Chapter 5 Mossbauer Spectroscopy

113 Chapter 5

MOSSBAUER SPECTROSCOPY

B.A.

GOODMAN

Department o f S p e c t r o c h e m i s t r y , The Macaulay I n s t i t u t e f o r S o i l Research C r a i g i e b u c k l e r , Aberdeen, AB9 2QJ, S c o t l a n d . 5 . 1 INTRODUC I ON The aim o

t h i s c h a p t e r i s t o p r e s e n t an a c c o u n t o f t h e c u r r e n t a p p l i c a t i o n s

o f Mossbauer s p e c t r o s c o p y i n t h e s t u d y o f c l a y m i n e r a l s . I t i s n o t i n t e n d e d as a r e v i e w a r t c l e , p a r t l y because a comprehensive r e v i e w a r t i c l e would be beyond t h e scope o f t h i s book and, more i m p o r t a n t l y , because t h e Mossbauer 1 i t e r a t u r e has been we1 -covered elsewhere. A p a r t f r o m t h e Mossbauer E f f e c t Reference and Data J o u r n a l (Stevens e t a l . ) , which l i s t s a l l papers p u b l i s h e d on Mossbauer spectroscopy, a number o f r e v i e w s o f a p p l i c a t i o n s t o t h e s t u d y o f c l a y m i n e r a l s has

been p u b l i s h e d i n r e c e n t y e a r s (e.g.

Heller-Kallai,

B a n c r o f t , 1973; 1979; Coey, 1975;

1980; Goodman, 1980). Thus, r e f e r e n c e s used h e r e w i l l be t o

i l l u s t r a t e t h e d i s c u s s i o n o r t o g i v e examples o f areas o f a p p l i c a t i o n , and a l a r g e number o f i m p o r t a n t p u b l i c a t i o n s w i l l n o t be r e f e r r e d t o . For t h i s I a p o l o g i s e i n advance, b u t , a p a r t f r o m i n d i c a t i n g t h e g e n e r a l areas i n which Mossbauer s p e c t r o s c o p y has p r o v e d u s e f u l , i t i s my i n t e n t i o n t o d e v o t e some o f t h e space a v a i l a b l e t o a c r i t i c a l e x a m i n a t i o n o f some o f t h e approaches t h a t have been g e n e r a l l y adopted and t o i n d i c a t e some e x p e r i m e n t s t h a t s h o u l d be performed i n t h e f u t u r e .

I n o r d e r t o do t h i s s a t i s f a c t o r i l y , i t will a l s o be

necessary t o i n c l u d e an account o f some aspects o f t h e t h e o r y , so t h a t t h i s c h a p t e r can be r e a d on i t s own w i t h o u t c o n t i n u a l r e f e r e n c e s t o t e x t s on t h e subject. Mossbauer s p e c t r o s c o p y i s a branch o f a b s o r p t i o n s p e c t r o s c o p y i n which t r a n s i t i o n s between t h e energy l e v e l s i n an a t o m i c n u c l e u s a r e s t u d i e d by means o f t h e r e s o n a n t a b s o r p t i o n o f y - r a d i a t i o n . A l t h o u g h w i d e l y separated, t h e n u c l e a r energy l e v e l s a r e v e r y p r e c i s e l y d e f i n e d and t h e i r changes w i t h v a r i a t i o n s i n t h e environment o f t h e n u c l e u s a r e v e r y s m a l l . I t i s t h u s necessary t o use r a d i a t i o n t h a t has an e x c e e d i n g l y s m a l l spread o f e n e r g i e s i n o r d e r t o observe t h e h y p e r f i n e s t r u c t u r e i n Mossbauer s p e c t r a . T h i s i s achieved b y u s i n g as t h e e x c i t i n g r a d i a t i o n y - r a d i a t i o n t h a t has been e m i t t e d b y n u c l e i i n t h e i r e x c i t e d s t a t e s as t h e y decay t o t h e i r ground s t a t e s . T h i s energy may t h e n be modulated by i m p a r t i n g a Doppler v e l o c i t y t o the- source n u c l e u s and a Mossbauer spectrum i s o b t a i n e d as t h e v a r i a t i o n o f t h e percentage a b s o r p t i o n o r t r a n s m i s s i o n r e l a t i v e t o t h e magnitude o f t h e Doppler v e l o c i t y . F o r t h i s reason v e l o c i t y (mm s - l o r cm s

-1

114 i s c o n v e n t i o n a l l y used as t h e energy u n i t i n Mossbauer spectroscopy. Several f a c t o r s l i m i t t h e number o f i s o t o p e s t h a t e x h i b i t t h e Mossbauer e f f e c t o r can be c o n v e n i e n t l y s t u d i e d by i t . F i r s t l y , i t i s necessary f o r a s o u r c e t o e x i s t t h a t can decay t o produce an i s o t o p e o f t h e n u c l e u s under i n v e s t i g a t i o n i n an e x c i t e d s t a t e . T h i s i m m e d i a t e l y e l i m i n a t e s a l l elements o f l o w a t o m i c number (i.e.

<19). Secondly, f o r r o u t i n e a p p l i c a t i o n s ,

i t i s necessary t h a t t h e h a l f -

l i f e of t h e source n u c l e u s i s a t l e a s t s e v e r a l days and p r e f e r a b l y months o r years. T h i r d l y , f o r a p p l i c a t i o n s i n c l a y mineralogy, the n a t u r a l occurrence o f t h e Mossbauer i s o t o p e s h o u l d be h i g h enough f o r t h e r e s o n a n t a b s o r p t i o n t o be detected w i t h a conventional spectrometer (i.e. F o u r t h l y , on e m i s s i o n o f t h e y - r a y ,

>O.OlX o f t h e t o t a l material).

t h e source n u c l e u s tends t o r e c o i l ( c o n s e r v -

a t i o n o f momentum), and t h e Massbauer e f f e c t can o n l y be observed i f t h i s r e c o i l energy can be spread o v e r t h e l a t t i c e as a whole, by c o u p l i n g w i t h t h e l a t t i c e v i b r a t i o n s . Thus, a f r a c t i o n o f e m i s s i o n s can o c c u r w i t h o u t n u c l e a r r e c o i l and i t i s t h e s e y - r a y s t h a t produce t h e Mossbauer spectrum. Since a s i m i l a r process

o c c u r s on a b s o r p t i o n , t h e r e c o i l - f r e e f r a c t i o n ( o r f - f a c t o r ) f r o m t h e source and a b s o r b e r must be as h i g h as p o s s i b l e i n o r d e r t o observe a w e l l - d e f i n e d G s s b a u e r spectrum. The problem o f r e c o i l i n c r e a s e s w i t h i n c r e a s i n g energy o f t h e y - i r a d i a t i o n and an upper l i m i t o f about 140 KeV e x i s t s f o r t h e o b s e r v a t i o n o f t h e Mossbauer e f f e c t . However, f o r e n e r g i e s g r e a t e r t h a n about 50 KeV i t i s necessary t o work a t l o w temperatures ( i . e .

b o i l i n g p o i n t o f l i q u i d helium) i n order t o

observe r e c o i l l e s s e m i s s i o n and a b s o r p t i o n . F i n a l l y , t h e w i d t h o f t h e Mossbauer a b s o r p t i o n peak i s determined b y t h e l i f e t i m e o f t h e n u c l e a r e x c i t e d s t a t e a c c o r d i n g t o t h e Heisenberg u n c e r t a i n t y p r i n c i p l e : I'T

=

where

h/2n r i s the l i n e width,

(1) T

i s t h e mean l i f e o f t h e e x c i t e d s t a t e and h i s

P l a n c k ' s c o n s t a n t . Thus a s h o r t h a l f - l i f e o f t h e e x c i t e d s t a t e l e a d s t o broad a b s o r p t i o n peaks. A f t e r t a k i n g a c c o u n t o f t h e above-mentioned f a c t o r s , t h e o n l y i s o t o p e s u i t a b l e f o r r o u t i n e i n v e s t i g a t i o n i n c l a y m i n e r a l s i s 57Fe, which has a n a t u r a l abundance o f 2.245% o f t h e i r o n . The n u c l e a r p r o p e r t i e s o f 57Fe a r e such t h a t i t s f i r s t e x c i t e d s t a t e , which has a s p i n o f 3/2, l i e s a p p r o x i m a t e l y 14.4 KeV above i t s ground s t a t e , which has a s p i n o f 1/2.

I n t e r a c t i o n s w i t h t h e environment o f t h e n u c l e u s can cause t h e s e

energy l e v e l s t o s h i f t o r s p l i t as shown i n F i g . 5.1. The s h i f t i n t h e c e n t r e of t h e absorption from t h a t o f a reference (Fig. 5.la) 6, t h e d o u b l e t s p l i t t i n g ( F i g . 5 . l b )

i s known as t h e isomer s h i f t ,

as t h e e l e c t r i c quadrupole i n t e r a c t i o n , o r

quadrupole s p l i t t i n g , A , and t h e s e x t e t s p l i t t i n g ( F i g . 5 . 1 ~ ) as t h e magnetic hyperfine interaction. S p e c t r a can be observed f r o m e i t h e r t r a n s m i t t e d o r s c a t t e r e d r a d i a t i o n . The f o r m e r i s used i n most c o n v e n t i o n a l arrangements b u t b a c k - s c a t t e r e x p e r i m e n t s a r e u s e f u l i n t h e s t u d y o f s u r f a c e r e g i o n s o f a sample and a r e o f p a r t i c u l a r

115 value i n corrosion studies.

-6; m

I

m

I

7

7-

nl

-2

- -1 2

-2

1

Y d

C

F i g . 5.1 The h y p e r f i n e i n t e r a c t i o n s i n Mb'ssbauer spectroscopy. ( a ) The isomer s h i f t s 6 , ( b ) t h e WadruPOle s p l i t t i n g , A , ( c ) t h e m a g n e t i c h y p e r f i n e i n t e r a c t i o n , and ( d ) combined magnetic and quadrupole i n t e r a c t i o n s . 5 . 2 B A S I C THEORY 5.2.1

Isomer s h i f t , 6

The isomer s h i f t observed i n a Mossbauer e x p e r i m e n t i s determined by t h e e l e c t r o n d e n s i t y a t t h e nucleus, and i s g i v e n b y : 6 = (2*/5) Ze 2 i l $-( ~ ) 1 2 a - 1 ~ ( o 2) rl 1 (Re 2 - Rg 2 1 where Z i s t h e a t o m i c number, e i s t h e e l e c t r o n i c charge,

(2) 1'

and

1'

a (0) r a r e t h e e l e c t r o n d e n s i t i e s a t t h e n u c l e u s o f t h e a b s o r b e r and r e f e r e n c e m a t e r i a l

Re and R a r e t h e r a d i i o f t h e n u c l e a r e x c i t e d and ground s t a t e s , 9 r e s p e c t i v e l y . I n i r o n t h i s e l e c t r o n d e n s i t y can v a r y by two mechanisms: ( i ) d i r e c t respectively,and

changes i n 4s e l e c t r o n d e n s i t y t h r o u g h t h e i n v o l v e m e n t o f 4s o r b i t a l s i n m o l e c u l a r o r b i t a l s and ( i i ) i n d i r e c t changes i n t h e e l e c t r o n d e n s i t y o f t h e c o r e s - e l e c t r o n s Mechanism ( i ) can sometimes be ( i . e . 15, 2s and p a r t i c u l a r l y 3s e l e c t r o n s ) . i m p o r t a n t i n t h e h i g h l y c o v a l e n t l o w s p i n compounds b u t i s u s u a l l y small when i r o n i s i n t h e h i g h s p i n s t a t e . Thus, i n s i l i c a t e minerals,mechanism

( i i ) i s the pre-

dominant source o f t h e isomer s h i f t . -It o c c u r s because t h e r a d i a l d i s t r i b u t i o n s o f a t o m i c o r b i t a l s a r e such t h a t e l e c t r o n s i n t h e 3d o r b i t a l s spend a f r a c t i o n of t h e i r time c l o s e r t o t h e nucleus than electrons i n t h e core s - o r b i t a l s .

