

Lin

dsl

eyit

e(B

a, S

r)(T

i, C

r, Fe

,M

g)21

O38

17.6

9M

ulti

ple

oxid

es w

ith

Nb,

Ta, a

nd T

i8.

5.1.

8D

eBee

rs m

ine,

Kim

berl

y,

Rep

ublic

of

Sout

h A

fric

aFo

und

in k

imbe

rlit

es a

nd

peri

dot

ite

nod

ules

Lop

ezit

eK

2Cr 2

O7

35.3

5A

nhyd

rous

ch

rom

ates

35.2

.1.1

Ofic

ina

Mar

ia E

lena

, To

pcop

illa

and

Ofi

cina

R

osar

io, I

quiq

uw P

ampa

, Ta

rapa

cá, C

hile

Ata

cam

a D

eser

t, C

hile

Lov

erin

gite

(C

a ,C

e)(T

i, Fe

+3 ,

Cr,

Mg)

21O

38

6.81

Mul

tipl

e ox

ides

wit

h N

b, T

a, a

nd T

i8.

5.1.

2Ji

mbe

rlan

a in

trus

ion,

N

orse

man

, Wes

tern

A

ustr

alia

, Aus

tral

ia

Syno

nym

: chr

ompi

coti

te

Mac

qu

arti

tePb

3Cu(

CrO

4)(S

iO3)

(OH

) 4 ⋅ 2

H2O

5.

30C

ompo

und

ch

rom

ates

36.1

.2.1

Mam

mot

h m

ine,

Tig

er, P

inal

C

ount

y, A

ZE

ncru

sts

dio

ptas

e

Mag

nesi

ochr

omit

eM

gCr 2

O4

54.0

8M

ulti

ple

oxid

es—

sp

inel

gro

up7.

2.3.

1Sc

hwar

zenb

erg,

Sile

sia,

G

erm

any

Man

gano

chro

mit

e(M

n, F

e+2 )

(Cr,

V) 2

O4

43.8

0M

ulti

ple

oxid

es—

sp

inel

gro

up7.

2.3.

2N

airn

e d

epos

it, B

ruku

nga,

A

del

aid

e, S

outh

Aus

tral

ia,

Aus

tral

ia

Syno

nym

: chr

ompi

coti

te

Mat

hias

ite

(K, C

a, S

r)(T

i, C

rFe,

Mg)

21O

38

18.5

4M

ulti

ple

oxid

es w

ith

Nb,

Ta,

and

Ti

8.5.

1.7

Jage

rsfo

ntei

n d

iam

ond

min

e,

Ora

nge

Free

Sta

te, R

epub

lic

of S

outh

Afr

ica

Occ

urs

in k

imbe

rlit

e

Mcc

onne

llite

CuC

rO2

35.2

4M

ulti

ple

oxid

es7.

1.1.

2M

erum

e R

iver

, Kam

akus

a,

Mar

arun

i dis

tric

t, G

uyan

aM

olyb

dof

orn

acit

ePb

2Cu[

(As,

P)O

4][M

o,C

r)O

4](O

H)

1.32

Com

poun

d

phos

phat

es43

.4.3

.3Ts

umeb

, Nam

ibia



Mon

gsha

nite

(Mg,

Cr,

Fe2+

) 2(T

i, Z

r)5

O12

5.24

Mul

tipl

e ox

ides

7.11

.11.

1Y

imen

guan

are

a (?

), Sh

and

ong,

Chi

na

Con

sid

ered

a “

dou

btfu

l”

spec

ies.

Phy

sica

l pr

oper

ties

are

sim

ilar

to

ilmen

ite

(ilm

); fo

und

as

incl

usio

ns in

ilm

. Not

an

IM

A a

ppro

ved

nam

e.M

ount

keit

hite

(Mg,

Ni)

11(F

e3+, C

r)3

(SO

4, C

O3)

3.5(

OH

) 24 ⋅

11

H2O

3.

34C

ompo

und

sul

fate

s32

.4.3

.1M

t. K

eith

Ni d

epos

it, 4

00 k

m

nort

h-no

rthw

est

of

Kal

gooo

lie, A

ustr

alia

Occ

urs

in b

leac

hed

se

rpen

tini

te

Nat

alyi

teN

a(V

+3 ,C

r+3 )

Si2O

65.

74In

osili

cate

s (c

linop

yrox

enes

)65

.1.3

c.5

Slyu

dyan

ka c

ompl

ex, L

ake

Baik

al r

egio

n, Z

abai

kaly

e (T

rans

baik

al)

Sibe

ria,

Rus

sia

Occ

urs

in q

uart

z-be

arin

g sc

hist

s

Nic

hrom

ite

(Ni,

Co,

Fe+

2 )(C

r, Fe

+3 ,

Al)

2O4

30.6

5M

ulti

ple

oxid

es —

sp

inel

gro

up7.

2.3.

4B

on A

ccor

d n

icke

l dep

osit

, Tr

ansv

aal,

Rep

ublic

of S

outh

A

fric

aO

lkho

nski

te(C

r+3 ,

V+

3 )2T

i 3O9

19.9

4M

ulti

ple

oxid

es w

ith

Nb,

Ta,

and

Ti

8.4.

1.2

Wes

tern

sho

re o

f Lak

e B

aika

l, 4.

5 km

sou

th o

f O

lkho

n Is

land

, Rus

sia

Pett

erit

ePb

Cr3+

(CO

3)2

(OH

) 4 ⋅ H

2O20

.10

Car

bona

tes

—

hyd

roxy

l or

halo

gen

16b.

2.1

Red

Lea

d m

ine,

Zee

han-

Dun

das

min

ing

fiel

d,

nort

hwes

tern

Tas

man

ia,

Aus

tral

ia

Foun

d in

oxi

diz

ed z

one

Cr-

anal

og o

f d

und

asit

e Fo

rmer

ly I

MA

199

9-03

4

Ph

oen

icoc

hro

ite

Pb2(

CrO

4)O

9.

52A

nhyd

rous

ch

rom

ates

35.1

.2.1

Ber

ezov

, Eka

teri

nbur

g (S

verd

lovs

k), U

ral

Mou

ntai

ns, R

ussi

aR

edin

gton

ite

(Fe2+

, Mg,

Ni)

(Cr,

Al)

2

(SO

4)4 ⋅

22H

2O

8.49

Hyd

rate

d a

cid

and

su

lfat

es29

.7.3

.6R

edin

gton

min

e, K

noxv

ille,

N

apa

Co.

, CA

Hg

exha

lati

ve a

nd

sand

ston

e-ho

sted

U-V

d

epos

its

Red

led

geit

eB

aTi 6C

r 2O

16(H

2O)

12.9

6M

ulti

ple

oxid

es7.

9.5.

2R

ed L

edge

min

e so

uth

of

Was

hing

ton,

Nev

ada

Cou

nty,

CA

(Con

tinu

ed)

Au

: Ple

ase

chec

k fo

r th

e m

issi

ng

info

rmat

ion

.



TAB

LE 2

.8A

(Con

tinu

ed)

Min

eral

Nam

eFo

rmu

laC

r C

onc.

(%

)

Dan

a C

lass

ifica

tion

Typ

e L

ocal

ity/

Loc

alit

ies

En

viro

nm

ent/

Rem

ark

sM

iner

al C

lass

No.

Rila

ndit

e(C

r,Al)

6SiO

11⋅5

H2O

(?)

41.1

5M

ulti

ple

oxid

es7.

11.1

4.1

Rila

nd c

arno

tite

cla

ims

20 k

m E

NE

of

Mee

ker,

Coa

l Cre

ek, C

O

Sand

ston

e-ho

sted

U-V

d

epos

its

San

tan

aite

Pb11

CrO

62.

01A

nhyd

rous

ch

rom

ates

35.4

.1.1

Sant

a A

na m

ine,

Car

acol

es,

Sier

ra G

ord

a, C

hile

Scho

llhor

nite

Na 0

.3C

rS2⋅H

2O36

.87

Sulfi

des

— in

clud

ing

sele

nid

es a

nd

tellu

rid

es

2.9.

17.2

Ens

tati

te a

chon

dri

te

met

eori

te, N

orto

n C

ount

y,

KS

Wea

ther

ing

prod

uct

of

casw

ells

ilver

ite

Shui

skit

eC

a 2(M

g,A

l)(C

r,Al)

2

(SiO

4)(S

i 2O7)

(OH

) 2 ⋅

H2O

Trac

eSo

rosi

licat

es58

.2.2

.9B

iser

sk d

epos

it, U

ral

Mou

ntai

ns, R

ussi

a

Stic

htit

eM

g 6C

r 2(O

H) 1

6(C

O3)

⋅4H

2O15

.90

Car

bona

tes

—

hyd

roxy

l or

halo

gen

16b.

6.2.

2A

del

aid

e m

ine,

Sti

chti

te H

ill,

Dun

das

, Tas

man

ia,

Aus

tral

ia

Alt

erat

ion

prod

uct

of

serp

enti

ne

Tara

pac

aite

K2C

rO4

26.7

8A

nhyd

rous

ch

rom

ates

35.1

.1.1

Sant

a A

na m

ine,

Car

acol

es,

Sier

ra G

ord

a, C

hile

Ata

cam

a D

eser

t, C

hile

Ton

gbai

teC

r 3C

286

.66

Nat

ive

elem

ents

1.1.

17.1

Liu

Zhu

ang,

Ton

bai C

ount

y,

Hen

an, C

hina

Uva

rovi

teC

a 3C

r 2(S

iO4)

320

.78

Nes

osili

cate

s —

ga

rnet

gro

up51

.4.3

b.3

Bis

sers

k, S

aran

sk,

Mor

dov

skay

a, R

ussi

aM

etam

orph

osed

chr

omit

ed

epos

its

Vau

qu

elin

ite

Pb2C

u(C

rO4)

(PO

4)(O

H)

7.

