Effect of PH on the Formation of Goethite and Hematite From Ferrihydrite

8/10/2019 Effect of PH on the Formation of Goethite and Hematite From Ferrihydrite

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278 Schwer tmann and Mu rad

lays and lay Minerals

1 , 0

Feo

0 1

0 . 0 1

0 001

p H

\ ~

\

\

\

7

, , , , , , , 9

500 1000

T i m e d o y s )

Figure 1. First-order reaction plot for the transformation of

ferrihydrite a t various pH values.

p l if ie d v e r s i o n o f t h e h y p e r f i n e f i e ld d i s t r i b u t i o n m o d e l

u s e d t o c h a r a c t e r i z e t h i s m i n e r a l b y M u r a d , 1 9 82 ).

R E S U L T S

Rate of transformation

T h e r a t i o F e 0 / F e t ( F e t = t o t a l F e ) h a s p r o v e n u s e f u l

t o c h a r a c t e r i z e t h e s t a g e o f t r a n s f o r m a t i o n f r o m f e r r i -

h y d r i t e t o g o e t h i t e a n d / o r h e m a t i t e ( S c h w e r t m a n n a n d

F i s c h e r , 1 9 66 ; L a n d a a n d G a s t , 1 9 73 ). A l i n e a r o r n e a r l y

l i nea r r e l a t i on sh ip f o r a semi - log p lo t (F igu re 1 ) i nd i -

c a t e s a f ir s t - o r d e r t y p e o f r e a c t i o n ; t h e r a t e o f t r a n s f o r -

m a t i o n a t a n y t i m e is p r o p o r t i o n a l t o t h e a m o u n t o f f e r -

r i h y d r i te l e f t f o r t ra n s f o r m a t i o n . T h e r a t e s l o w s d o w n

a s t h e p H d e c r e a s e s ( F i g u r e 1 ). A p l o t o f h a f t - c o n v e r -

s i o n t i m e a g a i n s t p H ( F i g u r e 2 ) s h o w s a s l i g h t l y c u r v e d

r e l a ti o n s h ip . T h e d e v i a t i o n o f t h e p H 3 s a m p l e f r o m t h i s

r e l a t i o n c a n n o t b e e x p l a i n e d .

Products of transformation

R e s i d u a l f e r r i h y d r i t e i n t h e s y s t e m d e c r e a s e d w i t h in -

c r e a s i n g p H . A t o r b e l o w p H 6 t h e p r o p o r t i o n o f f e r-

r i h y d r i t e a f t e r 4 41 d a y s w a s b e t w e e n 1 5 a n d 3 8 . T h i s

c o m p o n e n t w a s r e m o v e d b y o x a la t e t re a t m e n t b e f o r e

f u r t h e r a n a l y s i s . A f t e r 9 7 0 d a y s t h e f e r r ih y d r i t e c o n -

300

2

1

T ime o f ho l f conve rs ion (doys )

i

6 8 1 o

pH

Figure 2. Len gth of t ime for half conv ersion of ferrihydrite

to goethite and hematite vs. pH.

t e n t s in t h e s a m p l e s s t o r e d a t p H ~ 6 h a d d e c r e a s e d t o

b e t w e e n 2 a n d 1 9 , i n d i c a t i n g f u r t h e r t r a n s f o r m a t i o n .

L e s s t h a n 4 f e r r ih y d r i t e w a s p r e s e n t i n t h e s a m p l e s

s t o r e d a t p H / > 7 a f t e r 4 41 d a y s , a n d l es s t h a n 2 a f t e r

9 7 0 d a y s .

B e s i d e s r e s i d u a l f e r r i h y d r i t e , h e m a t i t e a n d g o e t h i t e

w e r e t h e o n l y o x i d e s p r e s e n t . T h e q u a n t i t a t i v e d e t e r -

m i n a t i o n o f h e m a t i t e a n d g o e t h i te b y X R D a n d M 6 s s -

b a u e r s p e c t r o s c o p y a g r e e d r e a s o n a b l y w e l l ( T a b l e I ) .