When t h i s

occurs, t h e a t t r a c t i v e Coulomb p o t e n t i a l between t h e n u c l e u s and t h e s - e l e c t r o n s

116 decreases. Therefore, decreases I $ t e r m Re2-R

1'

any i n c r e a s e i n t h e number o f d - e l e c t r o n s ,

effectively

and r e s u l t s i n a change i n 6, which i s p o s i t i v e because t h e

2 ( y ~e q u a t i o n ( 2 ) i s n e g a t i v e . The number o f d - e l e c t r o n s on t h e i r o n

9 i s determined n o t o n l y b y t h e o x i d a t i o n s t a t e b u t a l s o by t h e n a t u r e o f any molec-

u l a r o r b i t a l s i n which t h e d - e l e c t r o n s a r e i n v o l v e d . 5.2.2

The e l e c t r i c quadrupole i n t e r a c t i o n

I f t h e n u c l e u s has a n o n - u n i f o r m charge d e n s i t y , which i s t h e case when t h e

nuclear spin, I >

3,

then t h e energy l e v e l s may be s p l i t by an i n t e r a c t i o n w i t h

a non-cubic e l e c t r i c f i e l d g r a d i e n t . The magnitude o f t h e s p l i t t i n g , i l l u s t r a t e d i n Fig. 5 . l b , i s

k n o w n a s t h e quadrupole s p l i t t i n g , A , and i s p r o p o r t i o n a l t o t h e

p r i n c i p a l component o f t h e e l e c t r i c f i e l d g r a d i e n t t e n s o r . Thus, 2 A = $e q Q ( 1 + n2/3)'

(3) where Q i s t h e n u c l e a r quadrupole moment, e i s t h e charge on a p r o t o n , -eq i s t h e z-component o f t h e e l e c t r i c f i e l d g r a d i e n t t e n s o r (Vzz) parameter and i s equal t o (Vxx

-

Vyy)/Vzz.

and II i s t h e a s y m e t r y

Axes a r e c h o s e n so t h a t 0

n

Q

1. The

e l e c t r i c f i e l d g r a d i e n t i s composed o f terms f r o m t h e valence e l e c t r o n s and f r o m t h e charge d i s t r i b u t i o n i n t h e s u r r o u n d i n g c r y s t a l l a t t i c e . Thus, =

(1

where

q l a t t + (1 - R ) q v a l and R a r e t h e S t e r n h e i m e r f a c t o r s (Sternheimer, 1963) and qlatt

-Ym) y,

(4) and qval

a r e c o n t r i b u t i o n s f r o m t h e c r y s t a l l a t t i c e and t h e v a l e n c e e l e c t r o n s , r e s p e c t i v e l y The p r i n c i p a l ( V z z ) components o f t h e qval

terms a r e g i v e n i n T a b l e 5.1 f o r t h e

five d orbitals. TABLE 5.1 The c o n t r i b u t i o n s t o Vzz o f an e l e c t r o n i n each o f t h e 3d o r b i t a l s ( i n t h e i r usual forms each o f t h e s e o r b i t a l s has Wavefunction 1

d L z2 2 dx -Y d XY dxz dYz

0

= 0).

Vzz*

41 7 -417 -4/7

217 2/7

* i n u n i t s o f e < r-3 >, where e i s t h e charge o f an e l e c t r o n and i s t h e mean v a l u e o f r-3 f o r t h e o r b i t a l s

117 Ift h e l a t t i c e t e r m can be c o n s i d e r e d as a r i s i n g f r o m a s e t o f p o i n t charges, t h e n

f o r each charge q a t a d i s t a n c e r f r o m t h e n u c l e u s 2 2 -3 V x x = q r ( k i n ecos Q - 1 ) 2 . 2 v = qr-3(3sin esin 4 - 1) yy -3 2 Vzz = q r ( k o s e - 1 ) vXy = vyx = q r - 3 ( 3 s i n 2 e s i n Q c o s $ )

vXz

=

vYz =

vZx v ZY

= qr-3(3sinecosecosQ) = qr-3(3sinecosesin~)

where t h e p q l a r c o o r d i n a t e s r,

e

(5) and ,$ have t h e i r usual meaning.

Quadrupole i n t e r a c t i o n i n Fe3+. As t h e h i g h s p i n Fe3+ i o n has 1 e l e c t r o n i n each o f the d-orbitals,

t h e t o t a l qval

t e r m i s zero. Thus, f o r a c o m p l e t e l y i o n i c

s i t u a t i o n , t h e quadrupole s p l i t t i n g would be d e t e r m i n e d s o l e l y by l a t t i c e terms. However, i n a r e a l s i t u a t i o n t h e bonds w i l l be p a r t l y c o v a l e n t and t h e p o p u l a t i o n s o f these d - o r b i t a l s w i l l no l o n g e r g e n e r a l l y be u n i t y , and, f o r any s i t u a t i o n i n which t h e Fe3+ i o n i s n o t bound t o i d e n t i c a l groups i n c u b i c symmetry, t h e r e may be a c o n t r i b u t i o n t o A f r o m valence terms. W i t h low s p i n Fe3+ a non-zero qval i s expected (see T a b l e 5.1),but t h e s i t u a t i o n i s c o m p l i c a t e d b y t h e h i g h degree o f covalency i n t h e m o l e c u l a r o r b i t a l s o f such s p e c i e s . Quadrupole i n t e r a c t i o n i n Fez+. The h i g h s p i n Fez+ i o n has 1 e l e c t r o n o v e r t h e h a l f - f i l l e d s e t o f d - o r b i t a l s and would, t h e r e f o r e , b e expected t o have a l a r g e valence e l e c t r o n c o n t r i b u t i o n t o A . T h i s i s t h e case f o r t h e ground s t a t e i o n , b u t u s u a l l y t h e r e i s an e x c i t e d e l e c t r o n i c s t a t e o f o n l y a s l i g h t l y h i g h e r energy t h a n t h e ground s t a t e which produces a c o n t r i b u t i o n t o A o f t h e o p p o s i t e s i g n . Thus, i f a t t h e t e m p e r a t u r e o f o b s e r v a t i o n o f t h e spectrum, an e l e c t r o n i c e x c i t e d s t a t e i s populated, A w i l l be composed of v a l e n c e terms f r o m b o t h s t a t e s , i n a d d i t i o n t o any l a t t i c e terms - a f a c t o r t h a t can c o m p l i c a t e t h e i n t e r p r e t a t i o n o f t h e spectrum (see s e c t i o n 5.3.3).

The l o w s p i n Fe2+ i o n would be expected t o have a z e r o qval

t e r m (see T a b l e ' 5 . 1 ) , l a r g e effect.

b u t as f o r

low s p i n Fe3+, c o v a l e n t i n t e r a c t i o n s may have a

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

t h e y w i l l n o t be c o n s i d e r e d f u r t h e r here. The i n t e n s i t i e s o f t h e two peaks o f a quadrupole s p l i t spectrum v a r y w i t h t h e angle, 8 , between t h e p r i n c i p a l a x i s o f t h e e l e c t r i c f i e l d g r a d i e n t and t h e d i r e c t i o n o f t h e y - r a y as shown i n

T a b l e 5.2.

Thus, f o r 0 = 0, t h e i n t e n s i t y r a t i o .

TABLE 5.2 Angular dependence o f t h e peaks i n a q u a d r u p o l e - s p l i t spectrum Transition

5

4 +&

3/2

+t++t

* e

Relative Intensity*

1 5/3

+

-

cos

2

e

2 cos e ~

_

_

_

i s t h e a n g l e between t h e y - r a y and t h e z - a x i s o f t h e e l e c t r i c f i e l d g r a d i e n t

118 o f t h e peaks i s 3:1,

whereas f o r

e

= 90°,

t h e r a t i o i s 3:5. Consequently, w i t h a

s i n g l e c r y s t a l , t h e d i r e c t i o n of t h e p r i n c i p a l a x i s o f t h e e l e c t r i c f i e l d g r a d i e n t can be r e a d i l y determined. I n a p o l y c r y s t a l l i n e sample w i t h randomly o r i e n t e d c r y s t a l l i t e s , a summation o v e r a l l angles i s r e q u i r e d and an i n t e n s i t y r a t i o o f 1 : l i s o b t a i n e d . I n most work on c l a y m i n e r a l s p o l y c r y s t a l l i n e samples a r e used and an i n t e n s i t y r a t i o o f 1 : l f o r t h e peaks i s o f t e n assumed. However, i f t h e r e i s p r e f e r e n t i a l o r i e n t a t i o n i n t h e a b s o r b e r t h e n t h e peaks may n o l o n g e r have t h e same i n t e n s i t y , and l a c k o f r e c o g n i t i o n by t h e r e s e a r c h e r can l e a d t o t h e number of components i n t h e spectrum b e i n g i n c o r r e c t l y r e p o r t e d . However, i t has been shown by E r i c s o n and Wappling (1976) t h a t o r i e n t a t i o n o f t h e p l a n e o f t h e a b s o r b e r a t 54.7'

t o t h e d i r e c t i o n o f t h e y-beam r e n d e r s t h e peak i n t e n s i t y r a t i o s a g a i n

equal. 5.2.3

The magnetic h y p e r f i n e i n t e r a c t i o n

A c o u p l i n g o f t h e n u c l e a r magnetic moment w i t h any l o c a l o r a p p l i e d m a g n e t i c f i e l d a t t h e n u c l e u s causes a s p l i t t i n g o f t h e energy l e v e l s as shown i n F i g . 5 . l ~ Where t h e r e i s a combined magnetic and e l e c t r i c quadrupole i n t e r a c t i o n t h e n u c l e a r s p l i t t i n g s a r e d i r e c t l y r e l a t e d t o t h e combined terms ( F i g . 5 . l d ) ,

but u n l e s s t h e

a n g l e between t h e p r i n c i p a l axes o f t h e magnetic f i e l d and t h e e l e c t r i c f i e l d g r a d i e n t i s known, i t i s n o t p o s s i b l e t o d e t e r m i n e t h e magnitudes o f b o t h f r o m such a spectrum. The r e l a t i v e e n e r g i e s and i n t e n s i t i e s o f t h e t r a n s i t i o n s shown

i n F i g . 5 . l d a r e g i v e n i n T a b l e 5.3 f o r t h e case where t h e energy o f t h e m a g n e t i c h y p e r f i n e i n t e r a c t i o n i s much g r e a t e r t h a n t h a t o f t h e e l e c t r i c f i e l d g r a d i e n t . TABLE 5.3 R e l a t i v e e n e r g i e s and i n t e n s i t i e s o f t h e peaks i n a spectrum o f a sample h a v i n g a l a r g e magnetic f i e l d whose e n e r g i e s a r e p e r t u r b e d by t h e presence o f an e l e c t r i c f i e l d gradient. Transition

R e l a t i v e Energy

Relative Intensity

i s t h e a n g l e between t h e m a g n e t i c f i e l d and t h e p r i n c i p a l a x i s o f t h e e l e c t r i c f i e l d gradient e i s t h e a n g l e between t h e m a g n e t i c f i e l d and t h e y - r a y

JI

The parameters go and ge a r e t h e ground and e x c i t e d s t a t e gyromagnetic r a t i o s ,

119 respectively

-

t h e e n e r g y l e v e l s b e i n g s e p a r a t e d b y ggnH b y t h e magnetic f i e l d ,

ti, where 6, i s t h e n u c l e a r magneton. F o r a s i n g l e c r y s t a l when t h e a n g l e

t h e magnetic f i e l d and t h e y - r a y d i r e c t i o n i s Oo, a r e 3:0:1:1:0:3

and f o r e = 90'

e between

t h e i n t e n s i t y r a t i o s o f t h e peaks

t h e s e become 3:4:1:1:4:3.

Summing o v e r a l l angles

f o r a p o l y c r y s t a l l i n e sample g i v e s a r a t i o o f 3:2:1:1:2:3. 5.2.4

L i n e Shapes

Over a range o f e n e r g i e s , E , t h e a b s o r p t i o n , u, i s g i v e n b y + 4(E-Eo) 2 / r 2 J

u = uo[ 1

where Eo i s . t h e energy o f t h e t r a n s i t i o n ,

r

(6) i s d e f i n e d i n e q u a t i o n ( 1 ) and uo i s

given by u0

=-l.h;lcz.

