Mineral Matter and Ash in Coal - American Chemical Society

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Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on December 17, 2017 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch003

Geologic Controls on the Inorganic Composition of Lower Kittanning Coal Susan M. Rimmer and Alan Davis 1

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Department of Geology, University of Kentucky, Lexington, Κ Y 40506 Department of Geosciences, The Pennsylvania State University, University Park, PA 16802

Lateral trends in mineral composition of the Lower Kittanning seam can be related to depositional environments. Syngenetic pyrite formation occurred in areas where the overlying shales indicate brackish conditions existed; marcasite formed e p i g e n e t i c a l l y . Quartz content increases towards a northern source area which is coincident with a basement high. K a o l i n i t e , the major clay component, is predominantly authigenic and increases towards the margins of the depositional basin where it was stable in the low pH, freshwater areas of the peat swamp. I l l i t e / m i c a is mostly d e t r i t a l (as indicated by the high-temperature polytype) and increases towards the center of the basin due to i t s greater s t a b i l i t y in marine -influenced environments. Low expandable c l a y contents are consistent with the level of metamorphism experienced by t h i s c o a l .

The m i n e r a l c o m p o s i t i o n o f c o a l i s t h e r e s u l t o f p h y s i c a l , c h e m i c a l and b i o l o g i c a l p r o c e s s e s a c t i n g on t h e s y s t e m from t h e t i m e o f p e a t a c c u m u l a t i o n , t h r o u g h b u r i a l and subsequent i n c r e a s e i n metamorphic l e v e l , t o the present. With respect t o o r i g i n , i n o r g a n i c c o n s t i t u e n t s may b e c l a s s i f i e d a s d e t r i t a l ( t h o s e t r a n s p o r t e d i n t o t h e p e a t swamp) o r a u t h i g e n i c ( t h o s e f o r m e d w i t h i n t h e e n v i r o n m e n t ) . M a c k o w s k y (I) f u r t h e r d i f f e r e n t i a t e s b e t w e e n s y n g e n e t i c m i n e r a l s , formed d u r i n g t h e a c c u m u l a t i o n o f p e a t , and e p i g e n e t i c m i n e r a l s , which formed l a t e r . In r e c e n t y e a r s , t h e q u e s t i o n has been r a i s e d a s t o w h e t h e r most of the s i l i c a t e m i n e r a l s i n coal a r e d e r i v e d from i n o r g a n i c substances o r i g i n a l l y contained w i t h i n peat-forming plants (2^4), o r f r o m s o u r c e s o u t s i d e t h e p e a t swamp ( 5 - 6 ) . S t u d i e s o f modern p e a t - f o r m i n g e n v i r o n m e n t s have e m p h a s i z e d t h e importance o f d e t r i t a l i n f l u x ( 6 - 7 ) , syngenetic formation o f p y r i t e (8) and b i o g e n i c s i l i c a ( Λ 9 ) , and i n - s i t u m i x i n g w i t h u n d e r l y i n g sediments (7,10) t o account f o r mineral c o n s t i t u e n t s i n peat. Within 0097-6156/ 86/ 0301 -0041 $06.00/0 © 1986 American Chemical Society

