Semiquantitative Determination of Coal Minerals by X-ray Diffractometry

4

Downloaded by NANYANG TECHNOLOGICAL UNIV on November 19, 2015 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch004

Semiquantitative Determination of Coal Minerals by X-ray Diffractometry John J. Renton Department of Geology, West Virginia University, Morgantown, WV 26506

The most commonly employed analytical procedure for the quantification of minerals in coal is x-ray diffraction analysis of the low temperature ash. The magnitudes of the Bragg intensities used to evaluate mineral abundances are affected by inherent compositional and/or structural variations characteristic of a number of the common minerals found in coal and by non-random crystallite orientation within the sample mount. The subsequent variations in Bragg intensities cause the Bragg intensity-mineral abundance relationship to depart from linearity and result in errors of determination of mineral abundances of ±10 percent or more. The procedure must therefore be considered only semiquantitative. The purpose o f t h i s paper i s t o p r e s e n t and s u p p o r t t h e argument t h a t abundance e s t i m a t e s o f t h e m i n e r a l s i n c o a l based upon x - r a y d i f f r a c t i o n (XRD) d a t a c a n o n l y be c o n s i d e r e d s e m i - q u a n t i t a t i v e w i t h e x p e c t e d e r r o r s o f d e t e r m i n a t i o n o f 10 p e r c e n t o r more o f t h e r e p o r t e d v a l u e s . The c o m p o s i t i o n a l and p h y s i c a l c h a r a c t e r i s t i c s o f t h e low temperature ash components o f c o a l r e l a t i v e t o t h e p r e p a r a t i o n and mounting o f ash f o r XRD a n a l y s i s a l s o a f f e c t t h e p r e c i s i o n o f analyses. Minerals i n Coal The m i n e r a l s commonly found i n c o a l a r e l i s t e d i n T a b l e I . I n t h e average c o a l , c l a y m i n e r a l s may c o n s t i t u t e up t o 60 weight p e r c e n t o f the m i n e r a l m a t t e r ( 1 - 2 ) . Q u a r t z i s u s u a l l y t h e second most abundant m i n e r a l , w i t h up t o 20 weight p e r c e n t b e i n g common. The c a r b o n a t e m i n e r a l s ( c a l c i t e , s i d e r i t e and t o a l e s s e r e x t e n t , d o l o m i t e and a n k e r i t e ) and t h e i r o n d i s u l p h i d e m i n e r a l s ( p y r i t e and m a r c a s i t e ) make up, on t h e average, up t o 10 weight p e r c e n t o f each group. S u l p h a t e m i n e r a l s o f c a l c i u m and i r o n and t h e f e l d s p a r m i n e r a l s a r e commonly p r e s e n t b u t r a r e l y i n c o n c e n t r a t i o n s o f more t h a n a few weight p e r c e n t . E x c e p t f o r u n u s u a l cases such as t h e 0097-6156/ 86/ 0301 -0053506.00/ 0 © 1986 American Chemical Society

In Mineral Matter and Ash in Coal; Vorres, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

54

MINERAL

MATTER

A N D A S H IN C O A L

Table I


COMMON COAL

Silicates

-| C l a y Minerals

Carbonates

Disulfides

Sulfates

Feldspars

a

MINERALS

"Kaolinite 1 Illite Mixed Layer . Chlorite Quartz

AI Si 0 (OH)4 a b (MgFeAI) (SiAI) O (OH)

"Calcite Dolomite Ankerite _ Siderite

CaC0 (Ca, M g ) ( C 0 ) Ca(Fe, M g ) C 0 FeC0

3

Γ Pyrite

FeS FeS

(cubic) (orthorhombic)

2

H

2

5

6

Si0

4

10

2

3

H

3

L Marcasite

2

2

2

3

Fe (S0 ) -9H FeS0 -H O CaS0 -2H 0 CaS0 -1/2H 0

Coquimbite Szomolnokite Gypsum Bassanite Anhydrite Jarosite

2

4

3

4

2

a

4

2

4

2

CaS0 KFe (S0 ) (OH) 4

3

4

2

e

(NaCa)AI(AISi)Si 0

J~Plagioclase 1 Orthoclase

2

KAISi O 3

8

e

Illite has a composition similar to muscovite - K A I ( S i A I ) O ( O H ) , except for less K+and more S i 0 and H 0 . 2

2

3

10

2

2

b Mixed layered clays are usually randomly interstratified mixtures of illitic lattices with montmorillonitic and/or chloritic lattices.


