Mössbauer Spectroscopy and Its Chemical Applications - American

Mossbauer spectroscopy is a gamma ray of 14.4-keV energy to which ... (1,4,5). The Mossbauer parameters for most of the iron-sulfur minerals and ... t...
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10 Qualitative and Quantitative Analysis of

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Iron-Bearing Minerals in Fossil Fuels and Petroleum Source Rock by Iron-57 Mössbauer Spectroscopy

R. E .

1

KARL

and J. J. Z U C K E R M A N

Department of Chemistry, University of Oklahoma, Norman, O K 73019

The

Mössbauer

techniques described here have important

advantages over current methods for analyzing coal and petroleum source rock.

During the first stage of analysis,

the iron-containing minerals are identified by comparing the line positions with values for pure mineral samples. In the second stage, absorption intensities are used to deter­ mine relative amounts of mineral, and in the final step, absolute amounts of the mineral iron in the sample are found.

In oil shales,

four iron-containing minerals are

predominant: pyrite, dolomite, siderite, and illite.

The

results are related to those from x-ray diffraction analysis where the correspondence is not always complete.

n y spectroscopic m e t h o d is i n t r i n s i c a l l y c a p a b l e of y i e l d i n g o n l y t h r e e types o f i n f o r m a t i o n : ( 1 ) i d e n t i t y — o n t h e basis t h a t i d e n t i c a l sys­ tems b e h a v e i n a n i d e n t i c a l w a y ; ( 2 ) p u r i t y , o r t h e c o m p o s i t i o n o f a p h y s i c a l m i x t u r e — o n t h e basis t h a t p h y s i c a l m i x i n g does n o t alter spectro­ scopic properties; a n d ( 3 ) information concerning structure, bonding, a n d the forces b e t w e e n m o l e c u l e s .

I t is a strange c o m m e n t a r y t h a t t h e a p p l i ­

c a t i o n of M o s s b a u e r spectroscopy

t o t h e first t w o of t h i s list o f t h r e e

p o s s i b i l i t i e s has c o m e r a t h e r l a t e , f o r these a n a l y t i c a l a p p l i c a t i o n s w o u l d 1

Current address: Degussa Hanau, Postfach 1351, 6540 Hanau 1, West Germany.

©

0065-2393/81 /0194-0221$06.25/0 1981 American Chemical Society

In Mössbauer Spectroscopy and Its Chemical Applications; Stevens, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

222

MOSSBAUER

SPECTROSCOPY A N D

ITS

CHEMICAL

APPLICATIONS

seem to be the most s i m p l e a n d s t r a i g h t f o r w a r d , b u t interest i n t h e systematics of c h e m i c a l a n d m i n e r a l o g i c a l analysis b y M o s s b a u e r spectros­ c o p y is d e v e l o p i n g o n l y n o w .

Mossbauer Spectroscopy T h e M o s s b a u e r effect is i d e a l l y s u i t e d , a m o n g spectroscopic

tech­

n i q u e s , f o r analysis because of the h i g h e n e r g y of its e x c i t i n g l i n e , w h i c h is expressed i n w a v e n u m b e r s ( c m ) i n T a b l e I , f o r easy c o m p a r i s o n .

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- 1

T h e p e n e t r a t i n g n a t u r e of the g a m m a r a y s makes M o s s b a u e r spectroscopy effective for situations i n w h i c h the m a t e r i a l s t o b e a n a l y z e d are i n a d i l u t e m a t r i x , s u c h as r o c k or c o a l , w h i c h w i l l be l a r g e l y t r a n s p a r e n t to t h e e x c i t i n g g a m m a r a y . T h e s e a n d other advantages of u s i n g t h e M o s s ­ b a u e r effect to a n a l y z e i r o n m i n e r a l s are l i s t e d i n T a b l e I I . T h e M o s s b a u e r effect w a s d i s c o v e r e d i n 1957, a n d its p r i n c i p l e s h a v e b e e n d e s c r i b e d i n a n u m b e r of excellent texts ( 1 , 2 , 3 ) , i n c l u d i n g its m o r e recent a p p l i c a t i o n s to g e o l o g y a n d m i n e r a l o g y ( 1 , 4 , 5 ) . (6)

and magnetic

(7)

methods,

x - r a y p o w d e r d i f f r a c t i o n (9-12),

scanning electron

Thermal

microscopy

a n d i n f r a r e d spectroscopy

(13)

(8), have

b e e n a p p l i e d to the d e t e c t i o n a n d analysis of the m i n e r a l c o m p o n e n t s

of

c o a l , b u t the M o s s b a u e r m e t h o d has s o m e u n i q u e advantages n o t f o u n d i n other t e c h n i q u e s . H o w e v e r , t h e r e are also f o r m i d a b l e t e c h n i c a l p r o b ­ lems that w i l l n e e d to b e o v e r c o m e . U s i n g the M o s s b a u e r t e c h n i q u e f o r analysis, n o p r e c o n c e n t r a t i o n o r c h e m i c a l t r e a t m e n t steps are necessary, w i t h t h e i r a t t e n d a n t p o s s i b l e a l t e r a t i o n of c h e m i c a l state or e n v i r o n m e n t . M o s s b a u e r spectroscopy

T h e exciting line i n

5 7

Fe

is a g a m m a r a y of 1 4 . 4 - k e V e n e r g y to w h i c h

Table I.

Spectroscopic Energies Typical Energy of Radiation (cm' )

Spectroscopy X - r a y : i n n e r electronic states P h o t o e l e c t r o n : electronic states

1

^ /

V i s i b l e - u l t r a v i o l e t : v a l e n c e electronic states I n f r a r e d - R a m a n : v i b r a t i o n a l states

10,000,000 10,000 1,000 10

M i c r o w a v e : r o t a t i o n a l states E l e c t r o n s p i n resonance: electron s p i n states i n a m a g ­ netic field

0.1

N u c l e a r m a g n e t i c resonance: n u c l e a r s p i n states i n a m a g n e t i c field

0.001

M o s s b a u e r : n u c l e a r states

100,000,000

In Mössbauer Spectroscopy and Its Chemical Applications; Stevens, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

10.

KARL AND Z U C K E R M A N

Table II.

Mossbauer

Fossil Fuels and Petroleum

Spectroscopy

223

Source Rock

in the Analysis of Iron Minerals

N o p r e c o n c e n t r a t i o n or c h e m i c a l t r e a t m e n t C a r b o n a c e o u s a n d r o c k m a t r i x t r a n s p a r e n t to the e x c i t i n g l i n e (14.4keV) gamma ray A p p l i c a b l e t o c r y s t a l l i n e or a m o r p h o u s solids M e t h o d sensitive t o t i n y amounts of i r o n N o isotopic e n r i c h m e n t H i g h specific a c t i v i t y F e M o s s b a u e r sources a v a i l a b l e H i g h c o u n t rate spectrometers a v a i l a b l e S m a l l q u a n t i t i e s of s a m p l e M i n e r a l of greatest interest, p y r i t e , has a n intense a n d s i m p l e d o u b l e t spectrum

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5 7

t h e carbonaceous c o a l or c o a l process p r o d u c t m a t r i x w i l l b e t r a n s p a r e n t . T h e r e is no r e q u i r e m e n t for c r y s t a l l i n i t y , a n d i r o n i n a m o r p h o u s resonates as w e l l .

I n techniques

solids

b a s e d u p o n x - r a y d i f f r a c t i o n , o n the

other h a n d , crystallites of size < 200 n m g i v e diffuse l i n e s , a n d crystallites