37C

ompo

und

ph

osph

ates

43.4

.3.1

Ber

ezov

, Eka

teri

nber

g (S

verd

lovs

k), U

ral

Mou

ntai

ns, R

ussi

aV

olko

nsko

ite

Ca 0

.3(C

r+3 ,M

g,Fe

+3 )

2

(Si,A

l)4O

10(O

H) 2

⋅4H

2O

13.1

2C

lays

— s

mec

tite

gr

oup

71.3

.1a.

4M

ount

Efi

mya

tsk,

Ura

l M

ount

ains

, Rus

sia

Syno

nym

s: V

olch

onsk

oite

an

d W

olch

onsk

oite



Vuo

rela

inen

ite(M

n+3 ,F

e+2 )

(V+

3 ,Cr+

3 )2

O4

11.7

3M

ulti

ple

oxid

es —

spin

el g

roup

7.2.

4.1

Stät

a, D

over

stop

, Ber

gsla

gen,

Sw

eden

Wat

ters

ite

Hg+

4Hg+

2 Cr+

6 O6

9.47

Anh

ydro

us

chro

mat

es35

.4.2

.1C

lear

Cre

ek m

ercu

ry m

ine,

N

ew I

dri

a d

istr

ict,

San

Ben

ito

Cou

nty,

CA

Oxi

diz

ed z

one

of

exha

lati

ve H

g d

epos

its

Yed

linit

ePb

6CrC

l 6(O

,OH

) 8

3.17

Oxy

halid

es a

nd

hyd

roxy

halid

es10

.6.3

.1M

amm

oth

min

e, T

iger

, Pin

al

Co.

, AZ

Yim

engi

teK

(Cr,T

i,FeM

g)12

O19

29.1

9M

ulti

ple

oxid

es7.

4.1.

2Y

imen

gsha

n ar

ea, S

hand

ong,

C

hina

Zha

nghe

ngit

e(C

u,Z

n,Fe

,Al,C

r)4.

28N

ativ

e el

emen

ts1.

1.6.

2X

iaoy

anzh

uang

, Box

ian

Co.

, A

nhui

, Chi

naZ

inco

chro

mit

eZ

nCr 2

O4

44.5

6M

ulti

ple

oxid

es -

sp

inel

gro

up7.

2.3.

6O

nega

dep

ress

ion,

Kar

elia

, R

ussi

a

Not

e:M

iner

als

in b

old

typ

e ar

e C

r(V

I) m

iner

als.

Sour

ce:

Hur

lbut

(19

63);

Mar

tin

and

Bla

ckbu

rn (

1999

and

200

1); P

erro

ud (

2001

); W

ebm

iner

al (

2002

).



TAB

LE 2

.8B

Chr

omiu

m M

iner

als:

Cry

stal

logr

aphy

and

Phy

sica

l Pro

pert

ies

Min

eral

Nam

eC

ryst

al

Sys

tem

Ph

ysic

al P

rop

erti

es

Cle

avag

eC

olor

Den

sity

(S

.G.)

Dia

ph

anie

tyH

abit

Moh

sH

ard

nes

sL

ust

erS

trea

k

Ank

angi

teTe

trag

onal

-d

ipyr

amid

alno

neB

lack

4.44

Opa

que

—6.

5V

itre

ous:

adam

anti

neG

rayi

sh-

blac

kB

arb

erto

nit

e H

exag

onal

-d

ihex

agon

al

dip

yram

idal

[001

]-pe

rfec

tV

iole

t; pi

nk; p

ink

viol

et2.

1Tr

ansp

aren

t to

tran

sluc

ent

—1.

5−2.

0Pe

arly

Whi

te

Ben

tori

teH

exag

onal

-d

ihex

agon

al

dip

yram

idal

[101

0]-p

erfe

ctV

iole

t; lig

ht v

iole

t2.

025

Tran

spar

ent

—2.

0V

itre

ous:

gla

ssy

Vio

let

Bra

cew

ellit

eO

rtho

rhom

bic-

dip

yram

idal

—D

arkr

edd

ish-

brow

n;re

dd

ish-

brow

n;bl

ack

4.45

–4.4

8(a

v =

4.4

6)Tr

ansl

ucen

t to

op

aque

—5.

5−6.

5A

dam

anti

ne;

met

allic

Dar

k br

own

Bre

zina

ite

Ort

horh

ombi

c-d

ipyr

amid

al—

Bro

wni

sh-g

ray

to

gray

4.12

Opa

que

—3.

5–4.

5M

etal

lic: d

ull

—

Cal

sber

gite

Isom

etri

c-he

xoct

ahed

ral

—G

ray

5.9

Opa

que

—7.

0M

etal

lic—

Car

mic

hael

ite

Mon

oclin

ic-

pris

mat

icno

neC

inna

mon

to

blac

k—

Tran

sluc

ent

to

opaq

ue—

6.0

Met

allic

—

Cas

send

anne

ite

Mon

oclin

ic-

pris

mat

ic—

Red

dis

h-or

ange

——

mic

rosc

opic

cr

ysta

ls; p

laty

: sh

eets

3.5

Res

inou

s:

grea

syYe

llow

ish-

oran

ge

Cas

wel

lsil

veri

teTr

igon

al-

hexa

gona

l sc

alen

ohed

ral

—Ye

llow

ish-

gray

3.21

Opa

que

—1.

2M

etal

lic—

Ch

rom

atit

eTe

trag

onal

-d

itet

rago

nal

dip

yram

idal

none

Lem

on y

ello

w3.

142

——

——

Chr

ombi

smit

eTe

trag

onal

none

Bro

wn;

ora

nge;

ye

llow

9.8

Tran

sluc

ent

—3.

0–3.

5A

dam

anti

neB

row

nish

-ye

llow

Chr

omce

lad

onit

eM

onoc

linic

-sp

hero

idal

—E

mer

ald

-gre

en t

o d

ark

gree

n—

Tran

spar

ent

——

Vit

reou

s: d

ull

—

Au

: Ple

ase

chec

k sp

ellin

g.

"Dia

ph

anie

ty"

or "

Dia

ph

anei

ty"?



Chr

omd

ravi

teTr

igon

al-

dit

rigo

nal

pyra

mid

al

Ind

isti

nct

Gre

enis

h-bl

ack

or

dar

k gr

een

3.4

Tran

sluc

ent

to

opaq

ue—

7.0–

7.4

Vit

reou

s:

resi

nous

Gre

enis

h-gr

ay

Chr

omfe

rid

eIs

omet

ric-

hexo

ctah

edra

lN

one

Gra

yish

-whi

te—

Opa

que

—4.

0M

etal

lic—

Chr

omit

e(C

hrom

e ir

on o

re;

chro

mic

iron

)

Isom

etri

c-he

xoct

ahed

ral

Non

eB

lack

or

brow

nish

-bl

ack

4.5–

5.09

(av

= 4

.79)

Opa

que

Mas

sive

gr

anul

ar5.

5M

etal

licB

row

n

Chr

omiu

mIs

omet

ric-

hexo

ctah

edra

l—

Whi

te7.

2O

paqu

e—

4.0

Met

allic

—

Chr

omph

yllit

eM

onoc

linic

-pr

ism

atic

[001

]-pe

rfec

tE

mer

ald

gre

en2.

88Tr

ansp

aren

tPl

aty:

she

ets

3.0

Vit

reou

s: g

lass

yW

hiti

sh-

gree

nC

ochr

omit

eIs

omet

ric-

hexo

ctah

edra

lIn

dis

tinc

tB

lack

5.22

Opa

que

—7.

0M

etal

licG

reen

Coc

roit

eM

mon

oclin

ic-

pris

mat

ic[1

10]-

dis

tinc

t[0

01]-

ind

isti

nct

[100

]-in

dis

tinc

t

Yello

w; o

rang

ish-

red

; red

dis

h-or

ange

5.9–

6.1

(av

= 6)

Tran

sluc

ent

Cry

stal

line:

ac

icul

ar2.

5–3.

0A

dam

anti

neYe

llow

ish-

oran

ge

Dau

bree

lite

Isom

etri

c-he

xoct

ahed

ral

—B

lack

3.81

Opa

que

—4.

5–5.

0M

etal

licB

row

nish

-bl

ack

Dea

nes

mit

hit

eTr

iclin

ic-

pina

coid

alG

ood

Ora

nge-

red

—Tr

ansp

aren

t—

4.5–

5.0

Ad

aman

tine

—

Die

tzei

teM

onoc

linic

-pr

ism

atic

[100

]-in

dis

tinc

tG

old

en y

ello

w3.

617

Tran

spar

ent

—3.

5V

itre

ous:

gla

ssy

Lig

ht y

ello

w

Don

ath

ite

Tetr

agon

al-

dit

etra

gona

l d

ipyr

amid

al

Non

eB

lack

5.0

——

6.5–

7.0

Met

allic

Bla

ckis

h-br

own

Du

kei

teTr

igon

al-

dit

rigo

nal

pyra

mid

al

—Ye

llow

; yel

low

ish-

brow

n—

Tran

spar

ent

——

——

Ed

oyle

rite

Mon

oclin

ic-

pris

mat

icG

ood

Gre

enis

h-ye

llow

; or

ange

—Tr

ansp

aren

t to

op

aque

——

Res

inou

s—

Em

bre

yite

Mon

oclin

ic-

pris

mat

icN

one

Ora

nge

6.45

Tran

spar

ent

to

tran

sluc

ent

—3.

5E

arth

y: d

ull

Yello

w

Esk

olai

teTr

igon

al-

hexa

gona

l sc

alen

ohed

ral

Non

eB

lack

5.18

Opa

que

—8.

0–8.