8


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s u l p h i d e r i c h c o a l s o f the n o r t h e r n I l l i n o i s B a s i n , the o c c u r r e n c e of the o t h e r m i n e r a l s i n c o n c e n t r a t i o n s e x c e e d i n g a few p e r c e n t i s rare. I t must, however, be kept i n mind t h a t t h e m i n e r a l o g y o f the i n o r g a n i c p o r t i o n o f c o a l shows s y s t e m a t i c v a r i a t i o n b o t h geographi c a l l y and l o c a l l y r e f l e c t i n g the g e o c h e m i s t r y o f the o r i g i n a l peat f o r m i n g environment ( 3 ) . As a r e s u l t , "average" v a l u e s o f c o n c e n t r a t i o n may have l i t t l e p r a c t i c a l meaning. Most c o a l s c o n s i d e r e d f o r c o n v e r s i o n p r o c e s s e s such as l i q u i f a c t i o n , and t h e r e b y t h o s e o f prime i n t e r e s t to c h e m i s t s , a r e g e n e r a l l y h i g h i n ash (>10 weight p e r c e n t ) and s u l f u r (>1 weight percent). I n such c o a l s , i l l i t e would i n v a r i a b l y be the dominant c l a y m a t e r i a l , c o n s t i t u t i n g , i n some c o a l s , up to h a l f o r more o f the m i n e r a l c o n t e n t . Most o f the s u l f u r c o n t a i n e d i n t h e s e c o a l s w i l l be i n the form o f p y r i t e a l t h o u g h m a r c a s i t e may be l o c a l l y dominant ( 4 ) . Q u a n t i f i c a t i o n by X-ray

Diffraction

The most commonly employed q u a n t i t a t i v e XRD p r o c e d u r e used t o e v a l u a t e the c o n c e n t r a t i o n o f m i n e r a l components i n a multicomponent m i x t u r e o f m i n e r a l s compares the Bragg i n t e n s i t y d a t a o f unknowns t o those g e n e r a t e d from a s u i t e o f known s t a n d a r d samples. Mineral specimens a r e a c q u i r e d t o r e p r e s e n t each o f the m i n e r a l s expected i n the unknowns. The specimens a r e ground t o a u n i f o r m l y s m a l l s i z e ( l e s s t h a n 44 m i c r o n s ) and mixed t o g e t h e r i n c o n c e n t r a t i o n s which r e p r e s e n t t h e range o f c o n c e n t r a t i o n s expected f o r each m i n e r a l . An i n t e r n a l s t a n d a r d such as c a l c i u m f l u o r i d e , aluminum o x i d e o r powdered aluminum i s u s u a l l y added i n o r d e r to m o n i t o r and c o r r e c t f o r v a r i a t i o n s i n sample a b s o r p t i o n and i n s t r u m e n t a l v a r i a b l e s . Working c u r v e s a r e t h e n p r e p a r e d by p l o t t i n g the r a t i o o f i n t e n s i t y ( p r e f e r a b l y i n t e g r a t e d i n t e n s i t y ) o f t h e Bragg r e f l e c t i o n chosen to q u a n t i f y the m i n e r a l to t h a t chosen f o r the s t a n d a r d v e r s u s the weight p e r c e n t o f the m i n e r a l i n the s t a n d a r d samples. T h i s procedure w i l l provide analyses of h i g h p r e c i s i o n provided c e r t a i n b a s i c assumptions a r e met: ( 1 ) the c o m p o s i t i o n and c r y s t a l l i n i t y (degree o f o r d e r i n g ) o f the i n d i v i d u a l m i n e r a l s i n t h e unknowns a r e both REASONABLY c o n s t a n t from sample to sample and ( 2 ) the c o m p o s i t i o n and c r y s t a l l i n i t y o f t h e s t a n d a r d m i n e r a l s chosen f o r the p r e p a r a t i o n o f the s t a n d a r d samples REASONABLY d u p l i c a t e the c o m p o s i t i o n and c r y s t a l l i n i t y o f the r e s p e c t i v e m i n e r a l s i n the unknowns. The purpose o f the f o l l o w i n g d i s c u s s i o n i s to demonstrate t h a t , i n the case o f c o a l m i n e r a l s , n e i t h e r o f the above assumptions i s v a l i d and as a r e s u l t , any such q u a n t i t a t i v e p r o c e d u r e w i l l r e f l e c t the i n h e r e n t degree o f d e p a r t u r e from t h e s e b a s i c assumptions and w i l l t h e r e f o r e be s e m i - q u a n t i t a t i v e . Other p r o c e d u r e s u s i n g d a t a n o r m a l i z e d t o the t o t a l i n t e g r a t e d i n t e n s i t y and q u a n t i f i c a t i o n p r o c e d u r e s u t i l i z i n g w e i g h t i n g f a c t o r s based upon s t a n d a r d c h e m i c a l formulae f o r the m i n e r a l s can be used but w i t h no improvement i n quantitative errors ( 5 - 6 ) . I l l i t e and p y r i t e were s p e c i f i c a l l y c i t e d i n the above d i s c u s s i o n t o make a p o i n t r e l a t i v e t o t h e p r e c i s i o n and a c c u r a c y w i t h which c o a l m i n e r a l s can be q u a n t i f i e d by XRD. First, i l l i t e is NOT a m i n e r a l . I l l i t e i s " . . . a g e n e r a l term f o r the c l a y m i n e r a l c o n s t i t u e n t s o f a r g i l l a c e o u s sediments b e l o n g i n g to the m i c a group"



