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