5M

etal

licG

ray

(Con

tinu

ed)



TAB

LE 2

.8B

(Con

tinu

ed)

Min

eral

Nam

eC

ryst

al

Sys

tem

Ph

ysic

al P

rop

erti

es

Cle

avag

eC

olor

Den

sity

(S

.G.)

Dia

ph

anie

tyH

abit

Moh

sH

ard

nes

sL

ust

erS

trea

k

Ferc

hrom

ide

Isom

etri

c-he

xoca

tahe

dr

al

Non

eG

rayi

sh-w

hite

—O

paqu

e—

6.5

Met

allic

—

Flor

enso

vite

Isom

etri

c-he

xoct

ahed

ral

Non

eB

lack

—O

paqu

e—

5.0

Ad

aman

tine

: m

etal

licB

lack

Forn

acit

eM

onoc

linic

-pr

ism

atic

Non

eG

reen

; yel

low

; oliv

e br

own

6.27

Tran

spar

ent

—2.

0–3.

0A

dam

anti

neG

reen

ish-

yello

wG

eorg

eeri

ckse

nit

eM

onoc

linic

-pr

ism

atic

Non

ePa

le y

ello

w; b

righ

t le

mon

yel

low

3.03

5Tr

ansp

aren

t to

tr

ansl

ucen

tPr

ism

atic

cr

ysta

ls3.

0–4.

0V

itre

ous:

gla

ssy

Pale

yel

low

Gri

mal

diit

eTr

igon

al-

hexa

gona

l sc

alen

ohed

ral

—D

ark

red

or

red

dis

h-br

own

4.11

Opa

que

—3.

5–4.

5M

etal

licR

ed

Guy

anai

teO

rtho

rhom

bic-

dip

yram

idal

—G

rayi

sh-b

row

n;

gold

en b

row

n;

red

dis

h-br

own

4.53

——

——

Bro

wn

Has

hem

ite

Ort

horh

ombi

c-d

ipyr

amid

alG

ood

Bro

wn;

gre

enis

h-br

own;

yel

low

br

own;

ligh

t yel

low

br

own;

dar

k gr

eeni

sh-b

row

n

4.59

Tran

spar

ent

to

tran

sluc

ent

—3.

5A

dam

anti

neB

row

nish

-w

hite

Haw

thor

init

eH

exag

onal

-d

ihex

agon

al

dip

yram

idal

Non

eB

lack

6.0–

6.5

——

—M

etal

lic—

Hei

dei

teM

onoc

linic

-pr

ism

atic

Non

eSt

eel g

ray

4.1

Opa

que

—3.

5–4.

5M

etal

lic—

Hem

ihed

rite

Tric

linic

-pi

naco

idal

—O

rang

e; b

row

n;

blac

k6.

42—

—3.

0V

itre

ous:

gla

ssy

Saff

ron

IMA

199

8-02

9M

onoc

linic

-pr

ism

atic

—B

lack

—O

paqu

e to

tr

ansl

ucen

t—

—R

esin

ous

—

IMA

199

9-01

8Tr

igon

al—

Coa

l bla

ck—

Opa

que

——

Subm

etal

lic—



IMA

199

9-03

4O

rtho

rhom

bic

—Pa

le g

ray;

pin

kish

-vi

olet

—Tr

ansl

ucen

t—

—Pe

arly

—

IMA

200

0-04

2Tr

igon

al-

hexa

gona

l sc

alen

ohed

ral

—M

agen

ta; p

urpl

e—

Tran

spar

ent

——

Vit

reou

s: g

lass

y—

Iqu

iqu

eite

Trig

onal

-d

itri

gona

l[0

001]

-per

fect

; [0

110]

-ind

isti

nct

Yello

w2.

05Tr

ansp

aren

t—

2.0

Vit

reou

s: g

lass

yYe

llow

ish-

whi

teIr

anit

eTr

iclin

ic-p

edia

l—

Bro

wni

sh-y

ello

w;

yello

w; h

oney

br

own

5.8

——

3.0

Vit

reou

s: g

lass

yYe

llow

Isov

ite

Isom

etri

c-he

xoct

ahed

ral

—Ir

on g

ray

—O

paqu

eG

ranu

lar

8.0

Met

allic

—

Kal

inin

ite

Isom

etri

c-he

xoct

ahed

ral

—B

lack

—O

paqu

e—

5.0

Ad

aman

tine

—

Kno

rrin

gite

Isom

etri

c-he

xoct

ahed

ral

Non

eB

lue

gree

n; g

reen

3.75

6Tr

ansp

aren

t to

tr

ansl

ucen

t—

6.0–

7.0

Vit

reou

s: g

lass

y

Kos

moc

hlor

Mon

oclin

ic-

pris

mat

ic—

Bri

ght

gree

n—

Tran

spar

ent

to

tran

sluc

ent

—6.

0–7.

0V

itre

ous:

gla

ssy

Lig

ht g

reen

Kri

novi

teTr

iclin

ic-

pina

coid

alN

one

Dar

k gr

een;

em

eral

d

gree

n3.

38Su

btra

nslu

cent

to o

paqu

eM

icro

scop

ic

crys

tals

6.0–

7.0

Suba

dam

anti

neG

reen

ish-

whi

teL

ind

sley

ite

Trig

onal

-rh

ombo

hed

ral

Non

eB

lack

4.63

Opa

que

Mic

rosc

opic

cr

ysta

ls7.

5M

etal

licG

ray

Lop

ezit

eTr

iclin

ic-

pina

coid

al[0

10]-

perf

ect;

[100

]-d

isti

nct;

[001

]-d

isti

nct

Ora

nge

red

; red

2.69

Tran

spar

ent

—2.

5V

itre

ous:

gla

ssy

Lig

ht y

ello

w

Lov

erin

git

e(C

hrom

pico

tite

)Tr

igon

al-

rhom

bohe

dra

lN

one

Bla

ck4.

41O

paqu

e—

7.5

Met

allic

Gra

yish

-bl

ack

Mac

qu

arti

teM

onoc

linic

-pr

ism

atic

[100

]-go

odO

rang

e5.

49—

Euh

edra

l cr

ysta

ls3.

5A

dam

anti

neL

ight

ora

nge

Mag

nesi

o-ch

rom

ite

Isom

etri

c-he

xoct

ahed

ral

Non

eB

lack

4.2

Opa

que

—5.

5M

etal

licD

ark

gray

Man

gano

-chr

omit

eIs

omet

ric-

hexo

ctah

edra

l—

Gra

yish

-bla

ck4.

86O

paqu

e—

5.5

Met

allic

—

Mat

hias

ite

Trig

onal

-rh

ombo

hed

ral

Non

eB

lack

4.6

Opa

que

—7.

5M

etal

licG

ray

(Con

tinu

ed)



TAB

LE 2

.8B

(Con

tinu

ed)

Min

eral

Nam

eC

ryst

al

Sys

tem

Ph

ysic

al P

rop

erti

es

Cle

avag

eC

olor

Den

sity

(S

.G.)

Dia

ph

anie

tyH

abit

Moh

sH

ard

nes

sL

ust

erS

trea

k

Mcc

onne

llite

Trig

onal

-he

xago

nal

scal

end

ohed

ral

—D

ark

red

5.49

——

—M

etal

lic—

Mol

ybd

ofor

naci

teM

onoc

linc-

pris

mat

icN

one

Lig

ht g

reen

6.66

——

2.0–

3.0

Ad

aman

tine

—

Mon

gsh

anit

eH

exag

onal

—B

lack

—O

paqu

e—

5.0–

6.0

Met

allic

—M

ount

keit

hite

Hex

agon

al[0

001]

-per

fect

Whi

te o

r lig

ht p

ink

2.12

—Fl

akes

: flat

thin

cr

ysta

ls2.

0Pe

arly

W

hite

Nat

alyi

teM

onoc

linic

-pr

ism

atic

[110

]-d

isti

nct

Yello

wis

h-gr

een;

lig

ht g

reen

3.55

Tran

spar

ent

—7.

0V

itre

ous:

silk

yG

reen

Nic

hrom

ite

Isom

etri

c-he

xoct

ahed

ral

Ind

isti

nct

Bla

ck5.

24O

paqu

e—

6.0–

6.5

Met

allic

Gre

enis

h-gr

ayO

lkho

nski

teM

onoc

linic

Non

eB

lack

4.48

Opa

que

Plat

y: s

heet

s8.

0M

etal

licB

lack

Pett

erit

eO

rtho

rhom

bic

—Pa

le g

ray;

pin

kish

vi

olet

—Tr

ansl

ucen

t—

—Pe

arly

—

Ph

oen

icoc

hro

ite

Mon

oclin

ic-

pris

mat

ic—

Dar

k re

d7.

01Tr

ansl

ucen

t—

2.5

Ad

aman

tine

: re

sino

usYe

llow

or

ange

Red

ingt

onit

eM

onoc

linic

—W

hite

; lig

ht v

iole

t1.

761

Tran

spar

ent

Fibr

ous

crys

tals

2.0

Silk

yW

hite

Red

led

geit

eTe

trag

onal

-d

ipyr

amid

alG

ood

Yello

w g

reen

;bl

ack

3.72

Opa

que

—6.

7A

dam

anti

neG

rayi

sh-

whi

teR

iland

ite

Unk

now

n—

Bla

ckO

paqu

eO

paqu

ePl

aty:

she

ets

2.0–

3.0

Res

inou

sG

rayi

sh-

brow

nS

anta

nai

teH

exag

onal

-tr

apez

ohed

ral

[000

1]-p

erfe

ctSt

raw

yel

low

9.15

5—

—4.

0—

—

Scho

llhor

nite

Trig

onal

-d

itri

gona

l py

ram

idal

[000

1]-p

erfe

ctG

ray

2.7

Opa

que

Mic

rosc

opic

cr

ysta

ls1.

5–2.