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M I N E R A L M A T T E R A N D A S H IN C O A L

(7). To a c l a y m i n e r a l o g i s t , the term i l l i t e i s synonomous w i t h v a r i a b i l i t y i n b o t h c o m p o s i t i o n and c r y s t a l l i n i t y ( 8 ) . The s i t u a t i o n i s even f u r t h e r c o m p l i c a t e d by t h e f a c t t h a t much o f the m a t e r i a l i n c o a l r e f e r r e d t o as " i l l i t e " i s a c t u a l l y an i l l i t e dominated mixed l a y e r e d c l a y w h e r e i n the i l l i t e l a t t i c e s a r e randomly i n t e r s t r a t i f i e d w i t h 14Â c l a y l a t t i c e s ; u s u a l l y c h l o r i t e . This mixing of clay m i n e r a l l a t t i c e s f u r t h e r adds to the i n h e r e n t v a r i a b i l i t y i n both c o m p o s i t i o n and c r y s t a l l i n i t y o f the i l l i t i c m a t e r i a l . The c o n s t i t u t i o n o f " i l l i t e " can t h e r e f o r e be expected t o v a r y s i g n i f i c a n t l y from sample t o sample. I t s h o u l d be q u i t e apparent from t h e above d i s c u s s i o n t h a t no " s t a n d a r d " i l l i t e e x i s t s t h a t c o u l d be used to r e p r e s e n t i l l i t e i n s t a n d a r d samples. The i r o n d i s u l p h i d e s may r e p r e s e n t 10 weight p e r c e n t o r more o f h i g h a s h - h i g h s u l f u r c o a l ashes. U s u a l l y , p y r i t e i s the major d i sulphide. P y r i t e o c c u r s i n c o a l i n a number o f m o r p h o l o g i c a l forms and s i z e s ( 9 ) . Not o n l y does the p y r i t e i n c o a l v a r y i n morphology and s i z e but a l s o i n s t o i c h i o m e t r y and c r y s t a l l i n i t y . S t u d i e s have been conducted i n the a u t h o r ' s l a b o r a t o r y on cut and p o l i s h e d s u r f a c e s o f c o a l b l o c k s w h e r e i n the b l o c k s have been exposed t o the atmosphere and the p y r i t e s o b s e r v e d o v e r a p e r i o d o f t i m e . Some p y r i t e g r a i n s , the e u h e d r a l forms ( t h o s e t h a t show d e f i n i t e c r y s t a l facesj, remain b r i g h t and show l i t t l e tendency to r e a c t . The m a s s i v e forms o f p y r i t e , on t h e o t h e r hand, show a wide v a r i a t i o n i n apparent r e a c t i v i t y , w i t h c r y s t a l s o f i r o n s u l f a t e s b e i n g observed to form on some p y r i t e s u r f a c e s w i t h i n a m a t t e r o f h o u r s and i n some c a s e s , w i t h i n minutes. Another study i n v o l v e d the q u a n t i f i c a t i o n of p y r i t e i n d i f f e r e n t coal lithotypes. C o a l s a r e d e s c r i b e d m e g a s c o p i c a l l y based upon the degree o f b r i g h t and d u l l b a n d i n g . Zones a r e d e l i n e a t e d w i t h i n the c o a l and d e s i g n a t e d as a " l i t h o t y p e " based on the r e l a t i v e p e r c e n t a g e of b r i g h t and d u l l bands w i t h i n t h e zone ( 1 0 ) . D o m i n a n t l y b r i g h t bands a r e c a l l e d "VITRAIN", d u l l bands, "DURAIN" and t h o s e i n t e r m e d i a t e between the two; "CLARAIN". A l t h o u g h the d e s i g n a t i o n as t o l i t h o t y p e i s s o l e l y made depending upon megascopic d e s c r i p t i o n , the l i t h o t y p e s d i f f e r i n b a s i c m a c e r a l c o m p o s i t i o n as i l l u s t r a t e d by some d a t a f o r the Waynesburg C o a l shown i n T a b l e I I . TABLE I I .