2 1 , + 1 . 1

(7)

Eo'

21 t 1 1 t a 9 where h i s P l a n c k ' s c o n s t a n t , c i s t h e v e l o c i t y o f l i g h t ,

2n

o f t h e n u c l e a r e x i c t e d and ground s t a t e s , r e s p e c t i v e l y , and

Ie and Ia are the spins i s th;

internal

conversion c o e f f i c i e n t . The L o r e n t z i a n l i n e shape d e s c r i b e d by e q u a t i o n ( 6 ) i s s t r i c t l y v a l i d o n l y f o r i n f i n i t e l y t h i n absorbers. However, i n o r d e r t o o b t a i n a reasonable s i g n a l - t o - n o i s e r a t i o f a i r l y t h i c k absorbers a r e g e n e r a l l y used ( u s u a l l y between 0.05 and 0.2 mg 2 o f 57Fe p e r cm ) , and t h e t r u e l i n e s h a p e i s m o d i f i e d by s a t u r a t i o n e f f e c t s . N e v e r t h e l e s s , i t i s g e n e r a l l y assumed t h a t peaks a r e o f L o r e n t z i a n shape, and, i n t h e a n a l y s i s o f p a r t i a l l y r e s o l v e d s p e c t r a , t h i s may l e a d t o i n a c c u r a c i e s i n t h e e s t i m a t i o n o f t h e parameters ( i . e .

p o s i t i o n s , w i d t h s and i n t e n s i t i e s ) o f t h e

componrnt peaks. T h i s problem can b e overcome by t h e use o f t h e t r a n s m i s s i o n i n t e g r a l i n t h e c o m p u t a t i o n procedures (Shenoy

Gal. 1975), b u t

i t r e q u i r e s much

more computer t i m e t h a n t h e s i m p l e L o r e n t i z i a n l i n e shapes and has n o t so f a r been g e n e r a l l y adopted i n c l a y - m i n e r a l work. Another case where t h e L o r e n t z i a n shape does n o t h o l d i s f o r r e l a x a t i o n s p e c t r a i.e.

-

where t h e r a t e o f f l u c t u a t i o n o f t h e h y p e r f i n e parameters i s comparable w i t h

the time o f the t r a n s i t i o n

(a.10-7-10-9

s ) . An example o f t h e dependence o f

t h e shape o f magnetic h y p e r f i n e s t r u c t u r e on t h e r e l a x a t i o n t i m e i s g i v e n i n Fig.5.2. T h i s b e h a v i o u r may r e s u l t e i t h e r f r o m t h e i n v e r s i o n i n d i r e c t i o n o f t h e magnetic h y p e r f i n e f i e l d i n a paramagnetic m a t e r i a l as a r e s u l t o f a s p i n - f l i p process o r by t h e c o l l e c t i v e r e o r i e n t a t i o n o f t h e m a g n e t i c moment d i r e c t i o n i n v e r y small p a r t i c l e s o f m a g n e t i c a l l y - o r d e r e d m a t e r i a l s . T h i s l a t e r process i s v e r y i m p o r t a n t

i n t h e s t u d y o f t h e c l a y m i n e r a l components o f s o i l s where s m a l l p a r t i c l e s a r e

comnon and w i l l be r e f e r r e d t o i n more d e t a i l l a t e r .

120

iW T ’ q -10

10 -10 VELOCITY/rnm

10

5-l

Fig.5.2

The dependence o f t h e shape o f t h e magnetic h y p e r f i n e s t r u c t u r e on r e l a x -9 a t i o n time, ( a ) t = 1 0 - l 2 s , ( b ) t = lO-’s, ( c ) t = 2.5 x lO-’s, ( d ) t = 5 x 10 s , ( e ) t = 7.5 x lO-’s, ( f ) t = 2.5 x 10-8s, (adapted f r o m Wickman, 1966).

5.2.5

-8

( 9 ) t = 7.5 x 10

s (h) t =

A b s o r p t i o n areas and t h e r e c o i l - f r e e f r a c t i o n The magnitude o f t h e resonance a b s o r p t i o n i s dependent on t h e e f f e c t i v e t h i c k -

ness, t, o f t h e absorber.

t =

(8)

nfoO

where n i s t h e number of atoms o f t h e MEssbauer i s o t o p e p e r u n i t area, f i s t h e r e c o i l - f r e e f r a c t i o n and u0 i s t h e a b s o r p t i o n c r o s s s e c t i o n as d e f i n e d i n e q u a t i o n ( 7 ) . On t h e assumption t h a t a L o r e n t z i a n shape i s v a l i d t h e a r e a under t h e a b s o r p t i o n peak i s g i v e n by

(9)

A = (n/2)fsrt where fs i s t h e r e c o i l - f r e e f r a c t i o n o f t h e source. F o r f a i r l y t h i n absorbers

2 ( < about 0.1 mg 57Fe/crn ) t h e L o r e n t z i a n shape i s a r e a s o n a b l e a p p r o x i m a t i o n t o

t h e e x p e r i m e n t a l peaks, b u t t h e l i n e w i d t h i s dependent upon t h e a b s o r b e r t h i c k n e s s . Thus , a c c o r d i n g t o B a n c r o f t ( 1 973)

rex = ra + rs

t

0.27 r t

(10)

121 where rex i s t h e e x p e r i m e n t a l peak w i d t h and ra and

rs

are the widths f o r t h i n

a b s o r b e r and source, r e s p e c t i v e l y . The e x p e r i m e n t a l l i n e w i d t h s may be broadened b y a number o f f a c t o r s o t h e r t h a n t h e a b s o r b e r t h i c k n e s s and some o f these can be e x t r e m e l y i m p o r t a n t i n t h e s t u d y o f c l a y m i n e r a l s . I n p a r t i c u l a r t h e e x i s t e n c e o f i n h o m o g e n e i t i e s i n t h e sample r e s u l t s i n t h e p r o d u c t i o n o f a range o f components which may n o t be m u t u a l l y r e s o l v e d . Broadened peaks, which may no l o n g e r approximate t o L o r e n t z i a n shape, r e s u l t and, i f t h e problem i s n o t r e c o g n i z e d , may l e a d t o erroneous c o n c l u s i o n s when c o m p u t e r - f i t t i n g o v e r l a p p i n g peaks. The magnitude o f t h e f - f a c t o r can v a r y a p p r e c i a b l y f r o m one sample t o a n o t h e r and may be expressed as 2 2 2 /A )

f = exp (-4n

(11 1 where A i s t h e wavelength o f t h e y - r a y and i s t h e mean square d i s p l a c e m e n t o f t h e Mossbauer atom f r o m i t s e q u i l i b r i u m p o s i t i o n under thermal v i b r a t i o n . The f-factor,

t h e r e f o r e , v a r i e s w i t h t e m p e r a t u r e and decreases r a p i d l y a t h i g h temper-

a t u r e s . A l s o may v a r y . a l o n g d i f f e r e n t d i r e c t i o n s i n a c r y s t a l ( G o l d a n s k i i e t al.,

1963), w i t h t h e r e s u l t t h a t unequal peak h e i g h t s can be o b t a i n e d i n a

quadrupole s p l i t spectrum f r o m a randomly o r i e n t e d p o l y c r y s t a l l i n e sample. F o r t u n a t e l y , t h i s phenomenon i s q u i t e r a r e and i s b e l i e v e d n o t t o be i m p o r t a n t i n t h e s t u d y o f c l a y m i n e r a l s a t ambient and l o w e r temperatures. 5.3 APPLICATIONS I N THE STUDY OF CLAYS

I n t h i s , t h e main s e c t i o n o f t h i s c h a p t e r , t h e p r i n c i p a l t y p e s o f a p p l i c a t i o n o f Mossbauer s p e c t r o s c o p y w i l l be discussed. F o r convenience t h e s e have been grouped under 3 main headings:( i ) q u a l i t a t i v e a n a l y s i s , which w i l l be concerned w i t h t h e i d e n t i f i c a t i o n o f o x i d a t i o n s t a t e s o r p a r t i c u l a r m i n e r a l phases; ( i i ) q u a n t i t a t i v e a n a l y s i s , where some o f t h e problems o f o b t a i n i n g q u a n t i t a t i v e i n f o r m a t i o n w i l l be discussed; (iii)

s t r u c t u r a l analysis,

i n which t h e n a t u r e o f t h e s t r u c t u r a l i n f o r m a t i o n

t h a t can be o b t a i n e d f r o m a Mossbauer spectrum w i l l be c r i t i c a l l y assessed. 5.3.1 (i)

Applications i n Q u a l i t a t i v e Analysis I d e n t i f i c a t i o n o f Oxidation States.

The most comnon a p p l i c a t i o n o f

Miissbauer s p e c t r o s c o p y i s i n t h e i d e n t i f i c a t i o n o f t h e o x i d a t i o n s t a t e s o f i r o n . As i n d i c a t e d i n S e c t i o n 5.2.1,

t h e h i g h s p i n i o n s Fe2+ and Fe3+ can be r e a d i l y

d i s t i n g u i s h e d b y t h e magnitudes o f b o t h t h e i r isomer s h i f t s and t h e i r quadrupole s p l i t t i n g s . The isomer s h i f t i s t h e more r 6 l i a b l e parameter, s i n c e p o p u l a t i o n o f t h e 4s e l e c t r o n i c l e v e l i s n e v e r l i k e l y t o be l a r g e and t h e v a r i a t i o n i n t o t a l e l e c t r o n d e n s i t y i n t h e 3d o r b i t a l s , as a r e s u l t o f changes i n t h e c o v a l e n t c h a r a c t e r o f t h e bonds, i s much s m a l l e r t h a n t h e d i f f e r e n c e s between t h e two s t a t e s .

122 Thus isomer s h i f t s a t room t e m p e r a t u r e r e l a t i v e t o i r o n m e t a l a r e i n t h e ranges

-

0.1

0.5 mm

5-l

f o r Fe3+ and 0.7

-

1.3 mm s-'

f o r Fez+. W i t h i n these groups t h e r e

a r e v a r i a t i o n s w i t h c o o r d i n a t i o n number ( d i s c u s s e d i n S e c t i o n

5.3.3 below) and w i t h

t h e n a t u r e o f t h e atom bound t o t h e i r o n , so t h a t f o r c l a y m i n e r a l s , where t h e i r o n i s u s u a l l y bound t o oxygen atoms, t h e isomer s h i f t s f o r m d i s c r e t e groups w i t h i n t h e ranges mentioned above. The quadrupole s p l i t t i n g i s o f t e n a good g u i d e t o t h e o x i d a t i o n s t a t e , p a r t i c u l a r l y when t h e c r y s t a l l o g r a p h i c s i t e s c o n t a i n i n g t h e i r o n a r e n o t g r e a t l y d i s t o r t e d f r o m c u b i c symmetry. I n such cases t h e quadrupole s p l i t t i n g f o r Fe3+ tends t o be small (<1

IMII

5 - l ) and t h a t f o r Fez+ l a r g e ( 2 . 5

-

3.5 mm s - ' ) .

However, as i n d i c a t e d e a r l i e r , t h e Fez+ v a l u e can be g r e a t l y reduced i f e x c i t e d e l e c t r o n i c s t a t e s a r e p o p u l a t e d . Such a phenomenon o f t e n o c c u r s a t q u i t e l o w temperatures (sometimes w e l l below a m b i e n t ) so t h a t i n t e r p r e t a t i o n o f t h e quadrupole s p l i t t i n g i s n o t always s t r a i g h t f o r w a r d . T h i s w i l l be d e a l t w i t h i n more d e t a i l i n s e c t i o n 5.3.3,

where t h e i m p l i c a t i o n i n s t r u c t u r a l a n a l y s i s w i l l be discussed.