0M

etal

lic—



Shui

skit

eM

onoc

linic

-pr

ism

atic

[001

]-pe

rfec

tD

ark

brow

n3.

24—

—6.

0—

Gre

enis

h-br

own

Stic

htit

eTr

igon

al-

hexa

gona

l sc

alen

ohed

ral

[000

1]-p

erfe

ctL

ilac;

ligh

t vi

olet

pi

nk2.

2Tr

ansl

ucen

t to

tr

ansp

aren

tE

ncru

stat

ions

no

dul

ar;

mic

aceo

us

1.5–

2.0

Vit

reou

s: g

lass

yPa

le v

iole

t bl

ue

Tara

pac

aite

Ort

horh

ombi

c-d

ipyr

amid

al[0

01]-

dis

tinc

t;[0

10]-

dis

tinc

tL

ight

yel

low

2.74

Tran

spar

ent

——

——

Ton

gbai

teU

varo

vite

Isom

etri

c-he

xoct

ahed

ral

Non

eG

reen

3.4–

3.8

Tran

spar

ent

to

tran

sluc

ent

Cry

stal

line:

fin

e la

mel

lar

6.5–

7.0

Vit

reou

s: g

lass

yW

hite

Vau

qu

elin

ite

Mon

oclin

ic-

pris

mat

icIn

dis

tinc

tB

row

n; b

row

nish

-gr

een;

bro

wni

sh

blac

k; g

reen

; bla

ck

6.0

Tran

spar

ent

to

tran

sluc

ent

Ren

ifor

m:

mam

mill

ary-

botr

yoid

al

2.5–

3.0

Ad

aman

tine

: re

sino

usB

row

nish

Vol

kons

koit

e (V

olch

onsk

oite

, W

olch

onsk

oite

)

mon

oclin

ic-

pris

mat

ic[?

]-pe

rfec

tYe

llow

; oliv

e gr

een;

gr

een

2.0–

2.5

(av

= 2

.25)

Subo

paqu

e—

1.5–

2.0

Res

inou

sB

lue

gree

n

Vuo

rela

incn

ite

Isom

etri

c-he

xoct

ahed

ral

—B

row

nish

-gra

y;

gray

ish-

blac

k4.

64O

paqu

e—

6.5

Met

allic

—

Wat

ters

ite

Mon

oclin

ic-

pris

mat

icN

one

Bro

nze;

red

dis

h-br

own;

bla

ck8.

91O

paqu

e to

tr

ansl

ucen

tC

ryst

allin

e:

encr

usta

tion

s4.

5Su

bmet

allic

Dar

k br

ick

red

Yed

linit

eTr

igon

al-

rhom

bohe

dra

l[1

120]

-dis

tinc

tR

ed v

iole

t5.

85Tr

ansp

aren

t to

tr

ansl

ucen

t—

2.5

Vit

reou

s: g

lass

yW

hite

Yim

engi

teH

exag

onal

-d

ihex

agon

al

dip

yram

idal

[000

1]-p

erfe

ctB

lack

4.34

Opa

que

—4.

0M

etal

licB

row

n

Zha

nghe

ngit

eIs

omet

ric-

hexo

ctah

edra

l—

Non

e—

Gol

den

yel

low

—3.

5M

etal

licB

ronz

e

Zin

coch

rom

ite

Isom

etri

c-he

xoct

ahed

ral

—B

row

nish

-bla

ck—

Tran

sluc

ent

to

opaq

ue—

5.8

Subm

etal

lic—

Sour

ces:

Bla

ckbu

rn a

nd D

enne

n (1

997)

; Mar

lin (

1999

); M

and

arin

o (2

001)

; Mar

tin

and

Bla

ckbu

rn (

2001

); Pe

rrou

d (

2001

); B

arth

elm

y (2

002)

.

Not

e:av

= a

vera

ge I

MA

= I

nter

nati

onal

Min

eral

Nam

es A

ssoc

iati

on.

Au

: Ple

ase

chec

k fo

r th

e m

issi

ng

valu

e.



Act was phased out, which had subsidized many chromite mines. The UnitedStates currently has known chromite deposits in California, Maryland,Montana, North Carolina, Oregon, Pennsylvania, Texas, Washington, andWyoming, but the ore’s low chromium content makes these deposits uneco-nomical for mining. There are essentially three grades of chromite ore: (1)chemical-grade, which averages 28.6% chromium; (2) metallurgical grade,which also averages 28.6% chromium; and (3) refractory grade, which aver-ages 23.9% chromium. In 2001, chromite ore and chromium ferroalloys andmetal were imported from the Republic of South Africa (48%), Kazakhstan(16%), Russia (9%), Turkey (9%), Zimbabwe (9%), and other nations (9%).Current world resources exceed 11 billion tons of shipping-grade chromite(USEPA, 1984; Papp, 2002). In 2001, 22% of chromium metal was recycledmostly in the form of stainless steel scrap. In 2002, 63% of chromium oreswere imported, primarily from South Africa, Kazakhstan, Zimbabwe, Turkey,and Russia (McCartan et al., 2003).

Primary chromite deposits only exist in certain types of ultramafic orclosely related anorthositic rocks, of which there are two main types: (1) thestatiform or layered intrusion and (2) the pod shaped (podiform) type(Thayer, 1973). The main ore mineral is chromite [(Mg, Fe2+) (Cr, Al, Fe3+)2O4],which has a variable composition. The Cr2O3 content of most chromite oreranges from about 15 to 64%; however, in mixtures of chromite and othersilicate minerals, Cr2O3 contents range from 7 to 55% (Thayer, 1973; Lipinand Page, 1982). Thicknesses of ore grade zones vary from 0.5 to 10 m. Thesedeposits are described in the following sections.

2.2.3.1 Stratiform Mafic-Ultramafic Chromite Deposits

These deposits are also known as chromium-platinum mafic-ultramafic com-plexes. They occur as igneous plutons intruded into tectonically stable por-tions of the crust or craton during the Achaean period (Precambrian) morethan 1.9 billion years ago. The elongated character of many of these complexessuggests possible intrusion during crustal rifting. As the magma cooled, itcrystallized with different minerals crystallizing sequentially at differentrates. Minerals thus preferentially settled out by the process known as mag-matic segregation (Bates and Jackson, 1987), producing a layered igneouscomplex. Such layered complexes generally cover thousands of square kilo-meters. Layering has resulted in more felsic (less dense) rock types in theupper portion of the complex and ultramafic (more dense) rocks near thebottom. However, the complexes’ overall composition is that of a gabbro.Secondary alteration in the form of serpentinization may occur in olivine-richwall rocks (Hughes, 1982; Hatch et al., 1972; Lipin and Page, 1982).

The stratiform mafic-ultramafic association includes world-class chromitedeposits, which also produce significant amounts of platinum group metals(PGMs); two examples are:

1. The Precambrian Stillwater Complex in Montana, which was orig-inally intruded as a sill. This basic layered intrusion composed of

Au: Pleasechecksententence forclarity.



approximately 15 repetitious layers of hartzburgites, chromatite,olivine chromatite, and bronzitite is exposed only over a 48.28 kmlength because it is bounded by faults at either end. The complexes’thickness has been measured from 4876.8 to 5486.4 m. Thirteenchromatite zones have been recognized but only one zone has beenmined. PGMs and disseminated nickel–copper sulfide mineralsalso occur within the chromite ore zones (Ridge, 1972; Lipin andPage, 1982; Page 1992a).

2. The Bushveld Igneous Complex of the Republic of South Africawhich covers an area that averages 280 km long by 160 km wide(with the greatest width approximately 400 km) and a thickness of9 km centered on the town of Rustenburg in the Transvaal State.The Bushveld complex contains cumulus ferrogabbro to diorite,which has vanadium–titanium magnetite layers. Chromite layersoccur in cumulate hartzburgite, dunite, and pyroxinite. The orescontain varying amounts of the minerals chromite, ilmenite, mag-netite, pyrrhotite, pentlandite, chalcopyrite, and PGMs. Chromiteore thicknesses increase in basal depressions within the layers(Page, 1992b). Ore reserves may be several billion metric tons.

2.2.3.2 Podiform- or Alpine-Type Chromite Deposits

Podiform-type deposits occur as podlike masses in the ultramafic portionsof ophiolite complexes. Local rock types include highly deformed duniteand hartzburgite, which may be locally serpentinized. The deposits generallyare highly deformed, having formed throughout the Phanerozoic in thelower part of the oceanic lithosphere along spreading plate boundaries. Theycan be divided into minor podiform chromites, most of which are found inCalifornia and Oregon, and major podiform chromite, occurring mostly inIran, Turkey, and Cuba (Singer and Page, 1992; Singer et al., 1992; Ash, 1996).

Ore mineralogy generally consists of chromite with possible ferrichromite,magmetites, and PGMs (ruthenium, osmium, and iridium) (Albers, 1992).Minor podiform deposits range from about 0.16 to 10,000 metric tons witha median of about 100 metric tons. Ore grades range from 10 to 56% Cr2O3

with a median grade of 44% Cr2O3. Major podiform deposits range fromabout 500 to 2.0 million metric tons with a median of about 2000 metric tons.Ore grades range from 22 to 56% Cr2O3 with a median grade of 46% Cr2O3

(Singer and Page, 1992; Singer et al., 1992).

2.2.4 Crude Oil, Tars and Pitch, Asphalts, and Coal

Crude oils (Table 2.9) contain many different, mostly metallic, elements;cobalt (Co), chromium (Cr), copper (Cu), molybdenum (Mo), nickel (Ni),lead (Pb), vanadium (V), and zinc (Zn) are commonly found. Ni and V aregenerally enriched with respect to most other elements. Crude oils are poorly

Au: Pleasecheck thespelling

Au: Pleasecheck thespellingchange.



enriched with six valence group metals: Cr, Mo, W, and uranium (U). Theratio of V/Cr is generally equal to or greater than 10 in most crude oils(Krejci-Graf, 1972).