Lithotype Vitrain Clarain Durain

Maceral Composition

%Vitrinite 93.1 84.2 43.4

o f L i t h o t y p e s o f the Waynesburg C o a l

%Exinite 1.8 4.4 17.8

%Inertinite 2.4 5.6 24.9

%Mineral 2.7 5.8 13.9

Matter

The low temperature ash o f each l i t h o t y p e was s u b m i t t e d t o XRD analysis. The i n t e g r a t e d i n t e n s i t i e s o f each o f t h e s e l e c t e d a n a l y t i c a l Bragg r e f l e c t i o n s s e l e c t e d f o r the i n d i v i d u a l m i n e r a l s were summed f o r a l l m i n e r a l s p r e s e n t i n each sample t o g i v e a "total integrated intensity". T h i s v a l u e was then d i v i d e d i n t o the i n t e g r a t e d i n t e n s i t y o f the p y r i t e a n a l y t i c a l Bragg r e f l e c t i o n t o g i v e the " p e r c e n t o f t o t a l i n t e g r a t e d i n t e n s i t y " . The d a t a a r e summarized i n F i g u r e 1. I t i s apparent t h a t t h e r e i s a s y s t e m a t i c r e l a t i o n s h i p between the c o m p o s i t i o n / c r y s t a l l i n i t y o f the p y r i t e and the b a s i c o r g a n i c makeup o f t h e c o a l . Most i m p o r t a n t i s the


4.

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Semiquantitative

Determination

of Coal

57

Minerals


30

t ι ι I I

*l

/ / /

/

/ /

4 I I

/

A

Vitrain

*

Clarain

•

Durain

10

12

Percent Pyrite I Actual) F i g u r e 1. R e l a t i o n s h i p between a c t u a l p e r c e n t p y r i t e and p e r c e n t o f t o t a l i n t e g r a t e d i n t e n s i t y f o r P y r i t e Bragg R e f l e c t i o n f o r v a r i o u s coal lithotypes.