With l o w e r symmetries, t h e r e may a l s o be a non-zero v a l e n c e c o n t r i b u t i o n t o t h e quadrupole s p l i t t i n g i n Fe3+, which, combined w i t h t h e l a r g e r l a t t i c e c o n t r i b u t i o n , can o c c a s i o n a l l y g i v e values o f A ( > 1.5 mm s - l ) comparable w i t h those o b t a i n e d from Fez+ i n low symmetry, where t h e valence and l a t t i c e c o n t r i b u t i o n s a r e u s u a l l y

o f opposite sign. Two p o t e n t i a l problems e x i s t , which cause d i f f i c u l t i e s i n t h e s i m p l e i d e n t i f i c ation o f oxidation states: (a)

I f a s p e c i e s has a low r e c o i l - f r e e f r a c t i o n a t

t h e temperature o f i n v e s t i g a t i o n , t h e n t h a t s p e c i e s w i l l make 1 i t t l e c o n t r i b u t i o n t o t h e o v e r a l l Mossbauer a b s o r p t i o n . An example h e r e i s t h e case o f a Fez+exchanged m o n t m o r i l l o n i t e (Fig.5.3).

A t 77K b o t h f e r r o u s and f e r r i c components

1.80

.83

3 1.78 E

.82

(o

0

K a 0

1.76 .81

-2

0

2 Velocity

/

-2

0

2

mm's-1

F i g . 5.3 S p e c t r a o f Fe*+-exchanged m o n t m o r i l l o n i t e a t ( a ) 77K and ( b ) ambient temperatures (Helsen and Goodman, u n p u b l i s h e d r e s u l t s ) .

123 can be c l e a r l y seen, b u t a t ambient temperature t h e f e r r o u s component, which a r i s e s e n t i r e l y f r o m i r o n i n exchange s i t e s , i s no l o n g e r v i s i b l e because o f i t s small f - f a c t o r .

On t h e b a s i s o f t h i s spectrum a l o n e i t c o u l d have been concluded

t h a t Fez+ was absent. ( b )

The l o w s p i n i o n s b o t h have parameters t h a t o v e r l a p

a p p r e c i a b l y w i t h those f r o m h i g h s p i n Fe3+. T h e r e f o r e , i f t h e r e i s t h e p o s s i b i l i t y o f low s p i n i o n s b e i n g p r e s e n t i n a sample, t h e n t h e i d e n t i f i c a t i o n o f t h e o x i d a t i o n s t a t e s i s less straightforward. Fortunately, low s p i n ions are u n l i k e l y t o o c c u r i n c l a y m i n e r a l s t r u c t u r e s , b u t where samples have been t r e a t e d w i t h c e r t a i n s t r o n g l y complexing r e a g e n t s , t h e p o s s i b i l i t y o f t h e i r f o r m a t i o n should n o t be i g n o r e d . (ii)

I d e n t i f i c a t i o n o f S p e c i f i c M i n e r a l Species.

I n t h e o r y , Mossbauer

spectroscopy has t h e p o t e n t i a l t o r e v e a l t h e e x i s t e n c e o f i r o n - c o n t a i n i n g m i n e r a l phases a t l e v e l s below t h e i r l i m i t s o f d e t e c t i o n by o t h e r more c o n v e n t i o n a l techniques such as XRD. T y p i c a l parameters f o r Fe i n some s i l i c a t e m i n e r a l s ( T a b l e 5.4) show t h a t t h e r e i s a l a r g e o v e r l a p o f parameters f r o m one s p e c i e s t o a n o t h e r . Thus Mossbauer s p e c t r o s c o p y has 1 i t t l e p o t e n t i a l f o r i d e n t i f y i n g a TABLE 5 . 4

T y p i c a l Mossbauer parameters f o r some l a y e r s i 1 i c a t e s a t ambient temperatures Fe3+ Kaol in it e Chamosi t e Muscovite Illite Glauconite Nontronite t Montmorillonite Biotite

*

6*

A*

0.36 0.38 0.40 0.33 0.36 0.36 0.32 0.40

0.52 0.78 0.72 0.65 0.40 0.30 0.52 0.52

Fe2+ 6*

0.38 0.35 0.36 0.37 0.38

A*

1.21 1.05 0.62 1.15 0.92

6*

A*

6*

A*

1.14 1.13 1.14 1.15

2.57 3.00 2.75 1.7-2.7

1.12 1.12

2.20 2.20

1.07 1.11

2.90 2.62

1.09

2.18

A l l parameters i n mm s - I ,

'Also

isomers s h i f t s r e l a t i v e t o Fe metal -1 and A = 0.5 3 r d component sometimes o b t a i n e d w i t h 6 i= 0.2 mm s

-

0.6 mn s

-1

s p e c i f i c s i l i c a t e m i n e r a l w i t h o u t a t l e a s t c o n s i d e r a b l e h e l p f r o m o t h e r techniques. On t h e o t h e r hand, parameters f o r c e r t a i n o t h e r groups o f m i n e r a l s may be d i s t i n c t enough f o r Mossbauer spectroscopy t o be used as a b a s i s f o r i d e n t i f i c a t i o n . An example i s t h e c a r b o n a t e m i n e r a l s where t h e Fez+ quadrupole s p l i t t i n g i s mm s-',

m. 1.9

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

To i l l u s t r a t e t h i s , t h e u s e f u l n e s s o f t h e t e c h n i q u e i n t h e i d e n t i f i c a t i o n of t h e v a r i o u s o x i d e s and h y d r o x i d e s i s d i s c u s s e d i n some d e t a i l . The parameters and magnetic p r o p e r t i e s f o r t h e h i g h l y - c r y s t a l l i n e m i n e r a l s a r e g i v e n i n Table 5.5.

124 TABLE 5.5 Mossbauer parameters f o r some c r y s t a l l i n e oxides and hydroxides o f i r o n Sample a-Fe203

Magnetic Properties

A

6-Fe2o3

Ordering Temperature ( K )

Experimental Temperature ( K )

956

300 77

107

295

H(T)

s(mm s - l )

51.6 52.7

0.36 0.48

4.2

49.6 51.9

0.324 0.331

y-Fe203

Fi

300

48.8 49.9

0.27 0.41

Fe3°4

Fi

300

49.3 46.0

0.25 0.65 0.37 0.48

a-FeOOH

A

393

300 77

38.4 50.4

6-FeOOH

A

295

300

-

77

v-FeOOH a-FeOOH

A

FO

A = antiferromagnetic

73

77 4.2

(455) (420) . ,

2 96

ao

Fo = ferromagnetic

A(IIIII

s-l)

0.72 0.97

0.37 0.38 0.52 0.48 0.48

0.55

46.0

-

0.48 0.51

0.55

38.0 52.5 50.5

0.35 0.7 0.7

47.3 46.3 43.7

F i = ferrimagnetic

Hematite (a-Fe203) i s c o n v e n i e n t l y i d e n t i f i e d by comparison o f s p e c t r a a t ambient and low temperatures because o f t h e e x i s t e n c e o f a Morin t r a n s i t i o n a t 260 K, which changes t h e angle between t h e p r i n c i p a l a x i s o f t h e e l e c t r i c a l f i e l d g r a d i e n t and t h e magnetic f i e l d . The spectrum o f magnetite, Fe304, a t ambient temperature has two components which r e p r e s e n t Fe3+ i n t e t r a h e d r a l s i t e s and a combination o f Fez+ and Fe3+ i n octahedral s i t e s . For the l a t t e r a s i n g l e s e t o f peaks i s observed because e l e c t r o n exchange between Fez+ and Fe3+ i s r a p i d . A t low temperatures, where t h e exchange i s slower, a f a r more complex spectrum i s obtained. The types o f magnetic o r d e r i n g shown i n Table 5.5,

can be d i s t i n g u i s h e d by o b t a i n i n g

s p e c t r a i n the presence o f a l a r g e a p p l i e d magnetic f i e l d . T h i s i s because f e r r o and f e r r i - m a g n e t i c samples r e o r i e n t i n t h e f i e l d w i t h subsequent l o s s o f i n t e n s i t y from peaks 2 and 5, whereas t h e a n t i f e r r o m a g n e t i c samples are u n a f f e c t e d . F o r ferromagnetic m a t e r i a l s , where t h e magnetic moments a r e a l i g n e d , t h i s r e o r i e n t a t i o n occurs w i t h much s m a l l e r f i e l d s than those r e q u i r e d f o r f e r r i m a g n e t i c m a t e r i a l s ,

125 where t h e magnetic moments a r e canted. When such experiments a r e combined w i t h t h e c h a r a c t e r i s t i c Mossbauer parameters, each o f t h e m i n e r a l s can be r e a d i l y d i s t i n g u i s h e d from t h e o t h e r s . Examples o f t h e i d e n t i f i c a t i o n o f h e m a t i t e and g o e t h i t e i n s o i l s have been g i v e n b y C h i l d s

Gal. (1978) and

(1978) and o f some o t h e r o x i d e s by Longworth

e.(1979).

Belozerskii

et.

However, i n n a t u r a l

samples, t h e problem i s much l e s s c l e a r c u t , s i n c e t h e e x i s t e n c e o f s m a l l p a r t i c l e s and isomorphous s u b s t i t u t i o n can have a marked e f f e c t on t h e magnetic p r o p e r t i e s of t h e m i n e r a l . As mentioned i n S e c t i o n 5.2.4,

the rate o f collective reorientation

o f m a g n e t i c moments i n m i c r o c r y s t a l s may be comparable t o t h e t i m e o f t h e Mossbauer t r a n s i t i o n a t temperatures w e l l below t h a t o f o n s e t o f t h e magnetic o r d e r i n g

-

a

phenomenon t h a t r e s u l t s i n a c o l l a p s e o f t h e 6-peak Mossbauer spectrum as shown i n Flg.5.2. W i t h isomorphous s u b s t i t u t i o n o f d i a m a g n e t i c i o n s i n t h e c r y s t a l l a t t i c e t h e s i z e s o f t h e domains o f m a g n e t i c o r d e r i n g decrease w i t h t h e same e f f e c t as a r e d u c t i o n i n p a r t i c l e s i z e . A c o m b i n a t i o n o f a h i g h l e v e l o f s u b s t i t u t i o n w i t h a s m a l l p a r t i c l e s i z e can l e a d t o v e r y d r a m a t i c r e d u c t i o n s i n t h e temperature a t which m a g n e t i c a l l y o r d e r e d s p e c t r a can be observed ( f o r example, Goodman and Lewis, 1981, have r e p o r t e d a c o l l a p s e d spectrum f r o m g o e t h i t e a t 77K). There i s a l s o an apparent r e d u c t i o n i n t h e magnetic f i e l d s f o u n d i n t h e s u b s t i t u t e d minerals (Janot

%,,

1971; Golden

u., 1979).

The presence o f a m i x t u r e o f phases may make t h e q u a l i t a t i v e i d e n t i f i c a t i o n o f t h e components d i f f i c u l t when t h e above-mentioned p a r t i c l e s i z e and isomorphous s u b s t i t u t i o n e f f e c t s a r e p r e s e n t . I n such circumstances t h e 6- and y-oxyhydroxides may n o t be d i s t i n g u i s h e d f r o m a-FeOOH. A l s o t h e p o o r l y - d e f i n e d m i n e r a l s f e r r i h y d r i t e and f e r r o x y h i t e b o t h o r d e r m a g n e t i c a l l y a t l o w temperatures w i t h magnetic f i e l d s (Murad and Schwertmann, 1980; C a r l s o n and Schwertmann, 1980) s i m i l a r t o t h a t o b t a i n e d f r o m aluminous g o e t h i t e , g i v i n g f u r t h e r p o s s i b i l i t i e s f o r m i s i d e n t i f i c a t i o n . I t would t h u s appear t h a t as t h e m i n e r a l s p e c i e s become l e s s w e l l - d e f i n e d ,

their

Mossbauer s p e c t r a become more a1 ike, and hence Mossbauer s p e c t r o s c o p y becomes l e s s u s e f u l f o r d i a g n o s t i c purposes. k s s b a u e r s p e c t r o s c o p y has f o u n d c o n s i d e r a b l e a p p l i c a t i o n s i n t h e s t u d y of s u r f a c e s when t h e s p e c t r a a r e o b t a i n e d i n t h e s c a t t e r i n g geometry (see e.g. T r i c k e r , 1977). I n a d d i t i o n t o t h e 14.4 KeV v - r a y ,

x - r a y s o r e l e c t r o n s e m i t t e d as

a r e s u l t o f t h e Mossbauer t r a n s i t i o n , may a l s o be d e t e c t e d . The mean escape depth o f t h e r a d i a t i o n v a r i e s w i t h i t s energy, t h u s a l l o w i n g s u r f a c e r e g i o n s t o be s e l e c t i v e l y s t u d i e d . W i t h t h e 14.4 KeV y - r a y t h e s p e c t r a a r i s e p r e d o m i n a n t l y f r o m the top

m o f t h e m a t e r i a l , whereas w i t h t h e l o w e r energy x - r a y s r e s u l t s a r e

o b t a i n e d f r o m t h e t o p few hundred nm. W i t h c o n v e r s i o n e l e c t r o n s i t may even be p o s s i b l e t o s t u d y monolayers p r o v i d e d t h i i r s p e c t r a do n o t o v e r l a p w i t h those from t h e i r substrates (Petrera

et., 1976).