Coal contains more than 50% carbonaceous material with the remainderoccurring as clay minerals, discrete mineral grains, and organically boundassociated elements. Seventy-nine such elements have been detected in coal.Chromium may be associated with the clays; the average chromium concen-tration for U.S. coals is 15 mg/kg (Finkelman, 1993).

2.2.5 Rock

Elemental chromium concentrations in crustal rocks range from 20 mg/kg infelsic igneous rocks such as granites (Table 2.10) to more than 2000 mg/kg inultramafic igneous rocks (and their metamorphosed equivalents). The crustalaverage (CA) is reported at approximately 100 mg/kg. The CA is the basis for

TABLE 2.9

Chromium Concentrations in Some Crude Oils, Tars and Pitch, Asphalts, and Coal

Organic Material Location Cr Concentration (ppm) Reference

Crude oil

Western Canada Basin

Average 0.05

Manning andGize (1993)

Maximum 1.68Saudi Arabia Average 0.65

Maximum 1.18Central European Mollase Basin, Germany

Average 0.11Maximum 0.45

Italy Average 3.15Maximum 10.7

Gösting II, Austria

Range 30–70

Krejei-Graf(1972)

N.E. Caucasus, Russia (13 samples)

Average 100

Urals, Russia (27 samples)

Average 100

Tars and pitch

Messel, Germany

Average 80

Kohtla, Estonia Range 70–350Kohtla, Estonia Range 35–70

Asphalts Matitza, Romania

Range 70–350

Selenia, Albania Range 35–70Coal United States

(7,847 samples)Average 15 Finkelman

(1993)Maximum 250

Au: InsertionOk?

Au: Pleasecheck thechange.



defining the chromium enrichment factor (EF) in rocks and soils, which rangesfrom 2 for felsic rock types, primarily granites, to 20 for ultramafic rocks.

Cr(III) is also a trace element constituent of igneous rocks, probably occur-ring as separate phase minerals such as chromite, chromium-bearing mag-netite, and/or ilmentite. It ranges from 100 mg/kg in amphiboles, pyroxenes(where it is actually dispersed in augite), biotite, magnetite, and olivine to1.0 mg/kg in plagioclase and potassium feldspars (Hughes, 1982; Brickerand Jones, 1995). Therefore, the largest source of chromium in minerals inthe crustal rocks is in the rock-forming minerals (Table 2.11).

Black shales (also known as metalliferous shales) form in anoxic (anaerobic)environments in fresh, brackish, marine, or hypersaline water. Black shales areenriched in arsenic (As), Cu, Cr, Mo, Ni, and U in concentrations ranging fromn × 101 mg/kg to n × 102 mg/kg (where n = 1 to > 9). Metals such as V and Znare further enriched to concentrations ranging to n × 103 mg/kg. These metalsgenerally are not visible with the naked eye or ordinary light microscopybecause they do not occur as discrete mineral phases; rather, they are micro-scopically dispersed in organic matter, clay, or sulfides. Chromium concentra-tions in black shale (Table 2.12) range from about 20 to 3000 mg/kg generally

TABLE 2.10

Total Chromium Concentrations in 1.0 km3 of the Crust for Different Rock Type

Rock Type

AverageDensity(g/cm3)

109 Metric Tons (Mg)/km3

Average CrConcentration

(mg/kg = g/Mg)1010 g-Cr/

km3

104 Metric Tons-

Cr/km3

Igneous:Granite 2.667 2.667 20 5.33 5.33Granodiorite 2.716 2.716 20 5.43 5.43Quartz diorite 2.806 2.806 20 5.61 5.61Diorite 2.700 2.700 20 5.40 5.40Gabbro 2.976 2.976 2000 595.20 595.20Peridotite 3.324 3.324 2000 664.80 664.80Dunite 3.277 3.277 2000 655.40 655.40Pyroxenite 3.231 3.231 2000 646.20 646.20

Volcanic:Basalt 2.965 2.965 220 65.23 65.23

Sedimentary:Limestone 2.400 2.400

2.3002.600

10 2.40 2.40

Shale 2.300 35 8.05 8.05Sandstone 2.600 100 26.00 26.00

Notes: There are 1.0 × 1015 cm3 in 1.0 km3; 106 g = 1.0 Mg.

Source: Rock densities: Daly et al. (1966).



averaging 100 to 111 mg/kg (Holland, 1979; Thornton, 1983; Leventhal, 1993;Quinby-Hunt et al., 1997).

2.2.6 Soil

Cr(III) coprecipitates with secondary soil clay minerals such as illites andsmectites (Sposito, 1983). For residual soils, chromium concentrations gen-erally reflect underlying bedrock concentrations; however, under tropicalweathering conditions, chromium in soil may be considerably enriched overbedrock, particularly in the B horizon (Hawkes and Webb, 1962; Mattigodand Page, 1983 and Table 2.7).

Worldwide chromium concentrations in soil average about 200 mg/kg.Scottish surficial soils range from 5.0 to 3000 mg/kg. In Canada, backgroundchromium concentrations (from 173 soil samples) range from 10 to 100 mg/kg with an average of 43 mg/kg (CCME, 1996). U.S. soils range from 1.0 to2000 mg/kg, averaging 54 mg/kg (Shacklette and Boerngen, 1984). Ninesamples of California residual soils had chromium concentrations rangingfrom 4.0 to 32 mg/kg, averaging 15.4 mg/kg (Pettygrove and Asano, 1985).

TABLE 2.11

Cr Concentration in Rock Forming Minerals in a Typical Granite

Mineral

AverageMineral

Fraction (%)

Cr Concentration in Mineral

(mg/kg)

Total Cr Concentration in Rock

mg/kg %

Quartz 35.0 Trace (approx. ~0.1) 0.035 0.32Feldspar 60.0 10 6.0 54.37Biotite 4.0 100 4.0 36.35Magnetite 1.0 100 1.0 9.06Total 100.0 11.035 100.10

TABLE 2.12

Chromium Concentrations for Various Shales

Shale TypeCr Concentration

(ppm)

Average shale 90Average black shale 100–111Atlantic (Cretaceous) 200Green River Formation, WY (Eocene) 40Black Sea (layer C) 150Appalachian (Devonian) 60Condar, Australia 55Alum (Cambrian) 94Mecca Quarry, PA 400New Albany (Devonian) 70Falling Run /Henryville 100Associated phosphates 20

Source: Leventhal (1993).

Au: InsertionOk?



2.2.7 Precipitation (Rain Water) and Surface Water

The average chromium concentration in rain water ranges from 0.2 to1.0 µg/l (Keiber and Heiz, 1992). Surface fresh water has total chromiumranging from approximately 0.5 to 2.0 µg/l with dissolved chromiumranging from approximately 0.02 to 0.3 µg/l. Chromium in remote areas,such as Antarctic lakes, ranges from less than 0.6 to 30 µg/l with con-centrations increasing with depth. Canadian surface waters range from0.2 to 44 µg/l. Surface water concentrations in industrialized regionsrange from 1.0 to 10 µg/l. U.S. surface water concentrations range to 84µg/l (WHO, 1996). Cr(VI) may be the dominant dissolved form in surfacewaters, particularly in oxygenated environments (CCME, 1996).

2.2.8 Groundwater

Background chromium groundwater concentrations generally follow themedia that it occurs in. Most chromium concentrations are low, averagingless than 1.0 µg/l (WHO, 1996). Typical ranges are from 0.02 to 6.0 µg/l witha median concentration of 0.2 µg/l (Allard, 1995).

2.2.9 Sea Water

Chromium in ocean or seawater averages about 0.3 µg/l with chromiumprobably occurring as Cr(OH)4

−and CrO42−. These species have an average

residence time of 6000 years (Henderson, 1982; Firestone, 2002). However,concentration variations occur in different oceans and seas. In the North Sea,a chromium concentration of 0.17 µg/l was detected (WHO, 1996). Chro-mium concentrations also vary with depth: in the North Pacific, chromiumconcentrations range from 0.156 µg/l at the surface to 0.26 µg/l in deeperwaters. In the North Atlantic, chromium ranges from 0.182 µg/l at the surfaceto 0.234 µg/l in deeper water (Donat and Bruland, 1995).

2.2.10 Air

In remote areas, chromium ambient air concentrations (Table 2.7) are rela-tively low: in Arctic air, concentrations range from 0.005 to 0.07 ng/m3. Inthe Canadian Arctic, air samples collected in the early 1980s had chromiumconcentrations at 0.26 ng/m3 (CCME, 1996). Most nonindustrialized areashave chromium air concentrations below 10 ng/m3.

In the United States, average chromium concentrations are generally below300 ng/m3 with median concentrations less than 20 ng/m3 (WHO, 1996). InCalifornia, the California Air Resources Board (CARB) routinely measuresatmospheric chromium [including Cr(VI)]. In 1986, CARB estimated that theambient chromium population-weighted annual concentration ranged from8.9 to 17.8 ng/m3. They also estimated that Cr(VI) comprised 3 to 8% of the



reported ambient chromium in air; this occurred in the form of respirableparticulates with a median diameter of approximately 1.5 to 1.9 µm.

Ambient chromium concentrations derived from soil particles are believedto be primarily composed of Cr(III) compounds. In the Regensburg area inthe Federal Republic of Germany, Nusko and Heumann (1997) found that thesurface layers of forest soils had a Cr(III)/Cr(VI) ratio of 1.47 as opposed toCr(III)/Cr(VI) in aerosol particles that ranged from 0.27 to 0.35. This sug-gested that Cr(III) predominates in organic-rich soil and that oxidation ofCr(III) to Cr(VI) occurs in the atmosphere. Cr(III) concentrations in aerosolparticles (measured in three seasons from August 1994 to September 1995)ranged from 0.05 to 36 ng/m3, and Cr(VI) concentrations in aerosol particles(for samples analyzed for the same period) ranged from 0.16 to 1.22 ng/m3.