A s s h o w n i n F i g u r e 3 , t h e h e m a t i t e : g o e t h i t e r a t i o H m /

( H m + G t ) v a r i e d s tr o n g l y w it h p H . W h e r e a s a t p H 1 2

o n l y g o e t h i t e w a s f o r m e d , m i x t u r e s o f g o e t h i t e a n d h e -

m a t i t e w e r e f o r m e d a t a l l o t h e r p H s . M a x i m u m h e m a -

t i t e f o r m a t i o n o c c u r r e d a t p H 7 a n d 8 . B e l o w t h i s p H

t h e H m / ( H m + G t ) r a t io d e c r e a s e d d r a s t ic a l ly , r e a c h -

i n g a m i n i m u m a t a b o u t p H 4 , b e l o w w h i c h t h e r a t i o

a g a in i n c r e as e d . A f t e r 9 70 d a y s t h e H m / ( H m + G t ) ra -

t i o w a s e s s e n t i a l l y t h e s a m e a s a f t e r 4 4 1 d a y s e x c e p t a t

p H 2 . 5, w h e r e t h e p r o p o r t i o n o f h e m a t i t e h a d i n c r e a s e d

r e l a t i v e t o t h a t o f g o e t h i t e a t t h e e x p e n s e o f re s i d u a l

f e r r i h y d r i t e .

B e s i d e s t h e p r o p o r t i o n o f t h e t w o i r o n o x i d e s , t h e i r

c r y s t al l in i t y a ls o v a r i e d w i t h p H . T h e X R D w i d t h a t h a l f

h e i g h t ( W H H ) o f th e l l 0 l i n e o f g o e t h i t e a n d t h e 0 1 2

l in e o f h e m a t i t e v a r i e d r e c i p r o c a l l y ( F i g u r e 4 ) . S i m i l a r

c o n c l u s i o n s w i t h r e g a r d t o c r y s t a l l i n i t y c a n b e d r a w n

f r o m t h e ir M 6 s s b a u e r s p e c t r a ( F i g u re 5 ). A s t h e X R D

l i n e s n a r r o w e d , b o t h t h e w i d t h s o f t h e h y p e r f i n e fi e ld



Vol. 31, No. 4, 1983 Effect of pH of ferrihydrite transformations 279

0 . 8

0,7

0,6

HI

05

-~ 0,4

0,3

0.2

0.1

01

3 4 5 6 7 8 9 10 11 12

pH

Figure 3. Hematite/ hematite + goethite) ratios vs. pH after

441 days of storage.

O.B )

4

}

g o 6t

~ o.~4

g

~ o 2t

370

l

~6f :Hi.x

\ o 3

i m Q x

x : WMH,,2 380

\ / . - - _ ._ .___J

i , 6 ~ 8 ' 1 0 ' 1 ' 2

385

Figure 4. Widths at half height WHH) of goethite 110 and

hematite 012 and magnetic hyperfine field Hi) of goethite vs.

pH. CoKc~ radiation.

distributions and the magnitudes of the hyperfine fields

of maximum absor ption Himax) decreased . Th e parallel

nature of these trends is demonstrated by the corre-

sponding curves in Figure 4, and is in agreement with

obse rvati ons by Murad 1982) that the hyperfine fields

of maximum absorption of pure goethites decrease and

the widths of hyperfine field distributions increase as

crystal linity beco mes poorer. A similar tre nd with re-

spect to the hyper fine field of hematite is also eviden t;

the highest field occurred where most hematite was

formed and where its XRD lines were sharpest, i.e.,

where the conditions of hematite formation relative to

those of goethite were m ost favorable.