58

MINERAL

MATTER

A N D A S H IN

COAL


o b s e r v a t i o n that equal c o n c e n t r a t i o n s of p y r i t e g i v e d i f f e r e n t i n t e n s i t y responses. Volume f o r volume, the p y r i t e c o n t a i n e d w i t h i n the b r i g h t c o a l ( v i t r a i n ) showing s i g n i f i c a n t l y h i g h e r Bragg i n t e n s i t i e s t h a n the p y r i t e c o n t a i n e d i n the d u l l e r c o a l s . To compound the problem, m a r c a s i t e f o r r e a s o n s unknown to the author does not show the i n t e n s i t y r e s p o n s e , volume f o r volume,as does p y r i t e . I t has been t h e a u t h o r ' s e x p e r i e n c e t h a t c o a l s t h a t have been shown by o p t i c a l e x a m i n a t i o n t o c o n t a i n m a r c a s i t e i n s i g n i f i c a n t c o n c e n t r a t i o n s show almost no i n d i c a t i o n o f the m i n e r a l b e i n g p r e s e n t on a d i f f r a c t o g r a m g e n e r a t e d from the low t e m p e r a t u r e ash. I t must be apparent from the above d i s c u s s i o n t h a t a number o f v a r i a b l e s o t h e r t h a n c o n c e n t r a t i o n a f f e c t the i n t e n s i t i e s o f m i n e r a l p a t t e r n s as o b s e r v e d on a d i f f r a c t o g r a m . Inasmuch as they cannot be m o n i t o r e d and compensated f o r m a t h e m a t i c a l l y , these v a r i a t i o n s must be r e f l e c t e d i n the e r r o r o f d e t e r m i n a t i o n . T h i s would be t r u e r e g a r d l e s s o f the q u a n t i f i c a t i o n p r o c e d u r e employed. The c o n c l u s i o n , t h e r e f o r e , i s t h a t the i n h e r e n t v a r i a b i l i t y i n c o m p o s i t i o n and/or c r y s t a l l i n i t y t h a t e x i s t s w i t h i n the major m i n e r a l components o f the low temperature ashes o f c o a l w i l l be r e f l e c t e d i n the s t a t i s t i c a l e r r o r o f d e t e r m i n a t i o n and t h a t e r r o r w i l l be o f s u f f i c i e n t magnitude to p r e c l u d e the use o f the term " q u a n t i t a t i v e " to d e s c r i b e the p r o cedure. T h e r e f o r e , any p r o c e d u r e u s i n g x - r a y d i f f r a c t i o n to d e t e r mine the m i n e r a l s i n c o a l must be c o n s i d e r e d s e m i - q u a n t i t a t i v e at best. Sample P r e p a r a t i o n and

Mounting

Any p r o c e d u r e f o r the p r e p a r a t i o n and mounting o f c o a l low temperat u r e ashes f o r XRD a n a l y s i s MUST take two p r o p e r t i e s o f the m a t e r i a l i n t o a c c o u n t : (1) m i n e r a l s e x i s t which r e a c t w i t h w a t e r to p r o d u c e a c i d i c s o l u t i o n s (the i r o n d i s u l p h i d e s ) which i n t u r n d i s s o l v e a c i d s o l u b l e components such as c a l c i t e and (2) the c l a y m i n e r a l s , by v i r t u e o f e x c e p t i o n a l l y w e l l d e v e l o p e d (001) c l e a v a g e s u r f a c e s , have a dominant p l a t e y c r y s t a l form. The s i g n i f i c a n c e o f the f i r s t a t t r i b u t e i s t h a t t h e ashes cannot be p l a c e d i n w a t e r t h e r e b y p r e c l u d i n g c e r t a i n sample p r e p a r a t i o n t e c h n i q u e s such as d i s p e r s i o n i n water f o l l o w e d by vacuum mounting on f i l t e r s o r c e r a m i c b l o c k s . The second c h a r a c t e r i s t i c , p o s s e s s i o n o f a p l a t e y c r y s t a l form, p r e c l u d e s the a t t a i n m e n t o f the t h e o r e t i c a l l y r e q u i r e d randomly o r i e n t e d sample. Those who work w i t h the c l a y m i n e r a l s , r e a l i z i n g a random sample cannot be p r e p a r e d and t h a t the c l a y p a r t i c l e s w i l l d e p o s i t i n p r e f e r r e d o r i e n t a t i o n , p u r p o s e l y p r e p a r e and mount the samples such t h a t the p r e f e r r r e d o r i e n t a t i o n o f the i n d i v i d u a l p l a t e l e t s i s maximized and t h e r e b y m i n i m i z e any v a r i a t i o n s i n d i f f r a c t i o n i n t e n s i t y due t o v a r i a t i o n s i n p a r t i c l e a l i g n m e n t w i t h i n t h e sample. The o r i e n t a t i o n o f the c l a y p l a t e l e t s p a r a l l e l t o t h e sample s u r f a c e p o s i t i o n s the "C" c r y s t a l l o g r a p h i c a x i s p e r p e n d i c u l a r to the sample surface. Because the d i a g n o s t i c i n t e r p l a n a r s p a c i n g f o r the c l a y m i n e r a l s i s a l o n g the "C" c r y s t a l l o g r a p h i c d i r e c t i o n , such an o r i e n t a t i o n i s ideal for clay mineral i d e n t i f i c a t i o n . The s i m p l e s t method t o mount a low t e m p e r a t u r e ash f o r XRD a n a l y s i s i s to p r e s s the ash onto the s u r f a c e o f a p e l l e t p r e p a r e d from t h e c o a l from which the ash was d e r i v e d .