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


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t h e p e a t e n v i r o n m e n t u n s t a b l e d e t r i t a l c l a y s , such as s m e c t i t e and i l l i t e , may u n d e r g o a l t e r a t i o n o r d i s s o l u t i o n ( 6 , 1 1 ) , w h e r e a s k a o l i n i t e may f o r m a u t h i g e n i c a l l y ( 2 , 1 2 ) . In a d d i t i o n , b i o g e n i c s i l i c a d i s s o l v e s , p o s s i b l y c o n t r i b u t i n g to l a t e r a u t h i g e n i c c l a y formation (6,9); i n c o a l s , a u t h i g e n i c k a o l i n i t e i n c l e a t s i s common (13-14). C l a y s may a l s o f o r m b y a l t e r a t i o n o f v o l c a n i c a l l y - d e r i v e d m a t e r i a l w i t h i n t h e p e a t swamp ( 1 5 - 1 6 ) . W h e r e a s many a u t h o r s h a v e e m p h a s i z e d t h e g r e a t c o n t r i b u t i o n o f d e t r i t u s t o t h e m i n e r a l c o n t e n t o f some c o a l s , s u c h a s t h e W a y n e s b u r g ( 5 J , t h e e x i s t e n c e o f c o a l s w i t h l i t t l e o r no i d e n t i f i a b l e d e t r i t a l i n p u t , such as t h e I n d i a n a B l o c k c o a l (Γ7), has l e d o t h e r a u t h o r s ( 3 - 4 ) t o s u g g e s t t h a t t h e i n o r g a n i c p o r t i o n o f swamp p l a n t s i s a n important source of ash c o n s t i t u e n t s i n the coal ( i n c l u d i n g q u a r t z and c l a y s ) . It i s suggested that t h i s i s p a r t i c u l a r l y important in the case of low-ash, c o m m e r c i a l - q u a l i t y c o a l s ( 2 ) . A recent cathodoluminescence study i n c i c a t e s t h a t most of the q u a r t z t h a t i s i n t i m a t e l y mixed w i t h the o r g a n i c p o r t i o n of the coal ( i . e . not a s s o c i a t e d w i t h a t t r i t a l c o a l l a y e r s ) i s i n d e e d . a u t h i g e n i c , and p r o b a b a l y d e r i v e d from c o n s t i t u e n t s o r i g i n a l l y c o n t a i n e d i n the plants (18). It appears, therefore, that several o r i g i n s are possible f o r m i n e r a l s i n p e a t and c o a l , i n c l u d i n g d e t r i t a l i n f l u x , b i o g e n i c i n p u t , and p r e c i p i t a t i o n e i t h e r d u r i n g o r a f t e r p e a t a c c u m u l a t i o n , i n c l u d i n g some c o n t r i b u t i o n f r o m i n o r g a n i c s u b s t a n c e s d e r i v e d f r o m p l a n t s . V a r i o u s s t u d i e s have a t t e m p t e d t o r e l a t e i n o r g a n i c c o m p o s i t i o n t o c o n d i t i o n s t h a t e x i s t e d at the time of peat accumulation. P y r i t e has been a s s o c i a t e d w i t h m a r i n e and b r a c k i s h c o n d i t i o n s ( 8 , 1 9 ) , and t h e p y r i t e c o n t e n t o f c o a l has been r e l a t e d t o r o o f l i t h o l o g i e s w h i c h r e f l e c t those conditions (20-21). C l a y a s s e m b l a g e s i n c o a l s and u n d e r c l a y s a l s o have been r e l a t e d t o d e p o s i t i o n a l e n v i r o n m e n t (14,22). The p u r p o s e o f t h i s p a p e r i s t o d e s c r i b e v a r i a b i l i t y i n t h e i n o r g a n i c c o n t e n t o f t h e L o w e r K i t t a n n i n g c o a l seam i n w e s t e r n P e n n s y l v a n i a and a t t e m p t t o e x p l a i n t h e d i s t r i b u t i o n o f m i n e r a l s , t a k i n g i n t o c o n s i d e r a t i o n d e p o s i t i o n a l e n v i r o n m e n t and l e v e l o f metamorphism. The L o w e r K i t t a n n i n g p e a t a c c u m u l a t e d o n a d e l t a p l a i n ; a s t h e s e a t r a n s g r e s s e d f r o m t h e w e s t , t h e p e a t was l a t e r c o v e r e d by the Lower K i t t a n n i n g S h a l e . Fossil invertebrates in this s h a l e u n i t i n d i c a t e f r e s h w a t e r , b r a c k i s h and m a r i n e c o n d i t i o n s e x i s t e d across the study area ( 2 3 ) . S t r u c t u r a l features of the basin may h a v e s t r o n g l y i n f l u e n c e d s e d i m e n t a t i o n ( 2 4 ) . During accumulation o f L o w e r K i t t a n n i n g s e d i m e n t s a n d p e a t , t h e g r e a t e s t amount o f s u b s i d e n c e i s b e l i e v e d t o have o c c u r r e d i n t h e w e s t e r n and s o u t h w e s t e r n p a r t s o f w e s t e r n P e n n s y l v a n i a , w h e r e t h e b a s e m e n t i s now more d e e p l y b u r i e d , l e a v i n g t h e edges o f t h e b a s i n t o p o g r a p h i c a l l y higher (24). Bouguer g r a v i t y and s e d i m e n t t h i c k n e s s d a t a p r o v i d e support f o r t h i s increased subsidence. Basement h i g h s , a l s o i n d i c a t e d by g r a v i t y a n o m a l i e s , r e p r e s e n t a second c o n t r o l on sedimentation. Such a h i g h e x i s t s i n t h e n o r t h - c e n t r a l p a r t o f w e s t e r n P e n n s y l v a n i a a n d may h a v e s u p p l i e d d e t r i t a l m a t e r i a l t o t h e swamp, s u p p l e m e n t i n g a p r e d o m i n a n t l y e a s t e r n s e d i m e n t s o u r c e . A d d i t i o n a l c o n t r o l s on s e d i m e n t a t i o n a p p e a r t o be a c t i v e f o l d s a n d local v a r i a t i o n s in paleotopography (24).