Both the proportions of the two minerals and their

crystallinity clearly indicate that conditions favorable

for the formation ofgoethite are unfavorabl e for the for-

Table I. Amounts and properties of goethite Gt) and

mation of hematite and vice versa. Goethite and he-

matite mu st therefore be formed throu gh a pair of com-

petitive reactions as postulated earlier Schwertm ann

and Fischer, 1966). The low er crystallin ity of the goe-

thite formed at pH 12, as indicated by both WH H and

Hima• seems to contradic t this statement, beca use no

hematite was formed at this pH. An exp lanat ion for this

is given below.

rystal morphology and size of goethite and hematite

In transmis sion electr on micrographs Figure 6), goe-

thite and hematite can be easily distinguished by their

crystal shapes. Hemat ite typically forms irregular hex-

agonal crystals whose diameters are about 3 times as

large -9 00 A) in the pH range 7-9 where maximu m

amou nts of hematite were formed) as at pH ~<4 where

hematite Hm) formed at various pH after 441 days.

Composition

Hm/(Hm + Gt)

HCT ~

pH (days) XRD2 MS3 XRD 4

Properties of goethite

Properties of hematite

WHH .o s Him.x6 HWHD 7 WH H~ s HiS

(~ (kOe) (kOe) (~ (kOe)

2.5 354 0.20 0.25 0.37

3.0 250 0.12 0.13 0.14

4.0 258 0.04 0.05 0.04

5.0 206 0.19 0.25 0.23

6.0 166 0.58 0.58 0.60

7.0 112 0.70 0.69

8.0 68 0.72 0.65 0.70

9.0 32 0.52 0.54

10.0 8 0.14 0.17 0.14

12.0 <4 0 0 0

0.77 375 43 0.23 497

0.37 381 22 0.34 504

0.31 383 22 0.41 504

0.32 377 22 0.22 508

0.45 373 24 0.13 508

0.49 0.11

0.45 373 33 0.11 510

0.37 0.09

0.21 381 24 0.18 507

0.42 378 27 - - - -

1 Half conversion time.

2 X-ray powder diffraction, CoKa radiation.

3 Mtssbauer spectroscopy.

4 After 970 days.

Corrected width at half height of XRD peak.

6 Maximum internal magnetic field.

7 Half width of magnetic field distribution.



28

Schwertmann and Murad

4

~ ~

l a y s a n d l a y M i n e r a l s

~~ ~ ~~

1

1 9

. .

1 0 5 0 5 1 0 1 0 5 0 5 1

o

V E L O C I T Y m m . s -1 )

Figure 5. Room temperature M6ssbauer spectra of goethite-hematite Gt-Hm) mixtures synthesized at various pH values.



Vol. 31, No. 4, 1983 Effect of pH of ferrihydrite trans form ations 281

Figure 6. Electron micrographs of goethites and goethite-hematite mixtures produced from ferrihydrite at various pH-v alues

inser ted figure).



282 Schw er tmann and Mu r ad lays and lay Minerals

~

z 9

~

9O

r

-10 -5 0 5 10

VELOCITY (ram s -t )

F igur e 7 . M6 ssbauer spec t r um a t 4~ of hemat i te - goe th i te

mixtur e synthes ized a t pH 2 .5 .

o n l y m i n o r a m o u n t s o f h e m a t i t e w e r e p r e s e n t ) . T h e a v -

e r a g e d i a m e t e r o f t h e h e m a t i t e p l a t e l e ts fo r m e d a t p H

9 ( c a l c u J a t e d f r o m t h e w i d t h a t h a l f h e i g h t o f t h e 1 I 0

a n d 3 0 0 l i n e s 1) i s 8 8 0 a n d 8 9 0 ~ , , w h i c h a g r e e s w e l l w i t h

t h e T E M o b s e r v a ti o n s .

T h e r o o m t e m p e r a t u r e M 6 s s b a u e r s p e c t r u m o f th e

s a m p l e s t o r e d a t p H 2 . 5 s h o w e d , e v e n a f t e r o x a l a t e

t r e a t m e n t , a s u p e r p a r a m a g n e t i c c o m p o n e n t ( F i g u r e 5).