Thus, w i t h b a c k s c a t t e r experiments

i t i s p o s s i b l e t o i d e n t i f y t h e i r o n - c o n t a i n i n g phases on t h e s u r f a c e s o f b u l k y m a t e r i a l s and examples o f a p p l i c a t i o n s i n c o r r o s i o n r e s e a r c h have been reviewed

126 by Graham and Cohen (1976) and i n t h e s t u d y o f f i n e a r t s by Keisch (1976).

5.3.2 Q u a n t i a t i v e A n a l y t i c a l A p p l i c a t i o n s Because o f t h e n a t u r e o f t h e t e c h n i q u e , any q u a n t i t a t i v e i n f o r m a t i o n o b t a i n e d by Mossbauer s p e c t r o s c o p y must r e l a t e s o l e l y t o t h e d i s t r i b u t i o n o f i r o n . Thus, i n a mixed-phase system i t i s o n l y p o s s i b l e t o o b t a i n t h e q u a n t i t a t i v e d i s t r i b u t i o n o f t h e i r o n components between t h e phases. W i t h o u t a knowledge o f t h e i r o n c o n t e n t s o f each phase i t i s i m p o s s i b l e t o determine t h e amounts o f each phase p r e s e n t . The q u a n t i t a t i v e a n a l y t i c a l a p p l i c a t i o n s a r e t h e r e f o r e e x t r e m e l y 1 i m i t e d , b u t t h e r e a r e areas, p a r t i c u l a r l y i n t h e d e t e r m i n a t i o n o f f e r r o u s / f e r r i c

ratios i n

u n s t a b l e phases, where t h e t e c h n i q u e i s a b l e t o make i n v a l u a b l e c o n t r i b u t i o n s . Examples here i n c l u d e redox r e a c t i o n s i n s i l i c a t e s (Rozenson and H e l l e r - K a l l a i , 1976 a and b; R u s s e l l

u., 1979), where Mossbauer s p e c t r o s c o p y has been success-

f u l l y used. One m a j o r d i f f i c u l t y i n t h i s t y p e o f work concerns t h e p o s s i b i l i t y

t h a t t h e r e c o i l - f r e e f r a c t i o n i s n o t i d e n t i c a l f o r a l l components i n t h e system b e i n g i n v e s t i g a t e d . Most workers have assumed t h a t by w o r k i n g a t low temperatures (e.g. 77K) any problems r e l a t i n g t o t h e f - f a c t o r s w i l l be i n s i g n i f i c a n t , o t h e r s have i g n o r e d t h e e f f e c t even a t room t e m p e r a t u r e . However, as t h e s p e c t r a i n Fig.5.3 show, m i s l e a d i n g r e s u l t s can sometimes be o b t a i n e d , p a r t i c u l a r l y when t h e n a t u r e s o f t h e l a t t i c e s i n which t h e i r o n i s h e l d d i f f e r a p p r e c i a b l y . A l t h o u g h t h i s example m i g h t be c o n s i d e r e d an extreme case, s i n c e t h e sample c o n t a i n e d b o t h i n t e r l a y e r and s t r u c t u r a l i r o n , q u a n t i t a t i v e problems r e l a t e d t o f - f a c t o r d i f f e r ences a r e by no means r a r e . TO f u r t h e r i l l u s t r a t e t h i s , F i g . 5.4 shows t h e d i f f e r e n c e s i n h e m a t i t e : g o e t h i t e r a t i o s d e t e r m i n e d f r o m Mossbauer s p e c t r a a t ambient t e m p e r a t u r e and 77K. I n t h i s case t h e m a j o r p r o b l e m a r i s e s f r o m t h e f a c t t h a t t h e g o e t h i t e peaks a t 77K a r e n o t t h e L o r e n t z i a n shape t h a t was assumed b y t h e c o m p u t e r - f i t t i n g program ( s e e S e c t i o n 5.2.4),

w i t h t h e r e s u l t t h a t some o f t h e a r e a under h e m a t i t e peaks i s

assigned t o g o e t h i t e . The room t e m p e r a t u r e r a t i o s may a l s o n o t be r e l i a b l e s i n c e f f a c t o r s a r e i n f l u e n c e d by p a r t i c l e s i z e and i n t h i s spectrum (Fig.5.4a)

there i s

evidence t h a t t h e g o e t h i t e p a r t i c l e s may be m i c r o c r y s t a l l i n e . I t has been shown r e c e n t l y t h a t Mossbauer s p e c t r o s c o p y can be used t o g i v e

a c c u r a t e q u a n t i t a t i v e i n f o r m a t i o n f r o m m u l t i p h a s e systems ( C o l l i n s , 1978, 1979; Bahgat, 1979). T h i s i n v o l v e d making measurements a t s e v e r a l t e m p e r a t u r e s and t h e n e x t r a p o l a t i n g t h e a b s o r p t i o n areas t o a t e m p e r a t u r e T* a p p r o p r i a t e f o r a z e r o v a l u e 2 f o r t h e mean square displacement, < x >, i n e q u a t i o n ( 1 1 ) . However, because o f t h e l a r g e number o f s p e c t r a r e q u i r e d t o c h a r a c t e r i z e one sample, t h e procedure i s v e r y t i m e consuming.

127

-10 -Fig.5.4

Vetocity

1

mm s-1

10

S p e c t r a from a m i x t u r e o f h e m a t i t e (a-Fe203) and g o e t h i t e (a-FeOOH) i n

a b a u x i t e sample ( a ) a t ambient temperatue and ( b ) a t 77K (Goodman, i i n p u b l i s h e d results). 5.3.3

Applications i n structural analysis

T h i s i s a v e r y wide f i e l d t h a t has developed o v e r t h e p a s t 20 y e a r s and, i n t h i s s e c t i o n , I s h a l l d i s c u s s s e v e r a l aspects o f t h i s t y p e o f a p p l i c a t i o n a l o n g w i t h t h e t h e o r e t i c a l j u s t i f i c a t i o n o f t h e procedures g e n e r a l l y adopted. These

w i l l be grouped under t h e f o l l o w i n g headings: ( i ) d e t e r m i n a t i o n o f c o o r d i n a t i o n number, ( i i ) d i s t r i b u t i o n o f i m p u r i t y i o n s i n o x i d e s , ( i i i ) i d e n t i f i c a t i o n o f the s i t e s containing octahedral i r o n i n s i l i c a t e s , ( i v ) e l u c i d a t i o n o f mineral a l t e r a t i o n r e a c t i o n s , and ( v ) t h e o r e t i c a l c o n s i d e r a t i o n s f o r s i l i c a t e s . By f a r t h e l a r g e s t amount o f e x p e r i m e n t a l work has been devoted t o t h e c h a r a c t e r i z a t i o n of i r o n - c o n t a i n i n g s i t e s , b u t t h i s w i l l be g i v e n p r o p o r t i o n a l l y l e s s space t h a n t h e o t h e r s u b - s e c t i o n s , because o f i t s b e t t e r coverage i n r e v i e w s pub1 i s h e d elsewhere ( B a n c r o f t , 1973; Coey, 1975; H e l l e r - K a l l a i , ( i ) D e t e r m i n a t i o n o f c o o r d i n a t i o n number.

1980).

I n s e c t i o n 5.2.1

i t was s t a t e d t h a t

t h e isomer s h i f t i s d e t e r m i n e d p r i n c i p a l l y ' b y t h e changes i n s h i e l d i n g o f t h e i n n e r - s h e l l s - e l e c t r o n s t h a t o c c u r as a r e s u l t o f v a r i a t i o n s i n d - e l e c t r o n d e n s i t y . T h i s a l l o w s t h e h i g h s p i n o x i d a t i o n s t a t e s o f i r o n t o be r e a d i l y d i s t i n g u i s h e d as

128 shown i n S e c t i o n 5.3.1.

However, t h e e f f e c t i v e d - e l e c t r o n d e n s i t y is dependent

upon t h e degree o f c o v a l e n c y i n t h e m o l e c u l a r o r b i t a l s i n which t h e d - e l e c t r o n s a r e i n v o l v e d . S i n c e t h e s e o r b i t a l s a r e e i t h e r non-bonding o r anti-bonding,any i n c r e a s e i n t h e degree o f c o v a l e n c y w i l l r e s u l t i n a decrease i n t h e d - e l e c t r o n d e n s i t y , and c o n s e q u e n t l y a s m a l l e r v a l u e f o r t h e isomer s h i f t . I n g e n e r a l f o r i r o n , where t h e t e r m R e 2 +

*

i n e q u a t i o n ( 2 ) i s n e g a t i v e , t h e r e appears t o be a 9 s t e a d y i n c r e a s e i n 6 w i t h i n c r e a s e i n c o o r d i n a t i o n number f o r a p a r t i c u l a r g r o u p bound t o t h e i r o n and, a l t h o u g h some workers have doubted t h e r e l i a b i l i t y o f u s i n g t h e isomer s h i f t as an i n d i c a t o r o f c o o r d i n a t i o n number (e.g.

Brown and

P r i t c h a r d , 1969; Richardson, 1975), i t has r e c e n t l y become i n c r e a s i n g l y a p p a r e n t t h a t i n s i l i c a t e m i n e r a l s i r o n i n t e t r a h e d r a l c o o r d i n a t i o n has i s o m e r s h i f t s 0.15

-

E.

0.20 mn s - l l o w e r t h a n t h o s e o f o c t a h e d r a l i r o n . Examples o f t h e u s e f u l n e s s

o f t h i s parameter have been i n t h e i d e n t i f i c a t i o n o f t e t r a h e d r a l i r o n i n n o n t r o n i t e s (Goodman

u., 1976 and Fig.5.5)

I

1

-1 F i g . 5.5

and i n p h l o g o p i t e (Sanz

1

e., 1978).

L

1

0 Velocity/mrn

s-1

2

1

Mossbauer spectrum o f a n o n t r o n i t e a t 77K ( f r o m Goodman

s., 1976).

There have been s u g g e s t i o n s t h a t t h e quadrupole s p l i t t i n g m i g h t be used as a g u i d e t o c o o r d i n a t i o n number (e.g.