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R I M M E R A N D DAVIS


S a m p l i n g and

Inorganic

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Methods

F o r t y - t h r e e channel samples of the P e n n s y l v a n i a n - a g e Lower K i t t a n n i n g c o a l ( K i t t a n n i n g F o r m a t i o n , A l l e g h e n y Group) were c o l l e c t e d i n western Pennsylvania. Samples a r e r e p r e s e n t a t i v e o f the t h r e e s u g g e s t e d d e p o s i t i o n a l e n v i r o n m e n t s ( F i g u r e 1) a n d a l s o o f t h e i n c r e a s e i n rank from high v o l a t i l e Β b i t u m i n o u s i n the west to low v o l a t i l e bituminous in the southeast. Chemical analyses i n c l u d e d m a j o r and m i n o r e l e m e n t s , t o t a l s u l f u r and s u l f u r f o r m s . L o w - t e m p e r a t u r e ashes ( L T A ' s ) were o b t a i n e d a c c o r d i n g t o p r o c e d u r e s d e s c r i b e d b y G l u s k o t e r (25). X-ray d i f f r a c t i o n a n a l y s i s of LTA's p r o v i d e d q u a l i t a t i v e , q u a n t i t a t i v e ( f o r q u a r t z and p y r i t e ) , and s e m i - q u a n t i t a t i v e ( f o r c l a y s in the l e s s than 2 micron f r a c t i o n ) data u s i n g p r o c e d u r e s m o d i f i e d f r o m R u s s e l l a n d Rimmer ( 2 6 ) . Kaolinite was q u a n t i f i e d u s i n g i n f r a - r e d s p e c t r o s c o p y . M i n e r a l c o m p o s i t i o n was a l s o c a l c u l a t e d b y n o r m a t i v e t e c h n i q u e s m o d i f i e d b y Rimmer (12J from methods used by e a r l i e r w o r k e r s (27-28). The s p a t i a l d i s t r i b u t i o n o f t h e s e m i n e r a l c o n s t i t u e n t s was a s s e s s e d b y c o n t o u r i n g and by t r e n d s u r f a c e a n a l y s i s . One p r o b l e m t h a t m u s t be a d d r e s s e d i n a s t u d y s u c h a s t h i s , i s w h e t h e r t h e d e n s i t y of data p o i n t s i s s u f f i c i e n t to j u s t i f y the d e s c r i p t i o n of trends across the b a s i n , or i s l o c a l v a r i a b i l i t y in the c o n s t i t u e n t s too g r e a t . A p p l i c a t i o n of trend surface a n a l y s i s permitted a s t a t i s t i c a l a s s e s s m e n t o f b a s i n a l t r e n d s , and i n d i c a t e d t h e e x t e n t o f local variation. I n t h i s p a p e r , c o n t o u r maps a r e p r e s e n t e d t o show important basinal trends; trend surface a n a l y s i s indicated that these t r e n d s were s t a t i s t i c a l l y s i g n i f i c a n t . D e t a i l s of the trend surface t e c h n i q u e a n d c o m p l e t e d a t a a r e d e s c r i b e d i n Rimmer (12). Results

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Discussion

The m a j o r m i n e r a l c o m p o n e n t s o f t h i s c o a l a r e q u a r t z , p y r i t e ( a n d m a r c a s i t e ) , and c l a y s ( p r e d o m i n a n t l y k a o l i n i t e and i l l i t e / m i c a , w i t h m i n o r amounts o f e x p a n d a b l e c l a y s ) . P y r i t e and M a r c a s i t e . High s u l f i d e c o n c e n t r a t i o n s are found towards the c e n t r a l p a r t o f the s t u d y a r e a , p a r t i c u l a r l y where the o v e r l y i n g s h a l e s i n d i c a t e b r a c k i s h c o n d i t i o n s e x i s t e d on t h e s o u t h e a s t e r n s i d e o f t h e b a s i n ( F i g u r e 2). This observation supports a s i m i l a r finding by Guber (29), r a t h e r than the c o n c l u s i o n of other workers t h a t higher p y r i t e contents are associated with marine rocks (20). F a c t o r s i n f l u e n c i n g t h e d i s t r i b u t i o n o f s u l f u r i n p e a t and c o a l s i n c l u d e a v a i l a b i l i t y o f i r o n and s u l f a t e , and pH. S u l f a t e i s thought t o be i n t r o d u c e d b y m a r i n e a n d b r a c k i s h w a t e r s ( 8 ) . R e c e n t w o r k on p y r i t e d i s t r i b u t i o n i n t h e F l o r i d a E v e r g l a d e s (30) indicates high p y r i t e contents are associated with b r a c k i s h c o n d i t i o n s r a t h e r than m a r i n e , and t h i s has been r e l a t e d t o t h e a v a i l a b i l i t y o f i r o n i n t h e d i f f e r e n t environments. In f r e s h w a t e r , i r o n i s t r a n s p o r t e d i n o r g a n i c c o l l o i d s w h i c h f l o c c u l a t e q u i c k l y upon e n t e r i n g b r a c k i s h water, r e s u l t i n g in a higher a v a i l a b i l i t y of i r o n in the b r a c k i s h e n v i r o n m e n t (31). I n a d d i t i o n , some i r o n may a l s o be t r a n s p o r t e d i n t o t h e swamp b y d e t r i t a l c l a y s w h i c h a l s o f l o c c u l a t e u p o n e n t e r i n g t h e swamp. A c i d i t y i s a l s o a f a c t o r , a s much o f t h e p y r i t e a p p e a r s t o f o r m as a b y - p r o d u c t o f s u l f a t e - r e d u c i n g b a c t e r i a (8). Compared




F i g u r e 1. D i s t r i b u t i o n o f Lower K i t t a n n i n g c o a l samples i n western Pennsylvania in r e l a t i o n to suggested environments of d e p o s i t i o n f o r the o v e r l y i n g s h a l e .

Figure basis,

2. P y r i t e c o n t e n t o f t h e L o w e r K i t t a n n i n g seam (%, as d e t e r m i n e d by x - r a y d i f f r a c t i o n a n a l y s i s ) .