A t 111 ~ a n d 4 . 2 ~ M 6 s s b a u e r s p e c t r a o f th i s s a m p l e

c o n s i s t e d o f t h r e e s e x t e t s . T h e s t r o n g e s t s e x t e t r e s u l t s

f r o m g o e t h i t e , w h i c h t h e s p e c t r a i n d i c a t e t o m a k e u p

a b o u t 75 t h e s a m p l e . T h e o t h e r t w o c o m p o n e n t s c a n

b e a t t r i b u t e d t o h e m a t i t e w h i c h h a s , a n d h e m a t i t e w h i c h

h a s n o t p a s s e d t h r o u g h a M o r i n t r a n s i t i o n ; a t 4 . 2 ~

( F i g u r e 7 ) t h e s e l a t t e r c o m p o n e n t s h a v e a p p r o x i m a t e l y

e q u a l a r e a s . A c c o r d i n g t o N i n i n g e r a n d S c h r o e e r ( 19 78 ),

t h i s w o u l d i n d i c a t e t h a t a b o u t h a l f o f t h e h e m a t i t e p a r -

t i c l e s h a v e s i z e s b e l o w 2 0 0 ~ , i . e . , a m e d i a n p a r t i c l e

s i z e o f a b o u t 2 0 0 ~ . T h i s v a l u e i s i n g o o d a g r e e m e n t

w i t h t h e m e a n c r y s t a l d i m e n s i o n p a r a l l e l t o t h e c - a x i s

o f 2 2 0 --- 4 0 ,~ c a l c u l a t e d f r o m t h e w i d t h s a t h a l f h e i g h t

o f s ix X R D l i n e s w i t h l ~ 0 .

T h e g o e t h i t e c r y s t a ls s h o w t h e u s u a l a c i c u l a r sh a p e ,

b u t v a r y i n s i z e a n d m o r p h o l o g y . A t p H 12 r a t h e r u n i -

f o r m n e e d l e s 5 0 0 - 1 5 0 0 , ~ l o n g a n d 1 5 0 - 5 0 0 ,~ w i d e w e r e

f o r m e d . A t p H 10 b r o a d c r y s t a l s , c o m m o n l y t w i n n e d

a n d w i t h t e r m i n a l c r y s t a l f a c e s - - - p r o b a b l y ( 01 2 ) a n d

( 0 ] 2 ) - - d o m i n a t e , a n d t h e n e e d l e s a re s o m e w h a t t h i n n e r

t h a n t h o s e a t p H 1 2. A t p H 8 , w h e r e m u c h l e s s g o e t h i t e

w a s f o r m e d , t w i n s a r e r a r e a n d t h e n e e d l e s a r e e x -

t r e m e l y t h i n .

T h e s e o b s e r v a t i o n s s u p p o r t t h e c o n c l u s i o n o f t h e

p r e v i o u s s e c t i o n : w h e n e v e r t h e c o n d i t i o n s f o r g o e t h i t e

c r y s t a l l iz a t i o n a r e f a v o r a b l e , t h i c k n e e d l e s a n d t w i n s

1 Calcula ted f rom the Sche rrer formula: M CD = 0.9k(54.7) /

b cos 0, w here h is the w avelength , b the w idth at half height

af ter cor rect io n for ins trumental l ine broade ning, and 0 the dif-

f ract ion angle .

t0g

c h v i f y

10 Fe OH)~

1 6

2 3 t~ 5 6 ? 8 9 10 11 12

pH

Figur e 8. Fe ( I I I ) ion spec ies a s a f unc tion of pH us ing r e -

ac t ion con s tan ts g iven by L indsay ( 1979) .