Taylor

et., 1968),

b u t , a l t h o u g h t h e r e may

be some e m p i r i c a l r e l a t i o n s h i p o v e r a s m a l l group o f samples, t h e t h e o r y o f t h e o r i g i n crf t h e e l e c t r i c quadrupole i n t e r a c t i o n ( S e c t i o n 5.2.2)

eliminates the

129 p o s s i b i l i t y o f any g e n e r a l r e l a t i o n s h i p between A and c o o r d i n a t i o n number. ( i i ) D i s t r i b u t i o n o f i m p u r i t y i o n s i n o x i d e s . I n S e c t i o n 5.3.1 i t was mentioned t h a t b o t h isomorphous s u b s t i t u t i o n o f d i a m a g n e t i c i o n s and t h e presence o f s m a l l p a r t i c l e s i z e s can have a marked e f f e c t on t h e t e m p e r a t u r e a t which 6-peak s p e c t r a a r e o b t a i n e d from m a g n e t i c a l l y o r d e r e d m i n e r a l s . A c o n s i d e r a b l e amount o f work has been c a r r i e d o u t i n o r d e r t o use t h e s e e f f e c t s i n t h e c h a r a c t e r i z a t i o n o f t h e o x i d e and o x y h y d r o x i d e m i n e r a l s , p a r t i c u l a r l y h e m a t i t e and g o e t h i t e . J a n o t and G i b e r t (1970) and J a n o t

M. (1973)

have shown t h a t t h e degree o f aluminium s u b s t i t u t i o n

has a measurable e f f e c t on t h e s i z e o f t h e magnetic f i e l d i n aluminous h e m a t i t e s and g o e t h i t e s . T h i s theme has been f u r t h e r extended by Golden

Gal. (1979), who

developed e q u a t i o n ( 1 2 ) f o r t h e r e l a t i o n s h i p between t h e computed magnetic f i e l d ( H ) , aluminium s u b s t i t u t i o n ( A A l ) , and s u r f a c e area (SA) f o r g o e t h i t e a t 77K

-

H ( T ) = 49.8 - o.136(:oA1) O.Oll(SA) (12) Although t h i s e x p r e s s i o n appears t o h o l d w e l l f o r many samples o f g o e t h i t e i t should be used w i t h c a u t i o n f o r t h e f o l l o w i n g reasons: ( a ) The r e l a t i o n s h i p i s e m p i r i c a l and i s n o t , t h e r e f o r e , v a l i d f o r samples w i t h c h a r a c t e r i s t i c s o u t s i d e t h e range o f t h o s e used i n i t s d e r i v a t i o n ; ( b ) As shown by Goodman and Lewis (1981), many s p e c t r a o f samples o f h i g h s u r f a c e a r e a s and moderate aluminium s u b s t i t u t i o n show s i g n s o f r e l a x a t i o n a t 77K. Peaks a r e , t h e r e f o r e , n o t o f L o r e n t z i a n shape and such an assumption l e a d s t o v a l u e s o f t h e magnetic f i e l d s f r o m computer f i t s t h a t a r e a p p r e c i a b l y l o w e r t h a n t h e t r u e f i e l d s . I n such c i r c u m s t a n c e s s p e c t r a s h o u l d be r e c o r d e d a t l o w e r temperatures (e.g.

t h e b o i l i n g p o i n t o f l i q u i d helium, 4.2K)

i n o r d e r t h a t e f f e c t s o f isomorphous s u b s t i t u t i o n on t h e magnitude of t h e i n t e r n a l magnetic f i e l d may be more f u l l y understood. (iii)

I d e n t i f i c a t i o n of s i t e s containing octahedral i r o n i n s i l i c a t e s . This

area o f work has been w i d e l y developed o v e r t h e p a s t 15

-

20 y e a r s . The b a s i c

p h i l o s o p h y has been t h a t i f t h e c o o r d i n a t i n g groups o r degree o f d i s t o r t i o n f r o m c u b i c symnetry vapy f o r d i f f e r e n t c r y s t a l l o g r a p h i c s i t e s , t h e n t h e e l e c t r i c f i e l d g r a d i e n t s a t those s i t e s w i l l a l s o v a r y . G e n e r a l l y - a c c e p t e d assignments o f compone n t s i n t h e Mossbauer s p e c t r a t o t h e c r y s t a l l o g r a p h i c s i t e s have been d e r i v e d t h r o u g h t h e s t u d y o f many samples o f each m i n e r a l s p e c i e s . T y p i c a l examples o f work on some l a y e r and c h a i n s i l i c a t e s i l l u s t r a t e t h e r e s u l t s t h a t can be o b t a i n e d . From t h e b a s i c s t r u c t u r e f o r a 2 : l l a y e r s i l i c a t e (Fig.5.6),

i t can be seen t h a t

two d i f f e r e n t t y p e s o f c o o r d i n a t i o n e x i s t a t t h e o c t a h e d r a l s i t e s , one s i t e h a v i n g

cis c o o r d i n a t i o n and b e i n g t w i c e as abundant as t h e o t h e r s i t e groups a r e i n a trans arrangement. F o r bond angles o f 90’ i t can

two OH groups i n where t h e OH

e a s i l y be shown, u s i n g t h e e x p r e s s i o n s f o r t h e components o f t h e e l e c t r i c f i e l d g r a d i e n t t e n s o r g i v e n i n s e c t i o n 5.2.2, t h a t t h e magnitude of,q5,tt f o r the trans s i t e . Thus, where two f e r r i c components with s i t e i s twice t h a t f o r the isomer s h i f t s c h a r a c t e r i s t i c o f o c t a h e d r a l c o o r d i n a t i o n have been observed i n t h e Mo’ssbauer s p e c t r a o f l a y e r s i l i c a t e s , t h e one w i t h t h e l a r g e r quadrupole s p l i t t i n g

130

Fig.5.6

The b a s i c s t r u c t u r e o f 2 : l l a y e r s i l i c a t e s .

has been assigned t o t h e

trans s i t e ,

(Rozenson and H e l l e r - K a l l a i ,

-e t a l . , 1976

and Fig.5.5),

commonly l a b e l l e d M1. Examples f o r g l a u c o n i t e

1978; McConchie

Gal., 1979), n o n t r o n i t e

m o n t m o r i l l o n i t e (Rozenson a n d - H e l l e r - K a l l a i ,

(Goodman 1977) and

b i o t i t e (Annersten, 1974; B a n c r o f t and Brown, 1975) i l l u s t r a t e t h i s approach. Assignments f o r Fez+ i o n s a r e u s u a l l y based on t h e arguments t h a t t h e c o n t r i b u t i o n t o t h e e l e c t r i c f i e l d g r a d i e n t from t h e from qval;

(Ilatt

term i s o f o p p o s i t e s i g n t o t h a t

thus, t h e component w i t h t h e s m a l l e r v a l u e o f A i s assigned t o t h e M1

s i t e . Experimental j u s t i f i c a t i o n f o r t h i s assignment has been g i v e n by Haggstrom

e t al.

(1969) who showed t h a t w i t h i r o n - r i c h b i o t i t e t h e amount o f i r o n correspond-

i n g t o t h e area o f t h e component w i t h t h e l a r g e r A was t o o g r e a t f o r i t t o be accomnodated i n t h e M1 s i t e s . A t y p i c a l spectrum o f a b i o t i t e i s shown i n F i g . 5.7, along w i t h the assignments o f the Fez+ s i t e s . A s i m i l a r approach has been adopted w i t h t h e c h a i n s i l i c a t e s . F o r pyroxenes,

t h e two f e r r o u s components observed i n t h e Mossbauer s p e c t r a can be assigned t o t h e two d i s t i n c t c r y s t a l l o g r a p h i c s i t e s on t h e b a s i s o f t h e degree o f d i s t o r t i o n from c u b i c symmetry (see e.g.

V i r g o and Hafner, 1969), and f o r amphiboles, components

can be observed (Goldman, 1979) t h a t can be assigned t o t h e f o u r types of octahedral s i t e i n t h e s t r u c t u r e . However, f o r both pyroxenes.and amphiboles t h e r e have been suggestions t h a t next-nearest-neighbour c a t i o n s may have some e f f e c t

131

5.2

< (0

0

In

4-

C

3

0

0

5 .l

-2 F i g . 5.7

0 Velocity/mm

2 s-1

The Mossbauer spectrum o f a b i o t i t e a t ambient temperature.

on t h e quadrupole s p l i t t i n g s and t h a t t h i s may l i m i t t h e accuracy w i t h which t h e s i t e d i s t r i b u t i o n s can be determined (Dowty and L i n d s l e y , 1973; Goldman 1979). T h i s problem o f n e x t - n e a r e s t - n e i g h b o u r e f f e c t s w i l l be d e a l t w i t h more f u l l y when t h e o r e t i c a l a s p e c t s a r e c o n s i d e r e d a t t h e end o f t h i s s e c t i o n . (iv)

The s t u d y o f m i n e r a l a1 t e r a t i o n r e a c t i o n s .

Because Mossbauer spectroscopy

has t h e a b i l i t y t o i d e n t i f y t h e c r y s t a l l o g r a p h i c s i t e s t h a t c o n t a i n i r o n i n m i n e r a l s as w e l l as t h e o x i d a t i o n s t a t e s o f t h a t i r o n , i t has c o n s i d e r a b l e p o t e n t i a l i n t h e s t u d y o f m i n e r a l r e a c t i o n s . Examples o f n a t u r a l w e a t h e r i n g i n c l u d e t h e s t u d y o f b i o t i t e (Yassoglou

e., 1972;

Goodman and Wilson, 1973) and hornblende

(Goodman and Wilson, 1976). Also, because o f t h e e f f e c t s o f temperature on t h e d i s t r i b u t i o n and o x i d a t i o n s t a t e s o f i r o n i n d i f f e r e n t m i n e r a l s p e c i e s , t h e r e have been s u g g e s t i o n s t h a t Mossbauer s p e c t r o s c o p y m i g h t be used as a geothermometer. Examples h e r e i n c l u d e t h e s t u d y o f pyroxenes ( V i r g o and Hafner, 1969) and cummingt o n i t e (Ghose and Weidner, 1972). The e f f e c t s o f p r e s s u r e have a l s o been s t u d i e d , (e.g.

Burns

Gal., 1972). R e v e r s i b l e

chemical r e a c t i o n s t h a t can t a k e p l a c e

w i t h i n m i n e r a l s t r u c t u r e s r e p r e s e n t an a r e a o f r e s e a r c h where Mossbauer s p e c t r o s copy can make an e x t r e m e l y v a l u a b l e c o n t r i b u t i o n . Reactions can b e f r o z e n a t i n t e r m e d i a t e stages, b y r a p i d c o o l i n g t o t h e t e m p e r a t u r e o f l i q u i d n i t r o g e n , a t which t h e Mossbauer s p e c t r a a r e o b t a i n e d . The s t u d y o f t h e r e d u c t i o n o f s m e c t i t e s b y v a r i o u s r e d u c i n g agents g i v e s examples o f t h e c h a r a c t e r i z a t i o n o f a i r - s e n s i t i v e

132 s p e c i e s formed d u r i n g redox r e a c t i o n s (Rozenson and H e l l e r - K a l l a i , Gal., 1979).

1976 a and b;

Russell

Thermal a l t e r a t i o n s o f m i n e r a l s i n a i r have been e x t e n s i v e l y s t u d i e d b o t h f r o m t h e academic p o i n t o f v i e w o f c h a r a c t e r i z i n g d e c o m p o s i t i o n r e a c t i o n s (examples i n c l u d e t h e s t u d y o f c h l o r i t e b y Goodman and Bain, 1978; and b i o t i t e , p h l o g o p i t e and v e r m i c u l i t e by T r i p a t h i

c,, 1978), and i n t h e a p p l i e d sense f o r

identifying

t h e t r e a t m e n t s t o which samples o f a r c h a e o l o g i c a l i n t e r e s t had been s u b j e c t e d . T h i s l a t t e r a r e a o f work has been r e v i e w e d b y K o s t i k a s

e. (1976) and has been

shown t o be s u c c e s s f u l i n t h e c h a r a c t e r i z a t i o n o f a n c i e n t p o t t e r y o f known provenance and s t y l e , and i n t h e i d e n t i f i c a t i o n o f changes i n manufacuuring t e c h n o l o g y . (v)

Theoretical considerations.

I n t h e d i s c u s s i o n above on t h e i d e n t i f i c a t i o n

o f i r o n - c o n t a i n i n g s i t e s i n s i l i c a t e m i n e r a l s t h e r e a r e a number o f weaknesses i n t h e arguments t h a t have been p r e s e n t e d t o s u p p o r t t h e i n t e r p r e t a t i o n s o f t h e s p e c t r a . F i r s t l y , most s p e c t r a a r e o b t a i n e d a t room temperature. There a r e a few examples

o f s p e c t r a a t 77K, b u t r e s u l t s f r o m l o w e r t e m p e r a t u r e s a r e e x c e e d i n g l y r a r e . T h i s i s an i m p o r t a n t o m i s s i o n s i n c e , as d i s c u s s e d i n S e c t i o n 5.2.2, o f f e r r o u s quadrupole s p l i t t i n g s t h e qval

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

t e r m i n room t e m p e r a t u r e s p e c t r a may be

c o m p l i c a t e d by t h e p o p u l a t i o n o f e x c i t e d e l e c t r o n i c s t a t e s . Such an e f f e c t can be e s t a b l i s h e d i f t h e quadrupole s p l i t t i n g shows a s i z e a b l e t e m p e r a t u r e dependence, e.g.

i n a b i o t i t e i n c r e a s e s i n A f r o m 2.61 t o 2.91 mm s - l f o r t h e o u t e r d o u b l e t

and f r o m 2.26 t o 2.59 mm s - l f o r t h e i n n e r d o u b l e t have been observed (Goodman, u n p u b l i s h e d r e s u l t s ) . T h e r e f o r e , i n o r d e r t o draw any c o n c l u s i o n s a b o u t t h e n a t u r e o f t h e qlatt

term, l o w t e m p e r a t u r e s p e c t r a must be c o n s i d e r e d . With f e r r i c i r o n

t h e r e i s n o t t h e same d i f f i c u l t y w i t h p o p u l a t i o n o f e x c i t e d s t a t e s a t ambient temperatures and t h e quadrupole s p l i t t i n g i s d e t e r m i n e d by a c o m b i n a t i o n o f qlatt and t h e qval

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

o r b i t a l s . I n many cases (e.g. Fig.5.7)

t h e f e r r i c components a r e l e s s c l e a r l y r e s o l v e d

t h a n t h e .ferrous a t room temperature, f u r t h e r i l l u s t r a t i n g t h e i m p o r t a n c e o f t h e e x c i t e d e l e c t r o n i c s t a t e s i n d e t e r m i n i n g t h e s i z e o f t h e l a t t e r components.