LTA


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Composition

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Coal

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t o t h e more a c i d i c f r e s h w a t e r e n v i r o n m e n t s , h i g h e r l e v e l s o f m i c r o b i a l a c t i v i t y w o u l d o c c u r i n t h e n e u t r a l t o b a s i c pH c o n d i t i o n s associated with brackish or marine waters. In the case o f the Lower K i t t a n n i n g seam, t h e r e f o r e , whether i r o n was t r a n s p o r t e d i n o r g a n i c - r i c h c o l l o i d s o r was a s s o c i a t e d w i t h c l a y s , i r o n a v a i l a b i l i t y w o u l d t e n d t o be h i g h e r i n t h o s e a r e a s w h i c h r e c e i v e d an i n f l u x o f f r e s h w a t e r and d e t r i t u s . The b r a c k i s h z o n e i n t h e s o u t h e a s t e r n p a r t o f t h e b a s i n t h u s r e p r e s e n t s an o p t i m i z a t i o n o f i r o n a n d s u l f a t e a v a i l a b i l i t y a n d pH l e v e l o f t h e swamp w a t e r s . The d i s c u s s i o n t h u s f a r h a s a s s u m e d a s y n g e n e t i c o r i g i n f o r t h e pyrite. Observations of s u l f i d e morphology suggest t h a t a t l e a s t some o f t h e s u l f i d e may be e p i g e n e t i c . E d w a r d s a n d B a k e r ( 3 2 ) showed t h a t p y r i t e forms i n marine environments whereas m a r c a s i t e forms u n d e r more a c i d i c c o n d i t i o n s . R e c e n t e x p e r i m e n t a l w o r k h a s shown t h a t p y r i t e f o r m s a t a pH o f 5 . 0 , w h e r e a s m a r c a s i t e f o r m s a t a pH o f 3.5 ( 3 3 ) . The o c c u r r e n c e o f m a r c a s i t e i n w h a t a p p a r e n t l y a r e m a r i n e f a c i e s may t h e r e f o r e i n d i c a t e s u b s e q u e n t a c i d i f i c a t i o n . In t h e Lower K i t t a n n i n g seam, m a r c a s i t e o c c u r r e n c e does not a p p e a r t o be c l o s e l y r e l a t e d t o d e p o s i t i o n a l e n v i r o n m e n t . Coal s a m p l e s a s s o c i a t e d w i t h f r e s h w a t e r , b r a c k i s h and m a r i n e f a c i e s c o n t a i n b o t h p y r i t e and m a r c a s i t e . Marcasite is absent or only a m i n o r c o n s t i t u e n t i n l o w - s u l f i d e ( l e s s t h a n 20%, LTA b a s i s ) s a m p l e s . I f t h e s e c o a l s had f o r m e d i n an a c i d i c swamp e n v i r o n m e n t , a d o m i n a n c e o f m a r c a s i t e o v e r p y r i t e m i g h t have been e x p e c t e d . In f a c t , m a r c a s i t e i s more a b u n d a n t i n h i g h - s u l f i d e c o a l s . A possible e x p l a n a t i o n i s t h a t t h e m a r c a s i t e i s e p i g e n e t i c , and i t s p r e s e n c e i n d i c a t e s an i n c r e a s e i n t h e a c i d i t y o f t h e i n t e r s t i t i a l w a t e r s following burial. This i n t e r p r e t a t i o n of the e p i g e n e t i c nature of m a r c a s i t e i n t h e L o w e r K i t t a n n i n g seam i s s u p p o r t e d b y p é t r o g r a p h i e observations. For example, marcasite f r e q u e n t l y occurs in s u l f i d e m a s s e s , s u c h a s s p h e r u l e s w h i c h may r e p r e s e n t r e c r y s t a l 1 i z e d f r a m b o i d s , o r as c e l l i n f i l l i n g s . Quartz. I n t h i s s e a m , q u a r t z c o n t e n t t e n d s t o be r e l a t i v e l y l o w , w i t h many s a m p l e s c o n t a i n i n g l e s s t h a n 15% ( F i g u r e 3 ) . High q u a r t z c o n t e n t s (15-20% and a b o v e ) a r e o b s e r v e d i n t h e n o r t h - c e n t r a l p a r t o f t h e r e g i o n , i n C l a r i o n and J e f f e r s o n C o u n t i e s , and i n s e v e r a l i s o l a t e d a r e a s a l o n g t h e e a s t e r n edge o f t h e f i e l d a r e a , such as Centre County. Low q u a r t z c o n t e n t s o c c u r i n a z o n e e x t e n d i n g f r o m t h e s o u t h and s o u t h e a s t t o w a r d s t h e n o r t h w e s t , and i n C l e a r f i e l d County. The d i s t r i b u t i o n o f q u a r t z may p r o v i d e c l u e s a s t o t h e o r i g i n o f t h i s m i n e r a l i n the Lower K i t t a n n i n g seam. Q u a r t z may h a v e b e e n t r a n s p o r t e d i n t o t h e p e a t swamp b y w a t e r a n d w i n d ( d e t r i t a l ) , o r may h a v e p r e c i p i t a t e d w i t h i n t h e swamp, p o s s i b l y f r o m s i l i c a t h a t was o r i g i n a l l y c o n t a i n e d i n t h e swamp p l a n t s . Some o f t h e q u a r t z g r a i n s o b s e r v e d i n t h i s s t u d y were 30-50 m i c r o n s i n s i z e , and most g r a i n s were c o n c e n t r a t e d i n a t t r i t a l c o a l bands. Both of these o b s e r v a t i o n s s u g g e s t t h a t much o f t h e q u a r t z i n t h e L o w e r K i t t a n n i n g i s o f detrital origin. W i l l i a m s and B r a g o n i e r (24) s u g g e s t e d a basement h i g h , s i t u a t e d i n t h e n o r t h - c e n t r a l p a r t o f w e s t e r n P e n n s y l v a n i a , was a c o n t r o l o n sedimentation during the P e n n s y l v a n i a n . It is possible that q u a r t z - r i c h s e d i m e n t s were shed from t h i s t o p o g r a p h i c h i g h i n t o t h e