d e v e l o p , i n d i c a t in g t h a t t h e c r y s t a l s g r o w w e l l n o t o n l y

i n t h e c r y s t a l l o g r a p h i c c -d i r e c t i o n ( p a r a l le l t o t h e n e e d l e

a x i s ) b u t a l s o i n t h e a - a n d b - d i r e c t i o n s . A s c o n d i t i o n s

b e c o m e s l e s s f a v o r a b l e ( i n d i c a te d b y t h e f o r m a t i o n o f

l e s s g o e t h i t e a n d m o r e h e m a t i t e ) t h e a - a n d b - d i r e c t io n

g r o w t h i s re t a r d e d r e l a t i v e t o t h e g r o w t h i n t h e c - d i r e c -

t i on . T h i s l e a d s t o g r o w t h o f a c i c u l a r c r y s t a l s o f s m a l l e r

w i d t h . T h e w i d t h o f t h e 1 10 l i n e , w h i c h i s r a t h e r s e n -

s i ti v e t o c r y s t a l d e v e l o p m e n t i n t h e a - a n d b - d i r e c t i o n ,

c l e a r l y r e f l e c t s t h i s s i t u a t i o n b y s h o w i n g m i n i m a b e -

t w e e n p H 3 a n d 5 a n d a t p H 1 0, w h e r e t w i n s d o m i n a t e ,

a n d b r o a d e r l i n e s a t p H 2 . 5, 6 - 9 , a n d 12 ( T a b l e 1 ), w h e r e

n e e d l e s d o m i n a t e . C o r r e s p o n d i n g l y , l a r g e r h e m a t i t e

c r y s t a l s w e r e f o rm e d w h e n t h e p r o p o r t i o n o f h e m a t i t e

i n t h e m i x t u r e w a s h i g h a n d v i c e v e r s a .

D I S C U S S I O N

T h e r e s u l t s s u p p o r t a n d r e fi n e a c o n c e p t d e v e l o p e d

e a r l ie r , a c c o r d i n g t o w h i c h g o e t h i t e a n d h e m a t i t e f o r m

f r o m f e r r i h y d r i t e b y t w o d i f f e re n t a n d c o m p e t i t i v e

m e c h a n i s m s : g o e t h i t e c r y s t a l s fo r m i n s o l u t i o n f r o m

d i s s o l v e d F e ( I I I ) i o n s p r o d u c e d b y t h e d i s s o l u t i o n o f

f e r r ih y d r i t e , w h e r e a s h e m a t i t e f o r m s t h r o u g h a n i n t e r-

n a l d e h y d r a t i o n a n d r e a r r a n g e m e n t w i t h i n th e f e r r i h y -

d r i t e a g g r e g a t e s ( S c h w e r t m a n n , 1 9 59 ; S c h w e r t m a n n a n d

F i s c h e r , 1 96 6; F i s c h e r a n d S c h w e r t m a n n , 1 97 5).

T h e r e f o r e , g o e t h i t e s h o u l d b e f a v o r e d a s t h e c o n c e n -

t r a t i o n o f F e ( I I I ) i o n s i n e q u i l i b r i u m w i t h f e r r i h y d r i t e

i n c r e a s e s , a n d h e m a t i t e s h o u l d b e f a v o r e d a s t h e c o n -

c e n t r a t i o n d e c r e a s e s .

T h e c o n c e n t r a t i o n a n d f o r m o f F e ( I I I ) io n s i n e q u i-

l i b ri u m w i t h f e r ri h y d r i t e d e p e n d s t r o n g l y o n p H . U s i n g

t h e a p p r o p r i a t e e q u i l i b r i u m c o n s t a n t s t h e d i a g r a m i n

F i g u r e 8, w h i c h s h o w s t h e p H - d e p e n d e n c e o f v a r i o u s

m o n o m e r i c F e ( I I I ) i o n s , h a s b e e n c o n s t r u c t e d . C o m -

p a r i s o n o f t h i s d i a g r a m w i t h t h e d i s t r i b u t i o n o f g o e t h i t e

a n d h e m a t i t e ( F i g u r e 3 ) s u g g e s t s t h a t g o e t h i t e i s s t r o n g -



Vol. 31, No. 4, 1983 Effect of pH of ferrihydrite transformations 283

ly favored where the concentration of monovalent

Fe III) ions, e ither Fe OH)2 § or Fe OH)4-, is at a max-

imum. The ma ximu m for Fe OH)2 +) [0 denot es ac tiv-

i ty] is at pH -4 , and that for Fe OH)a )-- withi n the

pH range tes ted- -at pH 12. On the other hand, hematite

shows maximum formation where these concentrations

are at their minimum, i.e., around pH 8 Figure 8), which

is also the point of zero charge of ferrihydrite.