I n any d i s c u s s i o n r e l a t i n g t h e s i z e o f f e r r i c quadrupole s p l i t t i n g s t o t h e c r y s t a l s t r u c t u r e i t i s necessary t o be s u r e t h a t t h e i r o n i s i n f a c t i n t h e m i n e r a l . T h i s may be o f p a r t i c u l a r i m p o r t a n c e i n t h e s t u d y o f specimens o f l o w i r o n c o n t e n t , where a c o m p a r a t i v e l y m i n o r amount o f an i r o n - r i c h i m p u r i t y phase c o u l d make a s i g n i f i c a n t c o n t r i b u t i o n t o t h e Mossbauer spectrum. I t has been suggested b y Goodman (1978a) on t h e b a s i s o f EPR work, t h a t some m o n t m o r i l l o n i t e s m i g h t have a c o n s i d e r a b l e percentage o f t h e i r i r o n i n a phase adsorbed on t h e i r s u r f a c e s . Even a f t e r s a t i s f y i n g t h e problems mentioned above, t h e r e a r e c e r t a i n d i f f i c u l t i e s i n j u s t i f y i n g t h e assignment o f components i n t h e Mossbauer s p e c t r a t o s p e c i f i c c r y s t a l l o g r a p h i c s i t e s . E x p e r i m e n t a l l y t h i s s i t u a t i o n has been most e v i d e n t i n t h e s t u d y o f pyroxenes. Dowty and L i n d s l e y (1973) r e p o r t e d , i n t h e i r work on c a l c i c pyroxenes, anomalies i n t h e r e l a t i v e areas a s s i g n e d t o t h e two s i t e s f r o m

133 two-component f i t s t o s p e c t r a compared t o r e s u l t s f r o m XRD. However, b y assuming t h a t each M1 s i t e was c h a r a c t e r i z e d b y f o u r components c o r r e s p o n d i n g t o f o u r d i f f e r e n t arrangements o f Fe and Ca i n t h e t h r e e n e a r e s t neighbour M2 p o s i t i o n s , i t was p o s s i b l e t o o b t a i n computer f i t s t h a t gave r e l a t i v e s i t e p o p u l a t i o n s

c o n s i s t e n t w i t h t h e XRD r e s u l t s . A l d r i d g e

u.

(1978) a l s o found i t necessary

t o use a s i m i l a r model t o i n t e r p r e t t h e s p e c t r a o f omphacite c l i n o p y r o x e n e s . The e s t a b l i s h m e n t o f t h e e f f e c t s o f n e x t - n e a r e s t - n e i g h b o u r i o n s on t h e quadrupole s p l i t t i n g i n one group o f s i l i c a t e s r a i s e s t h e q u e s t i o n o f t h e i r importance i n other minerals. I f a c r y s t a l s t r u c t u r e i s

a c c u r a t e l y known and i f a n e t charge

can be a s s i g n e d t o each atom w i t h i n t h e s t r u c t u r e t h e n i t i s a s i m p l e c o m p u t a t i o n a l problem t o use e q u a t i o n s ( 5 ) t o e s t i m a t e t h e qlatt

values f o r any p a r t i c u l a r s i t e

i n t h e s t r u c t u r e . I t i s a l s o a s i m p l e m a t t e r t o examine t h e e f f e c t s o f v a r i a t i o n s i n t h e charges on n e i g h b o u r i n g c a t i o n s i t e s on t h e ‘Ilattv a l u e s . I n c a l c u l a t i o n s

on b i o t i t e Mineeva (1978) has shown t h a t t h e range o f v a l u e s f o r A f o r each c r y s t a l l o g r a p h i c s i t e i s g r e a t e r t h a n t h e d i f f e r e n c e s between t h e s i t e s . Also, Goodman (1976) showed t h a t f o r f e r r i a n n i t e an e l e c t r i c f i e l d g r a d i e n t d i s t r i b u t i o n c o u l d be g e n e r a t e d by a Monte C a r l o method f o r each c r y s t a l l o g r a p h i c s i t e , and t h a t f o r t h i s m i n e r a l each r e s u l t i n g envelope c o u l d b e f i t t e d f a i r l y w e l l by 2 components, t h e r e l a t i v e i n t e n s i t i e s depending on t h e r a t i o o f d i v a 1 e n t : t r i v a l e n t ions:vacancies.

By a p p l y i n g t h e s e r e s u l t s t o t h e a n a l y s i s o f t h e spectrum o f b i o t i t e

shown i n F i g . 5.7 Goodman (1976) o b t a i n e d t h e computer f i t shown i n F i g . 5.8 which produced v e r y d i f f e r e n t c o n c l u s i o n s c o n c e r n i n g t h e r e l a t i v e p r e f e r e n c e o f Fe

2t

f o r t h e two s i t e s i n t h e s t r u c t u r e . A l s o i n t h e spectrum o f n o n t r o n i t e ( F i g . 5.5) i t has been suggested (Goodman 1978b) t h a t t h e two o c t a h e d r a l components do n o t

n e c e s s a r i l y a r i s e f r o m two d i f f e r e n t t y p e s o f c r y s t a l l o g r a p h i c s i t e , and t h a t t h e y c o u l d a t l e a s t i n p a r t be accounted f o r by t h e d i s t r i b u t i o n o f n e x t - n e a r e s t neighbour i o n s . I n some i n s t a n c e s i t m i g h t be p o s s i b l e t o r e s o l v e t h i s assignment problem b y d e t e r m i n i n g t h e s i g n o f i t w i l l be opposite f o r

Vzz f o r each component, s i n c e i n an i d e a l case

cis and trans c o o r d i n a t i o n s .

The s i g n i s most e a s i l y

measured e i t h e r by u s i n g a s i n g l e c r y s t a l o r b y t h e a p p l i c a t i o n o f a l a r g e magnetic f i e l d (see e.g.

C o l l i n s and T r a v i s , 1967). However, u n t i l a g r e a t e r u n d e r s t a n d i n g

o f t h e importance o f a l l o f t h e f a c t o r s a f f e c t i n g e l e c t r i c f i e l d g r a d i e n t s has been achieved, t h e use o f Mossbauer s p e c t r o s c o p y f o r t h e assignment o f i o n s t o c r y s t a l l o g r a p h i c s i t e s w i l l n o t be e n t i r e l y unambiguous.

4.

CONCLUSIONS Mossbauer s p e c t r o s c o p y can be used t o d e t e r m i n e t h e o x i d a t i o n s t a t e s o f i r o n i n

m i n e r a l s and t o i d e n t i f y t h e presence o f some m i n e r a l s p e c i e s i n samples o f unknown c o m p o s i t i o n , p a r t i c u l a r l y when t h e s e s p e c i e s e x h i b i t magnetic o r d e r i n g . However, s m a l l p a r t i c l e s i z e s and isomorphous s u b s t i t u t i o n s can d r a s t i c a l l y a l t e r t h e Mossbauer s p e c t r a a t a p a r t i c u l a r temperature. I n q u a n t i t a t i v e a n a l y s i s o n l y t h e

134

Fe3+ 1-

-1Ml

12% M1,MZ 43%

c C

a

0

0

5.1

I

F i q . 5.8

I

-2

0 Velocity/

mm 8

2

1

The Mijssbauer spectrum of t h e b i o t i t e i n F i q . 5.7 f i t t e d t o a model t h a t

assumes two doublets f o r Fez+ i n each c r y s t a l l o g r a p h i c s i t e ( f r o m Goodman 1976). d i s t r i b u t i o n o f i r o n can bedeterminedand even then t h i s can o n l y be determined a c c u r a t e l y when s p e c t r a a r e obtained over a range o f temperatures. With pure s i l i c a t e minerals, i t i s p o s s i b l e t o d i s t i n g u i s h i r o n - c o n t a i n i n g s i t e s according t o t h e i r c o o r d i n a t i o n numbers and t h e i r e l e c t r i c f i e l d g r a d i e n t s . The l a t t e r parameter i s determined by t h e c r y s t a l l a t t i c e and n o t j u s t by t h e i n e d i a t e environment o f t h e i r o n so t h a t each c r y s t a l l o g r a p h i c s i t e may have a range of quadrupole s p l i t t i n g s , t h e r e l a t i v e i n t e n s i t y o f which depends upon t h e d i s t r i b u t i o n o f next-nearest-neighbour

ions.

REFERENCES A l d r i d g e , L.P., Bancroft, G.M. , F l e e t , M.E. and Herzberg, C.T., 1978. Omphacite s t u d i e s , 11. Mossbauer s p e c t r a o f C2/c and P2/n omphacites. Am. Mineral., 63: 1107-1 115. Annersten, H. , 1974. Mgssbauer s t u d i e s o f n a t u r a l b i o t i t e s . Am. Mineral 59: 143-1 51 Bahgat, A.A., 1979. A new method f o r q u a n t i t a t i v e a n a l y s i s o f t h e Mossbauer e f f e c t Phys. Status S o l i d i ( A ) , 52:K217-220. Bancroft, G.M. , 1973. Mossbauer Spectroscopy: an i n t r o d u c t i o n f o r i n o r g a n i c chemists and geochemists. McGraw-Hill, London. Bancroft, G.M. , 1979. Mossbauer Spectroscopic s t u d i e s o f t h e chemical s t a t e of i r o n i n s i l i c a t e minerals. J. Phys. ( P a r i s ) C o l l o q . C2, 40: 464-471.

.