46



p e a t swamp. T r a n s p o r t a t i o n o f s e d i m e n t w i t h i n a swamp i s r e s t r i c t e d by t h e f i l t e r i n g e f f e c t o f the v e g e t a t i o n ( 7 ) . I t w o u l d be u n l i k e l y , t h e r e f o r e , t h a t d e t r i t a l q u a r t z w o u l d be d i s p e r s e d e v e n l y t h r o u g h o u t t h e swamp, b u t r a t h e r i t w o u l d c o n c e n t r a t e c l o s e t o t h e s o u r c e a r e a . The i r r e g u l a r d i s t r i b u t i o n o f q u a r t z a l o n g t h e e a s t e r n m a r g i n o f t h e basin again r e f l e c t s a d e t r i t a l i n f l u x . S e d i m e n t e n t e r i n g t h e swamp f r o m t h e e a s t was p r e d o m i n a n t l y c l a y , w i t h m i n o r i n f l u x e s o f q u a r t z occurring locally. Hoi b r o o k ( 3 4 ) o b s e r v e d t h a t q u a r t z c o n t e n t o f t h e Lower K i t t a n n i n g u n d e r c l a y was h i g h e s t d i r e c t l y o v e r c l a s t i c w e d g e s i n C l a r i o n a n d J e f f e r s o n C o u n t i e s a n d o v e r a n o t h e r wedge i n L a w r e n c e County. This coincides with areas of high quartz content in the Lower K i t t a n n i n g c o a l i t s e l f . Q u a r t z - r i c h sediment from the north may h a v e c o n t i n u e d t o be r e c e i v e d b y t h e s e a r e a s d u r i n g p e a t accumulation. A l t e r n a t i v e l y , mixing of the underlying sediments with t h e l o w e r p e a t l a y e r s , p e r h a p s a i d e d b y b i o t u r b a t i o n , may h a v e occurred; h o w e v e r , t h e v e r t i c a l d i s t r i b u t i o n o f q u a r t z i n t h e seam (12) does not s u p p o r t t h i s h y p o t h e s i s . Clay Minerals. C l a y m i n e r a l s may be t r a n s p o r t e d i n t o t h e swamp during peat accumulation ( d e t r i t a l ) , p r e c i p i t a t e from s o l u t i o n s r i c h i n a l u m i n u m , s i l i c o n and v a r i o u s c a t i o n s ( s y n g e n e t i c and e p i g e n e t i c ) , o r may f o r m b y t h e a l t e r a t i o n o f o t h e r m i n e r a l s e i t h e r w i t h i n t h e swamp o r d u r i n g b u r i a l ( t r a n s f o r m a t i o n a n d / o r d i a g e n e s i s ) ( 6 ) . The L o w e r K i t t a n n i n g c o a l c o n t a i n s k a o l i n i t e ( w e l l c r y s t a l l i z e d ) , i l l i t e / m i c a and e x p a n d a b l e c l a y s . Low t o t a l c l a y c o n t e n t s a r e o b s e r v e d i n the c e n t e r o f the s t u d y a r e a , where the c o a l i s r i c h i n p y r i t e , and i n t h e n o r t h - c e n t r a l r e g i o n where h i g h q u a r t z contents are observed. H i g h t o t a l c l a y c o n t e n t s a r e s e e n on t h e n o r t h w e s t e r n and e a s t e r n m a r g i n s o f t h e b a s i n . Because c l a y m i n e r a l data a r e p r e s e n t e d as a p e r c e n t a g e o f the l o w - t e m p e r a t u r e a s h , v a r i a t i o n s i n t h e q u a r t z and p y r i t e c o n t e n t s i n f l u e n c e t h e p r o p o r t i o n a t e d i s t r i b u t i o n of the c l a y s . To a v o i d t h i s p r o b l e m , t h e r e l a t i v e amounts o f i n d i v i d u a l c l a y m i n e r a l s i n t h e c l a y ( l e s s t h a n 2 m i c r o n ) f r a c t i o n were examined. As t h i s s i z e f r a c t i o n c o n t a i n s o n l y c l a y s , v a r i a t i o n s i n q u a r t z a n d p y r i t e do n o t i n f l u e n c e t h e r e s u l t s . K a o l i n i t e content i n c r e a s e s towards the margins of the b a s i n ( F i g u r e 4 ) , whereas i l l i t e and e x p a n d a b l e c l a y s i n c r e a s e t o w a r d s t h e center of the basin (not i l l u s t r a t e d ) . The d i s t r i b u t i o n o f k a o l i n i t e i n t h e L o w e r K i t t a n n i n g seam c l o s e l y r e s e m b l e s t h a t f o u n d i n t h e underclay. Holbrook (34) r e l a t e d h i g h k a o l i n i t e c o n t e n t o f the Lower K i t t a n n i n g u n d e r c l a y t o t o p o g r a p h i c h i g h s , where k a o l i n i t e e n r i c h m e n t r e s u l t e d from s u b a e r i a l l e a c h i n g . Thus, high k a o l i n i t e contents are f o u n d a l o n g t h e m a r g i n s o f t h e b a s i n and i n o t h e r a r e a s w h i c h exhibited positive relief. Comparison w i t h data presented i n H o l b r o o k ( 3 4 ) s u g g e s t s t h a t many o f t h e c o a l s a m p l e s w h i c h c o n t a i n more k a o l i n i t e o v e r l i e k a o l i n i t e - r i c h u n d e r c l a y s . This is p a r t i c u l a r l y t r u e i n t h e n o r t h , n o r t h w e s t and s o u t h e a s t . Mixing of p e a t w i t h t h e u n d e r l y i n g s e d i m e n t c o u l d have i n c r e a s e d t h e k a o l i n i t e content in these areas. Sharp c o n t a c t s were o b s e r v e d between c o a l a n d u n d e r c l a y , b u t e v i d e n c e o f s u c h i n t e r m i x i n g c o u l d be o b s c u r e d during compaction. In a s t u d y o f c l a y m i n e r a l v a r i a t i o n s i n u n d e r c l a y s i n the I l l i n o i s B a s i n , Parham ( 2 2 , 3 5 ) s u g g e s t e d t h a t d i f f e r e n t i a l