Below pH - 4 , although Fe OH)2 +) incre ases fur-

ther, it is overridden by the con centration of the diva-

lent F e OH) ~+ ions whic h ap pear to be less favo rabl e

for goethite crystal growth than the monovalent form.

This situation may retard the formation of goethite but

not that of hematite, so that relatively more hematite is

formed. The low er suitability of Fe OH) z+ compa red to

Fe OH)2 + can be explain ed as follows: Fe IlI) ions

feeding the growing goethite crystal must be discharged

at the crystal sur face befo re being built into the cryst al;

this discharge is probably easier for monoval ent than it

is for divalent ions.

This concept is further supported by earlier results

of Knight and Sylv a 1974) who obs erve d a posit ive re-

lationship betw een Fe OH)~ +) and the r ate of goethite

formation. It is also in agreement with recent work by

Hsu and Wan g 1980), who found more hematite to form

as acidity increased below pH 2.

At very high pHs > I M OH-), where crystallization

was very rapid and only goethite formed, the crystals

consisted of very thin -1 00 A) and long up to 5/xm)

needles, indicating an increasingly strong retardation of

crystal growth in the a- and b-directions and a rapid

growt h in the c-direction. In analogy to the strongly acid

range this could indicate the formation of divalent

Fe OH)52- ions under the influence of an ext reme ly high

OH-) instead of monovalent Fe OH)4- ions at lower

OH)-, the former being less suitable for crystal growth

than the latter. This could explain why the crystallinity

ofgo ethit e formed at pH 12 was lower than that formed

at pH 10 and below, although goethite was the only phase

to form at pH 12.

A C K N O WL E D G ME N T S

We are indebted to Prof. F. E. Wagner Physics De-

partment, Technisc he Universit/it M/inchen at Garch-

ing) for making the M6ssbauer measurements at 110~

and 4~ possible, to Dr. H. Ch. Barts cher er for taking

the electron micrographs, to Mrs. B. Gallitscher for

technical assistance, to Dr. W. R. Fischer for a critical

review of the paper, and to the Deutsche Forschungs-

gemeinschaft for financial support un der grant Schw 90/

32-5.

R E F E R E N C E S

B6hm, J. 1925) l~lber Aluminium- und Eisenoxide I: Z. An-

org. Chem. 149, 203-218.

Fischer, W. R. and Schwertmann, U. 1975) The formation

of hematite from amorphous iron III) hydroxide: Clays

Clay Minerals 23, 33-37.

Hsu, P. H. and Wang, M. K. 1980) Crystallization ofgoe-

thite and hematite at 70~ Soil Sci. Soc. Amer. J. 44, 143-

149.

Kiimpf, N. and Schwertmann, U. 1983) Goethite and he-

matite in a climosequence in southern Brazil and their ap-

plication in classification of kaolinitic soils: Geoderma 29,

27-39.

Knight, R. J. and Sylva, R. N. 074) Precipitation in hy-

drolysed iron III) solutions: J. Inorg. Nucl. Chem. 36, 591-

597.

Landa, E. R. and Gast, R. G. 1973) Evaluation of crystal-

linity in hydrated ferric oxides: Clays Clay Minerals 21,

121-130.

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New York, 449 pp.

Murad, E. 1982) The characterization ofg oethite by M6ss-

bauer spectroscopy: Amer. Mineral. 67, 1007-101 I.

Nininger, R. C. and Schroeer, D. 1978) M6ssbauer studies

of the Morin transition in bulk and microcrystalline a-Fe203:

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freien Eisenoxide in B6den, ihre mineralogischen Formen

und ihre Entstehungsweisen: Z. Pflanzenerniihr. Diingung

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Pflanzenerniihr. Diingung Bodenkunde 105, 194-202.