Worldwide, most chromium atmospheric source emissions are from wind-borne soil particles (62.4%) and volcanoes (34.67%) with the remaining emis-sions from sea salt spray, forest fires, and biogenic sources (Papp, 1994 andTable 2.13). Atmospheric persistence or residence time is estimated at anexperimental half-life (t1/2) of 13 h with total residence time estimated at lessthan 14 d. Chromium is removed from the atmosphere by dry and wet (rainout) deposition with most chromium deposition occurring through wet dep-osition although chromium particles less than 5.0 µm diameter may remainairborne for extended periods, allowing extended transport over large dis-tances by wind (CCME, 1996; CARB, 2002).

2.2.11 Biogeochemical Cycling

The determination of the natural biogeochemical cycling of chromiumbecomes somewhat difficult because of the large input of anthropogenic chro-mium, which has considerably perturbed the natural cycle. This is also typicalin the natural biogeochemical cycling of other metals such as aluminum, iron,copper, zinc, and lead in which atmospheric emissions from mining, smelting,

TABLE 2.13

Chromium Natural Atmospheric World Emissions

Source103 Metric Tons per Year

Median Percent Range

Wind-borne soil particles 27 62.40 3.6– 50Sea salt spray 0.07 0.16 0.03–1.4Volcanoes 15 34.67 0.81– 29Wild forest fires 0.09 0.21 0–0.18Biogenic: total 1.11 2.57 0.1–2.22Continental particulates 1.0 2.31 0.1–2.0Continental volatiles 0.05 0.12 0–0.10Marine 0.06 0.14 0–0.12Total 43.27 100.01 4.5–83

Source: Papp (1994).



and the burning of fossil fuels have substantially increased atmospheric, sur-face water, sediment, soil, plant and animal tissue levels (Schlesinger, 1991).

Dobrovolsky (1994) noted that most of the chromium total metal mass turn-over (calculated in millions of metric tons per year) occurs in stream loss to theoceans of suspended sediment input from streams and rivers. The amount ofsuspended load transported by this process is 2.460 million metric tons per year(Table 2.14b). This probably is correct in that most of the chromium metal mass,some 278,000 million metric tons, is concentrated in granites in the continentalcrust and another 132,000 million metric tons is concentrated in sediments eitheroverlying the granite crust or laying on the ocean floor. Recycling of this mate-rial is by subduction and reintroduction by magmatic activity (plutonic andvolcanic) back into the crust. This occurs only over a wide geologic time scale.

TABLE 2.14A

Chromium Concentrations inContinental Vegetation Annual Growth

ParameterCr Concentration

(mg/kg)

Ash 35Dry Phytomass 1.8Live Phytomass 0.7

Source: Dobrovolsky (1994).

TABLE 2.14B

Land Surface Chromium Fluxes

ParameterMetal Mass Turnover(106 Metric Tons/year)

World biological cycle on Land 0.309Stream loss:In solution 0.041In suspension 2.460Continental dust loss 0.19Transport from ocean to land MinorOceanic photosynthetic organismsbiological cycling

0.16


TABLE 2.14C

Chromium Mass Distribution in the Biosphere

ParameterMetals Mass

(106 Metric Tons)

Land vegetation 4.5Oceans (dissolved) 274Sedimentary 132,000Granites in continental crust 278,000




2.3 Chromium Geochemistry

2.3.1 Cr(III) Geochemistry

In its Cr(III) form, the Cr3+ has an ionic radius of 0.064 nm and it readilysubstitutes in crystal lattices for trivalent iron, which has an ionic radius of0.060 nm and trivalent aluminum, with an ionic radius of 0.050 nm (Smith,1972). Iron oxides and clays in soil and saturated zone alluvium thereforereadily sorb Cr(III).

Under standard conditions (at 25°C and 1.0 bar atmospheric pressure), inthe chromium–oxygen–hydrogen system (Figure 2.1), the Cr(III) stabilityzone occurs over a wide Eh and pH field under both reducing to oxidizingand acid to alkaline conditions. Cr(III) generally forms insoluble chromicoxide (Cr2O3), from approximately pH 5.0 to 13.5 and from an approximateEh ranging from +0.8 to −0.75 V (volts). At slightly less than pH 5.0, Cr2O3

dissolves to form soluble chromium hydroxide (CrOH2+). At a pH of approx-imately greater than 13.5 and an Eh ranging from 0.05 to −0.8 V, solubleCr(III) anion (CrO2

−) forms (Brookins, 1987). In aqueous environments underlow Eh conditions, the main Cr(III) species are the Cr(III) cations (Cr3+) andCrOH2+ (Richard and Bourg, 1991).

Under standard conditions, in the chromium–water–oxygen system, theCr(III) stability zone also occurs over a wide Eh and pH field under bothreducing to oxidizing and acid to alkaline conditions. Cr(III) generally formssoluble Cr3+ from about pH 0 to about pH 8 and at an Eh from approximately−0.4 to −1.2 V at the upper stability line. At a pH greater than 4 to aboutpH 7.5, Cr3+ dissolves to form soluble chromium hydroxide cations: CrOH2+

and Cr(OH)2+. At a pH of approximately 8.0, insoluble and amorphous

Cr(OH)3 forms, although small quantities of Cr(III) may be solubilizedwithin this stability zone. At extreme pH and reducing conditions (abovepH 12.0 and below Eh 0.0), soluble chromium hydroxide anions [Cr(OH)4

−

] form (Hem, 1977). In aqueous environments under low Eh conditions, themain Cr(III) species are the chromic cations Cr3+ and Cr(OH)2+ (Richard andBourg, 1991).

In the chromium–water–oxygen system (Figure 2.2), under standard con-ditions (predominant in groundwater), the governing reactions are:

Cr3+ + H2O ↔ CrOH2+ + H+ (2.1)

CrOH2+ + H2O ↔ Cr(OH)2+ + H+ (2.2)

Cr(OH)2+ + H2O ↔ Cr(OH)3

0 + H+ (2.3)

Cr(OH)30 + H2O ↔ Cr(OH)4

− + H+ (2.4)

Therefore, soluble chromium cations and anions are produced from insolubleCr(III) hydroxide. However, under most natural groundwater conditions, Cr(III)is relatively insoluble and rarely occurs above concentrations exceeding thedrinking water maximum contaminant level (MCL) of 50 ppb (Calder, 1988).



2.3.2 Cr(VI) Geochemistry

Under standard conditions, in the chromium–water–oxygen system(Figure 2.2), the Cr(VI) stability zone or field occurs over a much narrowerrange than the Cr(III) stability field. Cr(VI) species primarily occur underoxidizing (+Eh) and alkaline conditions (pH >6.0). In this field, Cr(VI) gen-erally forms soluble chromate (CrO4

2−) anions from approximately pH 6.0 to14.0 and at an Eh from approximately −0.1 to +0.9 V. At slightly below pH5.0, Cr2O3 dissolves to form soluble CrOH2+ (Brookins, 1987).

FIGURE 2.1Eh-pH diagram for the chromium–oxygen–hydrogen system. Eh values in volts (V). (Diagramfrom Brookins, D.G., 1987, Eh-pH Diagrams for Geochemistry, Springer-Verlag, New York, 176 p.With permission.)

1.2

1.0

0.8

0.6

0.4

0.2

0.0

–0.2

–0.4

–0.6

–0.80 2 4 6

pH

Eh

(V)

8 10 12 14

CrOH2+

Cr2O3

HCrO4–

SYSTEM Cr–O–H25°C, 1 bar

PO2 = 1 bar

PH2 = 1 bar

CrO2–

CrO42–

10–4

10–6



In aqueous environments, under oxidizing conditions, Cr(VI) is extensivelyhydrolyzed; therefore, it is present as an anion, generally forming CrO4

2− anddichromate (HCrO4

−) anions (Calder, 1988; Richard and Bourg, 1991). Accord-ing to Calder (1988), in the chromium–water–oxygen system (Cr + H2O + O2)and under standard conditions, the reaction that occurs is:

HCrO4− ↔ CrO4

2− + H+ (2.5)

FIGURE 2.2Eh-pH diagram for the chromium–oxygen–water system. Eh values in volts (V). (Diagrammodified from Hem, J.D., 1989, U.S. Geological Survey Water Supply Paper 2254, U.S. Govern-ment Printing Office, Washington, D.C., 263 p.)

1.2

1.0

0.8Dichromate

Cr2O72–

ChromateCrO4

2–

0.6

0.4

0.2

0

Eh

(vol

ts)

OX

IDIZ

ING

RE

DU

CIN

G

–0.2

–0.4

–0.6

–0.80 2 4 6 8

pH

10 12 14

Cr3

+

Cr(

OH

)2+

Cr(

OH

) 2+

Cr(

OH

)– 4

Cr(OH)3 (am)Insoluble Hydroxide

5 mg/L Dissolved Cr

5 mg/L Dissolved Cr

0.5 mg/L Dissolved Cr

WATER REDUCED



Cr(VI) substitutes for S(VI) because their ionic radii are similar: Cr(VI) anionshave ionic radii between 0.0325 and 0.052 nm and S(VI) anions have ionicradii between 0.029 to 0.034 nm (Robertson, 1976). This becomes importantbecause sulfate anions can replace chromate and dichromate anions.