Inorganic

Composition

of Lower Kittanning

Coal



Figure 4. K a o l i n i t e c o n t e n t o f the Lower the l e s s than 2 micron f r a c t i o n ) .

Kittanning

seam {% o f



48

m i n e r a l m a t t e r a n d

ash

in

c o a l

f l o c c u l a t i o n across the d e p o s i t i o n a l basin r e s u l t e d i n higher k a o l i n i t e c o n t e n t s t o w a r d s t h e b a s i n m a r g i n s , whereas i l l i t e and expandable c l a y contents i n c r e a s e towards the center of the b a s i n . A c c o r d i n g t o t h i s t h e o r y , some a u t h i g e n i c f o r m a t i o n o r r e g r a d i n g o f i l l i t e would a l s o take p l a c e . Holbrook (34) argued a g a i n s t t h i s p r o c e s s f o r t h e L o w e r K i t t a n n i n g u n d e r c l a y on t h e b a s i s t h a t m o s t o f t h e m i c a he o b s e r v e d was t h e 2NL p o l y t y p e , w h i c h f o r m s o n l y a t temperatures f a r in excess of those experienced during b u r i a l of t h e s e s e d i m e n t s , thus any mechanism t h a t i n v o l v e d f o r m a t i o n o f mica w i t h i n t h e b a s i n w o u l d n o t be v a l i d . S i m i l a r l y , t h e 2M\ i l l i t e / m i c a p o l y t y p e i s p r e s e n t i n the Lower K i t t a n n i n g c o a l , p a r t i c u l a r l y i n c l a y p a r t i n g s and h i g h - a s h samples ( 4 , 1 2 ) . I n some s a m p l e s , p o l y t y p e i d e n t i f i c a t i o n was d i f f i c u l t , h o w e v e r , o w i n g t o t h e degraded nature of t h i s mineral which r e s u l t e d i n broad d i f f r a c t i o n peaks. The p r e s e n c e o f d e g r a d e d i l l i t e / m i c a i s n o t s u r p r i s i n g b e c a u s e , u n d e r t h e c o n d i t i o n s t h a t e x i s t e d w i t h i n t h e p e a t swamp a n d d u r i n g b u r i a l , l e a c h i n g by o r g a n i c a c i d s would most l i k e l y have occurred. A l t e r n a t i v e l y , t h e d i s t r i b u t i o n o f c l a y s i n t h i s c o a l c a n be e x p l a i n e d i n terms of the chemical c o n d i t i o n s t h a t e x i s t e d i n the swamp e n v i r o n m e n t . A c o m p a r i s o n o f F i g u r e s 1 a n d 4 shows t h a t k a o l i n i t e o c c u r s i n a r e a s w h i c h may h a v e b e e n i n f l u e n c e d b y f r e s h w a t e r c o n d i t i o n s ; i l l i t e and e x p a n d a b l e c l a y c o n t e n t s a r e g e n e r a l l y h i g h e r i n a r e a s o f t h e b a s i n w h i c h may h a v e e x p e r i e n c e d more b r a c k i s h c o n d i t i o n s . Under the a c i d i c c o n d i t i o n s t h a t e x i s t e d i n t h e f r e s h w a t e r s w a m p s , k a o l i n i t e w o u l d be t h e s t a b l e c l a y m i n e r a l , f o r m i n g e i t h e r from s o l u t i o n o r as t h e a l t e r a t i o n p r o d u c t o f i n c o m i n g sediment. The h i g h d e g r e e o f c r y s t a l l i n i t y o f t h i s k a o l i n i t e suggests i n s i t u f o r m a t i o n which c o u l d r e s u l t from e i t h e r of these mechanisms ( 1 2 ) . T o w a r d s t h e c e n t e r o f t h e b a s i n , n e u t r a l pH c o n d i t i o n s e x i s t e d a n d i l l i t e / m i c a was l e s s a l t e r e d t h a n i n t h e m o r e acidic freshwater conditions. T h i s t r e n d w o u l d have been even more pronounced w i t h the onset of the marine i n v a s i o n . A l t e r a t i o n of the i l l i t e / m i c a s t i l l p r o c e e d e d t o some e x t e n t , p r o d u c i n g d e g r a d e d i l l i t e s and e x p a n d a b l e c l a y s . A r o u n d t h e m a r g i n s o f t h e swamp, i n f l u x e s o f s e d i m e n t , now r e p r e s e n t e d b y c l a y p a r t i n g s w i t h i n t h e c o a l , i n t r o d u c e d i l l i t e / m i c a - r i c h s e d i m e n t s , t h u s a c c o u n t i n g f o r some of the local decreases in k a o l i n i t e content. To e x a m i n e f u r t h e r t h e i n f l u e n c e o f p e a t - s w a m p c h e m i s t r y on t h e c l a y s , t h e c o m p o s i t i o n o f t h e c l a y f r a c t i o n o f t h e c o a l s a m p l e s was compared t o t h a t o f f l o o r , r o o f , and p a r t i n g s a m p l e s . Coal samples t e n d t o be e n r i c h e d i n k a o l i n i t e c o m p a r e d t o o t h e r s a m p l e t y p e s (Table I ) . F l o o r r o c k s t e n d t o have t h e h i g h e s t e x p a n d a b l e c l a y c o n t e n t s , whereas r o o f r o c k s have t h e h i g h e s t i l l i t e / m i c a c o n t e n t s . I f i t i s assumed t h a t t h e c o m p o s i t i o n o f t h e r o o f s h a l e s i s t h e c l o s e s t r e p r e s e n t a t i o n of parent mineralogy a v a i l a b l e , the c l a y sediments e n t e r i n g the d e p o s i t i o n a l basin included c o n s i d e r a b l e amounts o f i l l i t e / m i c a , moderate amounts o f k a o l i n i t e , r e l a t i v e l y s m a l l a m o u n t s o f e x p a n d a b l e c l a y s , a n d some c h l o r i t e . W h e r e a s some m o d i f i c a t i o n s may h a v e t a k e n p l a c e s i n c e d e p o s i t i o n o f t h e c l a y s i n t h e r o o f s h a l e s , t h e y p r o b a b l y have been a f f e c t e d l e s s by t h e o r g a n i c a c i d s than the c l a y s in the u n d e r c l a y s , coals or p a r t i n g s . The presence of c h l o r i t e in the roof rocks suggests that a c i d i c