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droxid. III: Z. Anorg. Allg. Chem. 346, 137-142.

(Received 8 July 1982; accepted 30 December 1982)

Pe31oMe----Pe3y.rlbTaTOM xpaHeHnfl qbeppnrn)lpnTa a ao~InblX cycneH3nax npn 24~ n Be.rIHqHnax pH

OT 2,5 ~10 12 a TeqeHne TaK ~nTenbnoro nepno~Xa raK Tpn ro~la 6blJIO o6pa3oaa~ne reTnTa n

reMaTnTa. IIponoplmn n rpncTa.q~nqnocTb aTnx npo~xyKToa naMeu~ncb 3naqHTeJIhHO C ae~nqnnofi

pH. MaKcnsia_~hnoe ro~nqecTao reuaTnTa o6paaoabiaaaocb npn pHOT 7 ~o 8, a MarcnMa~bnoe

KonnqecTao reTnTa npn pH = 4 tl pH = 12. Kar uoKa3ann pacmnpenne annnfi penTrenoacKofi

nopomKoso~ ~lnqbqbpart~nn, ae~nq~Hbl MarneTnqecv0ax caepxTonKnx nonefi ~ pacnpe~e~eRHe mnpun,

KpnCTa_aYll4qnOCT~, o6onx npo~lyKTOS 6b~na TeM cza6ee , qeM Menbme 6bI3Ia ilponopt~na COOTaeTCTayIo-

mero npo~yrTa s cMecn. Hpepjio~eno cyulecTBoaanne ~layx ronrypnpy~oulnx o6pa- 3oaaTe~bnb~x

upoueccoa: reT~IT o6pa3yeTca n3 pacxaopa, npenMyulecTaenno n3 MOnOBaaenTUbIX norton [Fe OH)2+

Fe OH)4-], co~ep~aumx Fe III) , a reMaTnT--a pe3y~bTaTe BnyTpennofi neperpynnnpoarn n

Jlerv~apaTatlHn anyTpn arrperaTos qbepporngpnTa. 3Ta ~ouuentms Caa3bIaaeT nponopunn reTnTa n

reMaTnTa c arTnaUOCTbtO norton, coJlep~aulnx Fe III) a pacxaope, n no~lpa3yMeaaeT, qTO yc~oana

6:iaronpn~Tn~ie ,n;na o6pa3oaaHn~ reTltTa aa.rLqloTCa ne6~aronpnaTnb~M~i ~.qa o6pa3onagn~ reMaTnTa

u Hao6opoT. [E.G.]



2 8 4 S c h w e r t m a n n a n d M u r a d lays and lay Minerals

R e s i i m ~ A u s F e r r i h y d r i t w i rd b e i 2 4~ u n d p H 2 . 5 b i s 12 i n n e r h a l b v o n b i s z u 3 J a h r e n G o e t h i t u n d

H a m a t i t . l h r M e n g e n v e r h ~ i l t n i s u n d i h r e K r i s t a ll i n it ~ i t v a r i i e r e n s t a r k m i t d e m p H . M a x i m a l e H ~ i m a t i t a n t e i le

t r a t e n z w i s c h e n p H 7 u n d 8 a u f , m a x i m a l e G o e t h i t a n t e i l e b e i p H 4 u n d b e i p H 1 2. D ie K r i s ta l li n it ~ .t b e i d e r

F e - o x id e , g e m e s s e n a n d e r R 6 n t g e n l i n i en v e r b r e i te r u n g , d e m m a g n e t i s c h e n H y p e r f e in f e l d u n d d e s s e n V e r -

t e i l u n g s b r e i te , w a r u m s o g e r i n g e r , j e g e r i n g e r i h r e n t s p r e c h e n d e r A n t e i l a n d e r M i s c h u n g w a r . F f i r d i e

B i l d u n g b e i d e r F e - o x i d e w i r d e i n k o m p e t i t i v e r M e c h a n i s m u s v o r g e s c h l a g e n : G o e t h i t b i l d e t s i c h f ib e r d i e