In soils and aqueous environments, Cr(III) adsorption by manganeseoxides is the first step toward its oxidation to Cr(VI) by manganese oxides.These typically accumulate on the surface of iron oxides and clay minerals(Bartlett and James, 1979). Mn(III) and Mn(IV) oxyhydroxides and Mn(IV)oxides (such as the mineral pyrolusite) oxidize Cr(III) to Cr(VI); oxidationrates tend to be higher with increasing pH (Eary and Rai, 1986; Hug et al.,1997). Fendorf and Zasoski (1992) noted the following overall reaction:

Cr3+ + 1.5δ−MnO2 + H2O → HCrO4− + 1.5Mn2+ + H+ (2.6)

A similar equation for this reaction was determined from experiments byPalmer and Puls (1994). Soil organic matter quickly adsorbs and reducesCr(VI) to Cr(III). It generally remains mobile only if its concentration exceedsthe adsorbing and reducing capacity of the soil (Bartlett and Kimble, 1979).Organic compounds also reduce Cr(VI) to Cr(III). Richard and Bourg (1991)noted that simple amino, humic, and fulvic acids produce intermediate Cr(V)species that changes to Cr(III) within a few days.

Cr(VI) reduction can also occur from reaction with ferrous [Fe(II)] iron.This generally involves a three-step process (Sedlak and Chan, 1997):

Fe2+ + Cr6+ → Fe3+ + Cr5+ (2.7)

Fe2+ + Cr5+ → Fe3+ + Cr4+ (2.8)

Fe2+ + Cr4+ → Fe3+ + Cr3+ (2.9)

2.3.3 Chromium Reaction Rates (Kinetics)

Chromium reaction rates or kinetics (how fast a reaction will occur), fromCr(III) to Cr(VI) by oxidation and back to Cr(III) by reduction, have beenextensively studied in the laboratory (Lin, 2000). As we have seen, Cr(III)can oxidize to Cr(VI) generally under the catalytic influence of Mn(IV)oxides. Eary and Rai (1986) noted that aqueous Cr(III) oxidation was notcaused by surface catalyzed reactions but by direct reactions of β -MnO2.

Kinetic experiments by Saleh et al. (1989) in lake water, sediment, and soilindicated that reaction rates for the oxidation of Cr(III) to Cr(VI) are relativelyslow with t1/2 ranging from 0.58 to 37.2 yr. Reaction rates for the reductionof Cr(VI) to Cr(III) tend to be very rapid with t1/2 ranging from instantaneousto 53 d under anaerobic or reducing conditions. For aerobic conditions, thet1/2 was measured from 15 min to 21.5 d.



2.4 Chromium Distribution in Primary Environments

2.4.1 Possible Sources of Natural Cr(VI) in Rocks

In the California Coast Ranges, the Franciscan complex contains ultramaficrocks that host chromite ore deposits. Davis (1966) described these depositsas magmatic segregations of chromite in peridotite and peridotite altered toserpentinite. Chromium minerals in these rocks, however, remain largelyunaltered, mostly occurring in the form of the mineral chromatite (FeCr2O3).Other chromium-bearing minerals include eskolaite (Cr2O3) and the chro-mium-bearing spinel minerals include rutile, ilmenite, and magnetite; theseare also contained within other silicate minerals such as the hornblendesand pyroxenes. Many ultramafic rock types contain significant amountsof chromium. Matzat and Shiraki (1974) reported that dunite, peridotite,pyroxentite, and serpentinite carry chromium concentrations to a maximumof 2400 mg/kg. Soils and sediments derived from these rocks also containchromium. Scott (1995) noted from the analysis of 158 soil samples, collectedfrom 25 locations in Sunnyvale and Mountain View (California), that theaverage chromium concentration was 51.28 mg/kg with a range from 30.5to 72.0 mg/kg.

Serpentinized and altered ultramafic rocks have been described in the liter-ature; these include the New Idria serpentinite body in San Benito County,which covers an approximate 124.3 km2 area along the ridge of the DiabloRange (Figure 2.3). About 18.1 km2 of the unit is located within the ArroyoPasajero’s Los Gatos Creek drainage basin. The serpentinite body contains anasbestos deposit of highly sheared and fractured, soft, friable, and powderyserpentinite containing blocks and fragments of harder rocks. The alteredmatrix includes plates and flakes of chrysotile, antigortite, magnesite, brucite,and talc. Asbestos, chromite, and mercury ore have been mined from thisportion of the upper Los Gatos Creek drainage area (Jones, 1988). However,much of the chromium contained in these minerals probably is Cr(III), althoughCr(VI)-bearing minerals may occur due to hydrothermal alteration of the sourcerocks (see the following paragraph; the next section; Steinpress, 2001).

The possible geochemical processes that transform Cr(III) to Cr(VI)include deuteric and hydrothermal alteration of the country rock containingchromium and chromium-bearing minerals. Hydrothermal alteration ofultramafic rocks, including serpentinite, could mobilize relatively insolubleCr(III) minerals by altering them into different mineral species. Numerousexhalative mercury-gold deposits, with associated carbonate-silicate alter-ation (producing a quartz, opal, and a carbonate-rich rock), occur through-out the California Coast Ranges. Examples include the Sulphur Bank minein Lake County, the New Almaden mine in Santa Clara County, and theNew Idria mining district in San Benito County, California (Figure 2.4).Mercury-gold deposits occur mostly in serpentinite associated with Fran-ciscan complex and associated ultramafic rocks. These deposits were formed

Au: Is the ref.to thesucceedingsection? Pls. check.



by hot-spring systems, from alkaline sulfide solutions at relatively lowtemperature and shallow depths, remnants of which are still present. [Inthe central and southern Coast Ranges, four hot springs with water tem-peratures 16.7 to 48.9°C are listed on the San Jose 1:250,000 scale map sheet.The Santa Cruz 1:250,000 scale map sheet lists 12 hot springs with watertemperatures ranging from 23.9 to 62.2°C (Jennings, 1985).] Widespreadalteration of country rock is in the vicinity of these hot-spring systems, withadvanced argillic alteration occurring in adjacent volcanic rocks at theMcLaughlin gold-mercury deposit in Lake County. That hydrothermal alter-ation has transformed chromium oxide minerals is evidenced by the pres-ence of redingtonite (a hydrous chromium sulfate) at the Manhattan minein the Knoxville mining district of Lake County, California (Davis, 1966;Albers, 1981; Vredenburgh, 1982; Peters, 1991; King and Rytuba, 1999).

The geochemical process of transforming Cr(III) to Cr(VI) in soil, sediments,and groundwater also would include oxidation by manganese dioxide(MnO2), and Franciscan complex rocks in the California Coast Ranges hostnumerous manganese mineral deposits. These occur as thin elliptical chertbodies containing the manganese minerals psilomelane, pyrolusite, rhodoch-rosite, hausmannite, and braunite. Such mineral assemblages occur in 70% of

FIGURE 2.3Ultramafic rock bodies in the central Coast Range of northwest-central California. (Map modifiedfrom Churchill, R.K. and Hill, R.K, 2000, California Division of Mines and Geology, Sacramento,California, Open File Report 2000–19, one map sheet, 1:1,000,000 scale. With permission.)

- ultramafic rock

LEGEND

MontereyBay

Monterey

SantaCruz

SanFrancisco

SanJose

NewIdria

101

101

101

580

5

5

99

99

Modified from: Churchil and Hill, 2000.

P a c i f i c

O c e a n



manganese deposits (Albers, 1981; Mosier and Page, 1988). In groundwater,manganese(IV) oxide occurs in the same Eh and pH ranges as the chromateand dichromate anions (Figure 2.5). Insoluble manganese dioxide coating sed-iments in the saturated zone may oxidize Cr(III) to Cr(VI).

FIGURE 2.4Location of mercury-gold ore deposits in the central Coast Range of California. (Map modifiedfrom Bailey et al., 1973, U.S. Geological Survey Professional Paper 820, U.S. GovernmentPrinting Office, Washington, D.C., pp. 401–414.)

Rocks younger thanCoast Range thrust

Rocks of Great Valley sequencelying above Coast Range thrust

Eugeosynclinal (Franciscan) rockslying beneath Coast Range thrust

Older, generally metamorphosedrocks and late Mesozoic granlticrocks

CO

AS

T

SIE

RR

A

PA

CIFIC

NEVADA

RANGESOCEAN

VALLEY

GR

EA

T

New Idria

SulphurBank

SanFrancisco

NewAlmaden

Santa Barbara

Sacramanto

LEGEND

Contact(Dashed where inferred, dotted where

concealed, hachures on overthrust side)

- Coast Range thrust- Major strike slip fault or fault zone

- Mercury - gold deposit- Less productive mercury deposit

Modified from: Bailey and others, 1973.

-

-

-

-



2.4.2 Known Sources of Natural Cr(VI) in Rocks

As of 2002, there were at least 24 known Cr(VI) minerals (Barthelmy, 2002);these occur as:

1. Cr(VI)-bearing minerals that are formed in the oxidized zones oflead (Pb) deposits, generally as the mineral crocoite (PbCrO4). In the

FIGURE 2.5Eh-pH diagram for the system manganese-oxygen-hydrogen. Eh values in volts (V). (Diagramfrom Brookins, D.G., 1987, Eh-pH Diagrams for Geochemistry, Springer-Verlag, New York, 176 p.With permission.)

1.2

1.0

0.8

0.6

0.4

0.2

0.0

–0.2

–0.4

–0.6

–0.80 2 4 6

pH

Eh

(V)

8 10 12 14

SYSTEM Mn–O–H25°C, 1 bar

PO2 = 1 bar

PH2 =1 bar

MnO2

Mn2+

Mn(

OH

) 2

Mn(

OH

) 3–

Mn3O4

MnO

Au: InsertionOk?



United States, crocoite occurs in the Vulture mining district of Arizona(Hurlbut, 1963). Hemihedrite [Pb10Zn(CrO4)6(SiO4)2F2] occurs at theFlorence lead-silver mine near Whickenburg, in Maricopa County,Arizona; deanesmithite (Hg2

+Hg32+Cr6+O5S2) and edoylerite

(Hg32+Cr6+O4S2) occur at the Clear Creek mercury mine in the New

Idria mining district, in the Diablo range of San Benito County,California (Perroud, 2001; Barthelmy, 2002).