3.

RIMMER AND

DAVIS

inorganic

Composition

of Lower Kittanning

c o n d i t i o n s have n o t been e x p e r i e n c e d , b e c a u s e u n s t a b l e i n such e n v i r o n m e n t s .


Table

Sample Type

Coals Floors Roofs Partings

49

Coal

this mineral

is

usually

I. C l a y C o m p o s i t i o n o f t h e L e s s Than 2 M i c r o n F r a c t i o n C o a l s and A s s o c i a t e d S e d i m e n t s : Summary S t a t i s t i c s .

Sample Size (n)

28 18 16 4

* η = Number o f

K a o l i ni te + Chlorite X s

62.2 42.2 31.2 43.5

12.58 11.53 11.03 17.89

S a m p l e s ; χ = Mean;

Expandable Clays χ s*

Illite/ Mica X s

23.6 33.7 50.4 41.5

8.03 12.45 12.71 9.88

s = Standard

of

14.1 23.9 18.3 15.0

8.55 8.94 10.00 8.16

Deviation

A c i d i c c o n d i t i o n s w i t h i n t h e p e a t swamp, p a r t i c u l a r l y i n t h e f r e s h w a t e r a r e a s , would f a v o r the f o r m a t i o n and/or p r e s e r v a t i o n of kaolinite. This would r e s u l t i n the observed enrichment i n k a o l i n i t e w i t h i n the coal r e l a t i v e to the surrounding sediments. Relatively l o w i l l i t e / m i c a c o n t e n t s a r e o b s e r v e d i n t h e c o a l s , p o s s i b l y due t o a l t e r a t i o n o f t h i s c l a y t o e x p a n d a b l e c l a y s and u l t i m a t e l y t o kaolinite. The t r a n s i t i o n f r o m e x p a n d a b l e c l a y s t o k a o l i n i t e h a s b e e n o b s e r v e d i n t h e S n u g g e d y Swamp b y S t a u b a n d C o h e n ( 1 1 ) . The i n o r g a n i c p a r t i n g s w i t h i n t h e c o a l h a v e s l i g h t l y h i g h e r i l l i t e contents; h o w e v e r , v e r y few s a m p l e s were examined and t h e r e s u l t s m u s t be i n t e r p r e t e d w i t h c a r e . Most of the c l a y i n these p a r t i n g s i s d e t r i t a l r a t h e r than the r e s u l t o f i n s i t u f o r m a t i o n , as s u g g e s t e d by t h e p r e s e n c e o f 2 M i l l i t e / m i c a and t h e low d e g r e e of c r y s t a l u n i t y of the k a o l i n i t e . T h i s i s i n c o n t r a s t t o most of the k a o l i n i t e w i t h i n the coal which i s probably a u t h i g e n i c . The f l o o r r o c k s c o n t a i n l a r g e r a m o u n t s o f e x p a n d a b l e c l a y s compared t o t h e c o a l s and r o o f r o c k s . K a o l i n i t e i s l e s s abundant t h a n i n t h e c o a l s , b u t m o r e so t h a n i n t h e r o o f s . O n l y one s a m p l e c o n t a i n e d any c h l o r i t e . These o b s e r v a t i o n s a r e c o n s i s t e n t w i t h a l t e r a t i o n o f u n d e r l y i n g sediment by o r g a n i c a c i d s as s u g g e s t e d by Rimmer a n d E b e r l (_36). A c t i n g on a t y p i c a l s e d i m e n t a r y c l a y a s s e m b l a g e , such as t h a t o b s e r v e d i n the r o o f s h a l e s , o r g a n i c a c i d s would l e a c h t h e c l a y s r e s u l t i n g i n t h e d e s t r u c t i o n o f c h l o r i t e and the gradual a l t e r a t i o n of i l l i t e to expandable c l a y s . As s u g g e s t e d b y t h e s l i g h t i n c r e a s e i n k a o l i n i t e , t h i s a l t e r a t i o n may h a v e progressed to k a o l i n i t e . A l t e r a t i o n of c l a y s in the f l o o r rocks i s l e s s s e v e r e t h a n was e x p e r i e n c e d b y c l a y s w i t h i n t h e p e a t swamp itself; t h u s , t h e f l o o r r o c k s have l o w e r k a o l i n i t e c o n t e n t s t h a n t h e coals. 1