L 6 s u n g u n d z w a r b e v o r z u g t a u s e i n w e r t i g e n F e ( I I I ) -I o n e n ( F e ( O H ) 2 § u n d F e ( O H ) 4 - ) u n d H a m a t i t d u r c h

i n t e r n e U m s t r u k t u r i e r u n g u n d E n t w ~ i s s e ru n g i n n e r h a l b d e r F e r r i h y d r i ta g g r e g a t e . D i e s e s K o n z e p t s t e l l t

e i n e B e z i e h u n g h e r z w i s c h e n d e m A n t e i l a n G o e t h i t u n d H a m a t i t u n d d e r A k t iv i t~ i t d e r F e ( I I I ) - I o n e n s p e z i e s

i n d e r L 6 s u n g u n d b e g r i i n d e t d ie A u s s a g e , d a b g / i n st i g e B e d i n g u n g e n f f ir d i e G o e t h i t b i l d u n g u n g f i n st i g e

f i ir d ie d e s H ~ m a t i t s s i n d u n d v i c e v e r s a .

R 6 s u m ~ - - -L a m i s e e n r 6 s e r v e d e f e r r i h y d r i t e d a n s d e s s u s p e n s i o n s a q u e u s e s a 2 4 ~ e t ~t d e s p H e n t r e 2 , 5

e t 1 2 p o u r u n e d u r 6 e a u s s i l o n g u e q u e t r o i s a n s a r e s u lt 6 e n l a f o r m a t i o n d e g o e t h i t e e t d h 6 m a t i t e . L e s

p r o p o r t i o n s e t l a c r is t a ll i n it 6 d e c e s p r o d u i t s o n t v a r i 6 l a r g e m e n t e n f o n c t i o n d u p H . U n m a x i m u m d h 6 -

m a t i t e a 6 t 6 f o r m 6 e n t r e p H 7 e t 8 e t u n m a x i m u m d e g o e t h i t e a 6 t 6 f o r m 6 a u p H 4 e t 1 2. L a c r i s ta U i n it 6

d e s d e u x p r o d u i t s s e s t a p p a u v r i e p r o p o r t i o n n e l l e m e n t ~t u n e d i m i n u t i o n d e l a p r o p o r t i o n d u p r o d u i t c o r -

r e s p o n d a n t d a n s l e m 6 1 a ng e , c o m m e l o n t i n d i q u 6 l 6 1 a r g i s s e m e n t d e s l i g n e s d e d i f f r a ct i o n p o u d r 6 e d e s

r a y o n s - X , e t l e s v a l e u r s d e c h a m p s e t l a d i s t r ib u t i o n d e l a rg e u r s m a g n 6 t i q u e s h y p e r f i n s . L e x i s t e n c e d e

d e u x p r o c 6 d 6 s c o m p 6 t i t i fs e s t s u g g e r6 : l a g o e t h i t e e s t f o r m 6 e v i a s o l u t i o n , p r 6 f 6 r a b l e m e n t ~t p a r t i r d i o n s

m o n o v a l e n t s F e ( I I I ) [ F e (O H ) 2 + e t F e ( O H ) 4 - ] , l h 6 m a t i t e p a r r 6 a r r a n g e m e n t i n t e r n e e t d 6 s h y d r a t i o n a u s e i n

d e s a g g r 6 g a t s fe r r i h y d r i te . C e c o n c e p t r e l i e l es p r o p o r t i o n s d e g o e t h i t e e t d h 6 m a t i t e ~ l a c t i v i t 6 d e l e s p ~ c e

d i o n F e ( l l I ) e n s o l u t i o n , e t i m p l i q u e q u e l e s c o n d i t i o n s f a v o r a b l e s p o u r l a f o r m a t i o n d e g o e t h i t e s o n t d 6 -

f a v o r a b l e s ~ c e l l e d e l h 6 m a t i t e e t v i c e v e r s a . [ D . J .]

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