Accessories & Oxides

7/30/2019 Accessories & Oxides

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Accessory(silicates)andOxideMinerals

Commonaccessoryandmetaloxidespeciesareoftenvaluableoreminerals

orimportantphasesforpetrographicinterpretation.

Anaccessorymineral


2/20

Titanite(sphene) CaTi[SiO4](O,OH,F)

Anesosilicatewherethedominantstructural

unitarecornersharingTiO6 octahedra.

WhatisspecialabouttheCa

coordination?


3/20

Sphene is monoclinic, +ve, with a 2V = 27.

Pale pinkish-brown to dark brown with 4th order birefringence.

It has a very distinctive form, particularly in thin section, and is a commonaccessory phase in many magmatic rocks. It will also form large crystals

under correct metamorphic terranes.

Its occurrence in alkaline (syenites nepheline) can be of economic value.


4/20

Zircon Zr SiO

,

mineralworld,

because

of

its

textural,

chemical

andisoto iccom lexit

It

is

also

extremely

stable:o p ys ca yan c em ca y.

systemtobesaturatedwithZrandSiO2.


5/20

These

very

small

crystals

contain

very

small

concentrations

of

238Uand235U.

Theelementsareradiogeneicanddecayto206Pband207Pb

respectively.

Itispossibletomeasuretheratiosoftheseelementsinazircon

againstaknownstandard(normallymanmade205Pb).Thenwe

canuse

the

very

well

constrained

half

lives

and

decay

constants

of238Uand235U,tocalculatetheageofthatgrain.


6/20

Thegreatstabilityofzirconmeansthatafterthe238Uand235Uare

enclosedinthestructure,aninternalclockisstartedandifoften

notstoppeduntilageologistwantsitstopped.

Himalaya ZirconsSIPHidraEast Greenland Naantali

10-25 Ma

a380 Ma ~1.8 Ga

Acasta

Gneiss~

Eastern Papua

New Guinea.4.3 Ma

Jack Hills

~4.4 Ga

Basement

460 Ma

Thetechnique

is

robust

from

shortly

after

Earths

formation

(4.5

Ga)

until

recently.


7/20


8/20

Zirconistetragonal(+ve)

an

growsin

many

differentforms.

, ,

andwhere

enclosed

in

phyllosilicates,maygiveriseto

Undercorrectconditions

itwill

form

perfect

.

3 cm

euhedralbipyramids.


9/20

Spinel Group (AB2O4)

Thespinelgroupmineralsconsistof6endmembers,with

variableoccupancyoftheA andBsites.

Spinel,(MgAl2O4)

and

hercynite,

(FeAl2O4)

make

up

the

simplest

solid

solution.

OthervariationsinthisseriesincludeZnandMnendmembers.

Analternative

substitution

occurs

at

.

Themostcommonformbeing2+ 3+

4

.

ThereisalsoaCr3+ substitution.


10/20

Ilmenite (FeTiO3)

Ilmenitehasahexagonalstructurecomparabletocorundum(Al2O3).

Itis

fairly

ubiquitous

in

many

igneous

and

metamorphic

rocks,

occurringasanaccessoryphase,orinveinsorstratiformlayers,

w ereitcanoccuraseconomic eposits.

Compositionally,it

will

partake

in

solid

solution

with

hematite

(Fe2O3).

uncommon,most

ores

are

dark

grey

andnondescript,whilegrainsinthin

sectionareo a ue.


11/20

ReflectedLightMicroscopy

Thisisatechniquewhichisapplicabletoopaquemineralsandusesasetof

observationsremarkablysimilartothosemadeduringtransmittedlight

work.

Colour Simpleobservation,orbycomparisonwithknowns.

Reflectivity Theamountoflightreflectedbacktophotocell.

.

Texture Cleavagetextures,orthogonalvsobliqueandpitting.

Bireflectance a

property

akin

to

pleochroism;

mineral

changes

colour

on

rotation.

Anisotropy apropertyakintobirefringenceincrosspolarisedlight.


12/20

Ilmenitetorutile

Whileilmeniteisfairlycommoninmostgreenschist,amphiboliteandeven

granulitefaciesrocks,itisfarlesscommonatpressuresabove~11kbar.

Atthese

pressures

it

is

typically

converted

to

rutile

(TiO2)

TiO2

Rutile

(tetragonal + ve)

Anatase

(tetragonal - ve) Brookite

parallel

perpendicular


13/20

Optical

properties

of

ilmenite

are

difficulttodetermineintransmitted

light.Possibleindicatorsareafaint

purpletingewhenviewedwithintense

ig t.

Rutile,on

the

other

hand,

has

a

characteristicdeepredbrowncolour,

extreme re r ngencean g re e .

.TiO2 isavaluableore,butitslimited

paragenesismakeeconomicdeposits

unviable.Instead,largeilmenitedeposits

areexp o te .

s

asan

a e

onus

ofhighpurityFeorebyproducts.

, ,

mostcommonlyasapigment.


14/20

Other important, but uncommon oxides.

Fe2O

3- Hematite

SnO2 - Cassiteritee r

2 4- rom e

Al2O

3

Corundum


15/20

HydroxideMinerals

Therearetwoimportanthydroxidemineralsthatyoushouldbeawareof.

Brucite(Mg[OH]2),

because

of

itspotentialroleinCO2

Gibbsite(Al[OH]2),

because

it

definestheoctahedrally

itdefinestheoctahedrally

coordinated

site

in

the

dioctahedralphyllosilicates,and

itistheworldsformostsource

. .


16/20

SulphidesandSulphatesImportantmineralsinoredepositgeologyandtheir

occurrenceiscontrolled,primarily,bytheREDOX conditions

.

,

thelossofanelectron.Oftentheyoccurtogether,"linked",inwhatare

referredto

as

reduction/oxidation

(REDOX)

reactions.


17/20

Anoxicenvironments sulphidesThebiggestgeneratorofsulphidemineralsareblacksmokers.

2

whichaccumulates

in

stratiform

deposits

that

may

result

inMississippiValleyTypeeconomicdeposits.


18/20

,largeeconomicdepositsisa

commonaccessorymineralismany

i neous,sedimentar and

metamorphicrocks.

Ithasahighmetalliclustre,isoftenyellow/gold

, . , .

ItsstructureiscloselyrelatedtoNaCl.

Pyriteisacommonmineralinfossils,using

replacetheskeletalstructureofdeadorganisms.


19/20

OtherSulphideMineralsCommonly occurring together are sphalerite (ZnS) and Galena (PbS)

Thesetwomineralsaretheprincipleoremineralsforbothmetalsandalso

.

operations

will

also

look

to

recover

significant

Cu,

Ag,

Sb

and

Bi.Observablepropertiesnotdissimilar,butPbSismuchheavier!

Znisthe4thmostcommonmetalinuse. Pb,perhapshasmoreformerusesthan

presentuses,butisstillimportant.


20/20

Sulphates hydrousoranhydrous

100sofspeciesknown,twoarerockforming,andonlyfour

willbecommonlyfoundinrockformingstrata.

Anhydrite CaSO4Celestite SrSO

Gypsum CaSO4.2H2O

EvaporiteDeposits

Baryte(Barite) BaSO4isaprimaryconstituentofwhite

.

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Accessories & Oxides