2. Cr(VI) minerals in nitrate-rich evaporite deposits in arid or desertenvironments such as the Chilean nitrate deposits in the AtacamaDesert. Chromate minerals are largely confined to iodine-bearingnitrate deposits where the annual precipitation is very low (approx-imately 50 mm per year). The ores consist of caliche-containingsodium nitrate (NaNO3), lautarite [Ca(IO3)2], other iodates, anddietzeite [Ca2(IO3)2(CrO4)] (Williams-Stroud, 1991; Barthelmy, 2002).The Peru-Chile Desert, which includes the Atacama Desert, formsone of a series of apparently long-lived, west coastal-type, subtrop-ical deserts such as those found in Australia and the Namib Desertof Africa. These are also known as hyperarid deserts and they mayhave begun forming as much as 14 million years ago. The Peru-ChileDesert is between 10° and 30° south latitude and 70° and 80° westlongitude. The Atacama Desert is centered at approximately 25°south latitude and 70° west longitude (Hartley and Chong, 2002).Similar climatic conditions may have occurred in the recent geologicpast (Cenozoic) in the Mojave Desert of California and Arizona.

3. Sheared, altered, and serpentinized ultramafic rocks, such as theFranciscan complex, containing Cr(III) minerals. Steinpress (2001;Section 3.1, this volume) reported the presence of 0.06 to 0.46 mg/kgCr(VI) in serpentinite. However, no specific mineral identificationwas made and the actual Cr(VI) mineral assemblages are not known.

2.5 Chromium Distribution In Secondary Environments

2.5.1 Known Natural Cr(VI) Occurrences in Surface Water and Groundwater

In surface water, Cr(III) generally is the predominant species; however,Cr(VI) has been reported in oxygenated fresh water lakes and estuaries.Kaczynski and Kleber (1993) noted natural Cr(VI) maximum concentrationsof 0.026 µg/l in Masonboron Inlet, North Carolina.

In the Paradise Valley, north of Phoenix, Arizona, groundwater collectedfrom wells with the pH approaching 9.0 contained 100 to 200 µg/l Cr(VI)(Robertson, 1976; Hem, 1989). Robertson believed that hydrolysis of feld-spars and common mafic minerals, such as augite and biotite comprising

Au: Ok?

Au: Ok?

Au: InsertionOk?

Au: Is itMasonboronInlet orMasonborinlet? Pls.check.



the alluvium, caused alkaline groundwater conditions in the middle of thevalley. This coupled with calcite produced highly alkaline conditions. Inaddition, there was an absence of ferrous iron, organic matter, and reducingorganisms, which allowed the groundwater to retain its oxidizing and alka-line character, transforming Cr(III) into Cr(VI).

Godgul and Sahu (1995) noted that in the Sukinda chromite belt of Orissa,India, serpentinization and magnesium ion releases during deuteric alter-ation of ultramafic rocks (peridotites) and associated extensive oxidation(laterization) created alkaline (high pH) pore water. This resulted in theproduction of Cr(VI) detected in stream, well, and quarry water. Cr(VI)concentrations ranged from 58 to 64 µg/l in stream water, not detected (ND)to 17 µg/l in well water, and ND to 1791 µg/l in quarry water. These watershad relatively high total manganese concentrations (7230 to 1,540,720 ppb)and relatively low total iron (ND to 12,950 µg/l) concentrations.

In the Presidio in San Francisco, Cr(VI) was present in background-oxidized groundwater with high pH’s, up gradient from any known con-tamination, with concentrations ranging from 52 to 98 µg/l (Steinpress,2001; Section 3.1, this volume). Possible natural background concentrationsof Cr(VI) occur in groundwater near Davis, California with concentrationsranging from 1.0 to 180 µg/l. It is believed that the overall regional Cr(VI)distribution in groundwater is random and therefore a natural condition(Davis, 1995).

In studies by the U.S. Geological Survey (USGS), Cr(VI) has been foundin western Mojave Desert, California groundwater (Ball and Izbicki, 2002).The USGS evaluated Cr(VI) in water sources from public water supplies,domestic and observation wells in alluvial aquifers. Total Cr concentrationsranged from 0.8 µg/l (the detection limit) to 60 µg/l; almost all of the totalCr was Cr(VI).

In 2001, routine sampling of water wells in the Soquel Creek Water District(SCWD) south of Santa Cruz, California determined that Cr(VI) occurred ingroundwater within the Aromas Red Sands (Aromas) aquifer at concentrationsranging from 6 to 38 parts µg/l. While these concentrations were below theMCL for total chromium of 50 µg/l, concerns about public health impactsfrom Cr(VI) in drinking water had been raised during public meetings. ToddEngineers (2002) conducted a focused Cr(VI) groundwater study to determinewhether the detected Cr(VI) was from anthropogenic (man-made) releases orfrom naturally occurring chromium in the Aromas Red Sands Formation. Thestudy found abundant evidence confirming that naturally occurring chromium-bearing minerals found in the Aromas Red Sands Formation were the possiblesources of the Cr(VI) detected in the Aromas aquifer. Environmental conditionsin Aromas aquifer water favor Cr(VI) production from dissolved Cr(III)because the Aromas aquifer waters are well oxygenated, generally have alka-line pH (greater than 7.0), and sediments are predominantly quartz-rich andlow in ferrous iron and aluminum oxides. No anthropogenic sources of Cr(VI)were identified in the inventory of possible contaminating activities in areassurrounding public water supply wells.



2.6 Forensic Geochemistry

Following the definition of forensic geology (Bates and Jackson, 1987) as“The application of the Earth Sciences to the law,” and of environmentalforensics by Stout et al. (1998): “… the systematic investigation of a contam-inated site(s) or events(s) that has impacted the environment which focuseson defensibly allocating liability for the contamination.”

Forensic geochemistry is the application of geochemical principles in thenatural environment to the law. Therefore, forensic geochemical investiga-tions require chemical and isotopic analysis of physical materials such as air,water, soil, and sediment. For metals, this usually involves the analysis oftheir stable isotopes and at times their radioactive isotopes. For example,Hurst et al. (1996) and Hurst (2002) have successfully used stable lead iso-topes to identify gasoline release sources and even provided an age for thesereleases. A complete explanation of environmental forensic and forensicgeochemical techniques is beyond the scope of this section and the reader isreferred to recent texts by Morrison (2000) and Murphy and Morrison (2002).

2.6.1 Soil

Very few forensic geochemical investigations involving Cr(VI) from contami-nated sites are cited in recent literature. This is largely because such forensicinvestigations are rather new. Recent experimental work by Prokisch et al.(2000) in soil showed that anthropogenic chromium could be distinguishedfrom geological chromium by analysis of yttrium (Y) and total chromium; theresults are plotted on an Y–Cr diagram (Figure 2.6). From experimental data,they found that soil collected and analyzed from noncontaminated areas (geo-logic chromium) had very close Cr–Y linear correlation. However, for areascontaminated by anthropogenic chromium, the Cr–Y values were scattered.

2.6.2 Groundwater

Ellis et al. (2001) reported the first measurements of chromium isotope frac-tionation of 53Cr and 52Cr during Cr(VI) reduction to Cr(III). Their studyshowed that dissolved Cr(VI) from groundwater at three contaminated sitesthat had delta (δ) 53Cr values ranging from 1.1%(per mil) to 5.8%. Thisindicated that reduction of Cr(VI) had occurred because the δ 53Cr valuesfrom plating bath solutions showed little or no fractionation for platingoperations ranging up to five years of use.

In a similar chromium isotope study, completed in the western MojaveDesert of California, Ball et al. (2001) noted that stable chromium isotopescould be used to determine the reduction of Cr(VI) to Cr(III) in a loweraquifer; they found that 53Cr was enriched in groundwater relative to 52Cr.This suggested that the Cr(VI) was from a natural source because anthropo-genic sources of chromium typically have δ53Cr values very near zero.



2.6.3 Air

As described in Section 2.2.10, Nusko and Heumann (1997) found that theCr(III)/Cr(IV) ratio in dust from soil erosion ranged from 0.27 to 0.35,whereas the same ratio in the atmosphere aerosol particles was much higher,ranging from 0.39 to 0.63, indicating oxidation of Cr(III) to Cr(VI) in theatmosphere. They also found that the soil surface layer at the forest’s edgewhich was high in organic material had a Cr(III)/Cr(VI) ratio of 1.47, indi-cating that Cr(VI) was preferentially reduced by the soil organic matter.Therefore, stable chromium isotopes may be used to determine the source(s)of Cr(VI) in atmosphere aerosol particles.

2.7 Acknowledgments

The author appreciates the editorial comments and suggestions made byDavid W. Abbott, R.G., C.H.G., Senior Geologist with Todd Engineers;Cynthia P. Avakian, Senior Project Manager with Hydro-EnvironmentalTechnologies, Inc.; and Dr. Jacques Guertin, Consulting EnvironmentalScientist. Alain E. Boutefeu, Graphics Coordinator/Technician with ToddEngineers, created and placed section figures into the required format.

FIGURE 2.6Plot of total chromium versus yttrium for soilscontaining chromium from anthropogenic andgeologic (geogenic) sources. (Diagram modified from Prokisch, J., Kovcs, Palencsr, A.J., Szegvri, I.,and Gyori, Z., 2000, Environ. Geochem. Health, 22, 317–323. With permission.)

60

50

40

30

20

10

07 8 9 10 11 12 13

Y (mg/kg)

Cr

(mg/

kg)

anthropogenicchromium

geologicalchromium

Au: InsertionOk?



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Au: Insertion Ok?

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