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Future Prospects A few y e a r s ago, an ad h o c group o f workers i n t e r e s t e d i n c o a l m i n e r a l s , The M i n e r a l M a t t e r i n C o a l Group, p r e p a r e d and d i s t r i b u t e d a r o u n d - r o b i n low temperature a s h t o t e n l a b o r a t o r i e s . Each l a b o r a t o r y was t o p r e p a r e , mount and q u a n t i f y t h e m i n e r a l components i n t h e ash by t h e i r r e s p e c t i v e XRD t e c h n i q u e s . The d a t a were t h e n compared. Even though a wide v a r i e t y o f t e c h n i q u e s was used f o r each phase o f the a n a l y s i s , w i t h t h e e x c e p t i o n o f t h e c l a y m i n e r a l e s t i m a t e s made by one l a b o r a t o r y ( s i g n i f i c a n t l y lower t h a n t h e o t h e r s ) and t h e p y r i t e e s t i m a t e made by a n o t h e r (too h i g h ) , t h e d a t a compared reasonably w e l l . The averages o f a l l t h e s u b m i t t e d e s t i m a t e s a r e summarized i n T a b l e I I I . TABLE I I I .

Mineral Illite+Mixed Kaolinite Quartz Calcite Pyrite

Layer

R e s u l t s o f Round-Robin L.T.A. A n a l y s i s Ave. Cone. WT% 30 18 21 10 18

STD Dev. 7.07 4.85 6.31 3.59 4.93

Coeff. of Variation 0.24 0.27 0.30 0.36 0.27

Another o b j e c t i v e o f t h e e x e r c i s e was t o d i s c u s s t h e r e s u l t s and p r o c e d u r e s used and come t o some agreement o n a " s t a n d a r d " p r o c e d u r e f o r sample p r e p a r a t i o n , mounting and q u a n t i f i c a t i o n t h a t would be acceptable t o a l l the workers. The agreement t h a t was reached was t h a t no agreement would be f o r t h c o m i n g on any o f t h e phases o f t h e analysis. With no one p r o c e d u r e demonstrably b e t t e r t h a n t h e o t h e r , each l a b o r a t o r y was expected t o m a i n t a i n t h e i r own p r o c e d u r e . As l o n g as a p r o c e d u r e i s s c i e n t i f i c a l l y and a n a l y t i c a l l y sound and r e f l e c t s a thorough u n d e r s t a n d i n g o f t h e c h a r a c t e r i s t i c s o f m i n e r a l s c o n t a i n e d i n c o a l and t h e r e q u i r e m e n t s and l i m i t a t i o n s o f x - r a y d i f f r a c t i o n , one p r o c e d u r e w i l l p r o b a b l y be as good as a n o t h e r b u t none w i l l be b e t t e r t h a n s e m i - q u a n t i t a t i v e . With a l l i t s s h o r t c o m i n g s , x - r a y d i f f r a c t i o n i s s t i l l t h e b e s t and most p r a c t i c a l method f o r t h e e s t i m a t i o n o f t h e abundance o f t h e individual minerals i n coal.

Literature Cited 1. Renton, J.J., 1982, in "Coal Structure", R. Meyers ed., Academic Press, p. 283-326. 2. Renton, J.J., 1978, Energy Sources, Vol. 43, No. 2, p. 91-112. 3. Renton, J.J., Cecil, C.B., 1979, in "Carboniferous Coal Guidebook", Donaldson, A . C . , Presley, M.W., Renton, J.J., eds., West Virginia Geological and Economic Survey, Morgantown, WV, p. 103-128. 4. King, H.M., 1978, Masters Thesis, Dept. of Geology, West Virginia University. 5. Renton, J.J., 1977, U.S. Dept. of Energy, MERC/CR-77/10, 20pp. 6. Renton, J.J., 1979, U.S. Dept. of Energy, MERC/CR-79/5, 22pp. 7. Grim, R . E . , et a l , 1937, Amer. Min., Vol. 22, pp. 813-829.


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8. Brown, G., 1961, Mineralogical Soc. of London, Clay Mineral Group, London, 544pp. 9. Grady, W.C., 1977, AIME Transactions, Vol. 262, p. 268-274. 10. Stopes, M.C., 1919, Proc. Roy Soc. Β., 90, p. 470-487, London.


R E C E I V E D August 1, 1985


Semiquantitative Determination of Coal Minerals by X-ray Diffractometry

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