.,

135 Bancroft, G.M. and Brown, J.R., 975. A Mossbauer s t u d y o f c o e x i s t i n g hornblendes 60: 265-272. and b i o t i t e s : q u a n t i t a t i v e FeAt/FeZt r a t i o s . Am. Mineral B e l o z e r s k i i , G.N. , Kazakov, M.I., Gagarina, E . I . and Khantulev, A.A. , 1978. Use o f Mossbauer spectroscopy f o r s t u d y i n g t h e forms o f i r o n i n f o r e s t s o i l s . S o v i e t S o i l Science, pp. 534-545 Brown, F.F. and P r i t c h a r d , A.M., 1969. The Mossbauer spectrum o f i r o n o r t h o c l a s e . E a r t h Planet. S c i . L e t t s . , 5: 259-260. Burns, R.G., Huggins, F.E. and Drickamer, H.G., 1972. A p p l i c a t i o n s o f h i g h pressure Mossbauer spectroscopy t o mantle mineralogy. 24th I G C Section 14, pp 113-123. Carlson, L. and Schwertmann, U. , 1980. N a t u r a l occurrence o f f e r o x y h i t e (6'-FeOOH). Clays Clay M i n e r a l s , 28: 272-280. Goodman, B.A. and Churchman, G.J., 1978. A p p l i c a t i o n o f Mossbauer Childs, C.W., spectroscopy t o t h e s t u d y o f i r o n oxides i n some r e d and yellow/brown s o i l samples from New Zealand. Proc. I n t e r . Clay Conf. 1978 (Pub. 1979): 555-565. Coey, J.M.0'. , 1975. Clay m i n e r a l s and t h e i r t r a n s f o r m a t i o n s s t u d i e d w i t h n u c l e a r techniques: t h e c o n t r i b u t i o n o f Mijssbauer spectroscopy. I n : 1 s t conference on Mossbauer Spectroscopy, Cracow, Poland. C o l l i n s , R.L., 1978. Q u a n t i t a t i v e Mossbauer a n a l y s i s . Phys. L e t t s . , 66A: 153-154. C o l l i n s , R.L., 1979. Q u a n t i t a t i v e Mossbauer spectroscopy. J . Phys. ( P a r i s ) Colloq. C2, 40: 36-38. C o l l i n s , R.L. and T r a v i s , J.C,., 1967. The e l e c t r i c f i e l d g r a d i e n t tensor. I n : I.J. Gruverman ( E d i t o r ) , Mossbauer E f f e c t Methodology. Plenum, New York, 3: 123-161. Dowty, E. and L i n d s l e y , D.H., 1973. Mossbauer s p e c t r a o f s y n t h e t i c hedenbergitef e r r o s i l i t e pyroxenes. Am. M i n e r a l , 58: 850-868. Ericson, T. and Wappling, R., 1976. Texture e f f e c t s i n 3/2-1/2 Mossbauer spectra. J . Phys. ( P a r i s ) Colloq. C6, 37: 739-722; o r d e r - d i s o r d e r i n cummingtonite, Ghose, S. and Weidner, J.R., 1972. Mg '-Fe (Ms,F~)S ~ i g 022(DH)2: a new geothermometer. E a r t h Planet. Sci. L e t t s . , 16: 346-354. Makarov, E.F,. and Khrapov, V.V., 1963. D i f f e r e n c e i n two peaks Gol'danskii, V.I., o f quadrupole s p l i t t i n g i n Mossbauer spectra. Phys. L e t t s . , 3: 344-346. Golden, D.C., Bowen, L.H., Weed, S.B. and Bigham, J.M. , 1979. kfossbauer s t u d i e s o f s y n t h e t i c and s o i l - o c c u r r i n g a l u m i n i u m - s u b s t i t u t e d g o e t h i t e s . S o i l Sci. SOC. Amer. J . , 43: 802-808. Goldman, D.S., 1979. A r e e v a l u a t i o n o f t h e Mossbauer spectroscopy o f c a l c i c amphiboles. Am. Mineral., 64: 109-118. Goodman, B.A., 1976. The e f f e c t o f l a t t i c e s u b s t i t u t i o n s on t h e d e r i v a t i o n o f q u a n t i t a t i v e s i t e p o p u l a t i o n s f r o m t h e Mossbauer s p e c t r a o f 2 : l l a y e r l a t t i c e s i l i c a t e s . J . Phys. ( P a r i s ) Colloq. C6, 37: 819-823. Goodman, B.A., 1978a. An i n v e s t i g a t i o n by Mdssbauer and EPR spectroscopy o f t h e p o s s i b l e presence o f i r o n - r i c h i m p u r i t y phases i n some m o n t m o r i l l o n i t e s . Clay M i n e r a l s , 13: 351-356. Goodman, B.A., 1978b. The Mossbauer s p e c t r a o f n o n t r o n i t e s : c o n s i d e r a t i o n o f an a l t e r n a t i v e assignment. Clays Clay M i n e r a l s , 26: 176-177. Goodman, B.A., 1980. Mossbauer Spectroscopy. I n : J.W. S t u c k i and W.L. Banwart ( E d i t o r s ) , Advanced Chemical Methods f o r S o i l and Clay M i n e r a l s Research, 0. Reidel, Dordrecht, pp 1-92. Goodman, B.A. and Bain, D.C., 1978. Mossbauer s p e c t r a o f c h l o r i t e s and t h e i r decomposition products. Proc. I n t e r . Clay Conf. 1978 (Pub. 1979): 65-74. Goodman, B.A. and Lewis, D.G., 1981. Mossbauer s p e c t r a o f aluminous g o e t h i t e s (a-FeOOH). J . S o i l Sci., i n press. Fraser, A.R. and Woodhams, F.W.D., 1976. A Mossbauer Goodman, B.A., R u s s e l l , J.D., and i n f r a r e d spectroscopic study o f t h e s t r u c t u r e o f n o n t r o n i t e . Clays Clay M i n e r a l s , 24: 53-59. Goodman, B.A. and I l i l s o n , M.J., 1973. A s t u d y o f t h e weathering o f a b i o t i t e u s i n g t h e Mossbauer e f f e c t . Mineral.. Mag., 39: 448-454. Goodman, B.A. and Wilson, M.J., 1976. A Mossbauer s t u d y o f t h e weathering o f hornblende. Clay Minerals, 11: 153-163. Graham, M.J. and Cohen, M., 1976. A n a l y s i s o f i r o n c o r r o s i o n products u s i n g Mossbauer spectroscopy. Corrosion, 32: 432-438.

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136 Haggstrom, L., Wappling, R. and Annersten, H., 1969. Mossbauer s t u d y o f i r o n - r i c h b i o t i t e s . Chem. Phys. L e t t s . , 4: 107-108. H e l l e r - K a l l a i , L., 1980. The use o f Mossbauer spectroscopy o f i r o n i n c l a y mineralogy. Presented a t 4 t h Meeting, European Clay Groups, F r e i s i n g , FRG, t o be published. Janot, C. and G i b e r t , H., 1970. Les c o n s t i t u a n t s du f e r dans c e r t a i n e s b a u x i t e s n a t u r e l l e s e t u d i e e s p a r e f f e t Mossbauer. B u l l . SOC. fr. M i n g r a l . C r i s t a l l o g r . , 93: 213-223. Janot, C., G i b e r t , H., De Gramont, X. and B i a i s , R., 1971. Etude des s u b s t i t u t i o n s A1-Fe dans des roches l a t 6 r i t i q u e s . B u l l . SOC. fr. Min6ral C r i s t a l l o g r . , 94: 36 7 3 80. Janot, C., G i b e r t , H. and Tobias,..C., 1973. C h a r a c t e r i s a t i o n de k a o l i n i t e s f e r r i f e r e s p a r s p e c t r o m e t r i e Mossbauer. B u l l . SOC. fr. M i n e r a l . C r i s t a l l o g r . , 96: 281-289. Keisch,..B., 1976. Analysis o f works o f a r t . I n : R.L. Cohen ( E d i t o r ) , A p p l i c a t i o n s o f Mossbauer Spectroscopy, Academic Press, New York, 1 : 263-286. Kostikas, A., Simopoulos, A. and Gangas, N.H. , 1976. A n a l y s i s o f a r c h a e o l o g i c a l a r t i f i c a t s . 1n:R.L. Cohen ( E d i t o r ) , A p p l i c a t i o n s o f Mossbauer Spectroscopy, Academic Press, New York, 1 : 241-261. Longworth, G., Becker, L.W., Thompson, R., O l d f i e l d , F., Dearing, J.A. and Rummery, T.A., 1979. M'ossbauer e f f e c t and magnetic s t u d i e s o f secondary i r o n oxides i n s o i l s . J. S o i l Sci., 30: 93-110. McConchie, D.M., Ward, J.B., McCann, V.H. and Lewis, D.W., 1979. A Mossbauer i n v e s t i g a t i o n o f g l a u c o n i t e and i t s g e o l o g i c a l s i g n i f i c a n c e . Clays Clay Minerals, 27: 339-348. Mineeva, R.M., 1978. R e l a t i o n s h i p between Mossbauer s p e c t r a and d e f e c t s t r u c t u r e i n b i o t i t e s from e l e c t r i c f i e l d g r a d i e n t c a l c u l a t i o n s . Phys. Chem. M i n e r a l s , 2: 267-277. Murad, E . and Schwertmann, U., 1980. The Mossbauer spectrum o f f e r r i h y d r i t e and i t s r e l a t i o n s h i p t o those o f o t h e r i r o n oxides. Am. Mineral., 65: 1044-1049. Petrera, M., Gonser, U., Hasmann, U., Keune, W. and Lauer, J., 1976. Are monol a y e r s d e t e c t a b l e by conversion e l e c t r o n Mossbauer-spectroscopy (CEMS)? J. Phys. ( P a r i s ) Colloq. C6, 37: 295-296. Richardson, S.M., 1975. A p i n k muscovite w i t h r e v e r s e pleochroism from Archers Post, Kenya. Am. M i n e r a l . 60: 73-78. Rozenson, I . and H e l l e r - K a l l a i , L., 1976a. Reduction and o x i d a t i o n o f Fe3+ i n d i o c t a h e d r a l s m e c t i t e s - 1: Reduction w i t h hydrazine and d i t h i o n i t e . Clays Clay Minerals, 24: 271-282. Rozenson. I . and H e l l e r - K a l l a i , - L., - 1976b. Reduction and o x i d a t i o n o f Fe3+ i n dioctahedral smectites 2: Reduction w i t h sodium s u l p h i d e s o l u t i o n s . Clays Clay Minerals, 24: 283-288. Rozenson, I. and H e l l e r - K a l l a i , L. , 1977. Mossbauer s p e c t r a o f d i o c t a h e d r a l smectites. Clays Clay M i n e r a l s , 25: 94-101. Rozenson, I.and H e l l e r - K a l l a i , L., 1978. tdossbauer s p e c t r a o f g l a u c o n i t e s r e examined. Clays Clay M i n e r a l s , 26: 173-175. Russell, J.D., Goodman, B.A. and Fraser, A.R., 1979. I n f r a r e d and Mossbauer s t u d i e s o f reduced n o n t r o n i t e s . Clays Clay M i n e r a l s , 27: 63-71. 1978. The l o c a t i o n and Sanz, J., Meyers, J., Vielvoye, L. and Stone, W.E.E., c o n t e n t o f i r o n i n n a t u r a l b i o t i t e s and ph1ogopites:a comparison o f several methods. Clay M i n e r a l s , 13: 45-52. Shenoy, G.K., F r e i d t , J.M., M a l e t t a , H. and Ruby, S.L., 1975. Curve f i t t i n g and t h e t r a n s m i s s i o n i n t e g r a l : warnings and suggestions. In: I.J. Gruverman ( E d i t o r ) , Mossbauer E f f e c t Methodology, Plenum, New York, 9: 277-305. Sternheimer, R.M., 1963. S h i e l d i n g and a n t i s h i e l d i n g e f f e c t s f o r v a r i o u s i o n s and atomic systems. Phys. Rev., 146: 140-160. Stevens, J.G., Stevens, V.E. and Gettys, W.L. Mossbauer E f f e c t Reference and Data Journal, Mossbauer E f f e c t Data Center, Univ. N o r t h Carolina, U.S.A. Taylor, G.L., RusQala, A.P. and Keeling, R.O., 1968. Analysis o f i r o n i n l a y e r s i l i c a t e s by Mossbauer Spectroscopy. Clays Clay M i n e r a l s , 16: 381-391. T r i c k e r , M.J., 1977. Iron-57 conversion e l e c t r o n Mossbauer !Pectroscopy. Surf. Defect Prop. S o l i d s , 6: 106-138.

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137 T r i p a t h i , R.P., Chandra, V., Chandra, R. and Lokanathan, S., 1978. A Mossbauer s t u d y o f t h e e f f e c t s o f h e a t i n g b i o t i t e , p h l o g o p i t e and v e r m i c u l i t e . J . I n o r g . Nucl. Chem., 40: 1293-1298. V i r g o , D. and Hafner, S.S., 1969. Fez+, Mg o r d e r - d i s o r d e r i n heated orthopyroxenes. M i n e r a l . SOC. Amer.,.Spec. Pap., 2: 67-81. Wickman, H.H., 1966. Mossbauer paramagnetic h y p e r f i n e s t r u c t u r e . I n : I . J . Gruverman ( E d i t o r ) , Mossbauer E f f e c t Methodology, Plenum, New York, 2: 39-66. Yassoglou, N.J., N o b e l i , C., K o s t i k a s , A.J. and Simopoulos, A.C. .. 1972. Weathering o f m i c a f l a k e s i n two s o i l s i n N o r t h e r n Greece e v a l u a t e d b y Mossbauer and c o n v e n t i o n a l t e c h n i q u e s . S o i l S c i . SOC. Amer. Proc., 36: 520-527.