50

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

A f i n a l c o n t r o l on c l a y m i n e r a l c o m p o s i t i o n i s t h e e x t e n t o f metamorphism e x p e r i e n c e d by the c o a l s . With the i n c r e a s e i n rank e x h i b i t e d by t h i s c o a l a c r o s s t h e s t u d y a r e a , c e r t a i n d i a g e n e t i c c h a n g e s m i g h t be e x p e c t e d . W h e r e a s no c o n s i s t e n t t r e n d s i n t h e t y p e o f e x p a n d a b l e c l a y s was o b s e r v e d , a g e n e r a l l a c k o f s m e c t i t e a n d e x p a n d a b l e c l a y s was n o t e d . T h e s e c l a y s do o c c u r i n l o w e r r a n k b i t u m i n o u s c o a l s , such as those i n the I l l i n o i s B a s i n ( H ) . Thus, i t a p p e a r s t h a t t h e r m a l m a t u r i t y ( c o a l r a n k ) may h e l p e x p l a i n t h e absence o f s m e c t i t e and t h e p a u c i t y o f m i x e d - l a y e r e x p a n d a b l e c l a y s i n t h e L o w e r K i t t a n n i n g c o a l a n d a s s o c i a t e d s e d i m e n t s , b u t no d i s t i n c t r e l a t i o n s h i p was f o u n d b e t w e e n t h e a m o u n t o f e x p a n d a b l e c l a y s and r a n k . The m o s t s i g n i f i c a n t c o n t r o l on c l a y m i n e r a l c o m p o s i t i o n , t h e r e f o r e , a p p e a r s t o be c h e m i c a l c o n d i t i o n s w i t h i n t h e swamp w h i c h produced a l t e r a t i o n of a parent m a t e r i a l t h a t included i l l i t e / m i c a , k a o l i n i t e , and l e s s e r amounts o f c h l o r i t e and expandabe c l a y s . Within the a c i d i c freshwater environments, a l t e r a t i o n of other c l a y s t o k a o l i n i t e may h a v e o c c u r r e d , i n a d d i t i o n t o t h e p r e c i p i t a t i o n o f k a o l i n i t e from s o l u t i o n . In b r a c k i s h o r m a r i n e c o n d i t i o n s , k a o l i n i t e f o r m a t i o n was n o t f a v o r e d , r e s u l t i n g i n a r e l a t i v e i n c r e a s e i n i l l i t e / m i c a content. Some i l l i t e may h a v e f o r m e d d i a g e n e t i c a l l y , b u t the presence of the high-temperature polytype i n d i c a t e s a d e t r i t a l o r i g i n f o r most of t h i s c l a y m i n e r a l . Regrading of degraded i l l i t e s may h a v e a l s o t a k e n p l a c e i n p o t a s s i u m - r i c h m a r i n e o r b r a c k i s h waters. L o c a l i n c r e a s e s i n expandable c l a y s a l o n g t h e edges o f t h e swamp may r e f l e c t c h e m i c a l a l t e r a t i o n o f m i c a - r i c h s e d i m e n t s i n t r o d u c e d i n t o t h e p e a t swamp. Summary The d i s t r i b u t i o n s o f m i n e r a l s w i t h i n t h e L o w e r K i t t a n n i n g seam c a n be r e l a t e d to depositional environment. Pyrite content is highest in a r e a s w h i c h may h a v e e x p e r i e n c e d b r a c k i s h c o n d i t i o n s . This d i s t r i b u t i o n r e f l e c t s t h e a v a i l a b i l i t y o f i r o n a n d s u l f u r , a n d pH c o n d i t i o n s w i t h i n t h e swamp. W h e r e a s much o f t h e p y r i t e f o r m e d s y n g e n e t i c a l l y , o b s e r v a t i o n s o f s u l f i d e modes o f o c c u r r e n c e s u g g e s t t h a t m a r c a s i t e formed e p i g e n e t i c a l l y . The r e l a t i v e l y h i g h q u a r t z c o n t e n t s o b s e r v e d t o w a r d s t h e n o r t h of the study area r e f l e c t the i n f l u x of q u a r t z - r i c h d e t r i t u s from a topographic high in that area. Most o f the d e t r i t u s e n t e r i n g the b a s i n f r o m t h e e a s t was c l a y - r i c h ; however, l o c a l i n f l u x e s of q u a r t z may h a v e o c c u r r e d p e r i o d i c a l l y . C h e m i c a l c o n d i t i o n s w i t h i n t h e p e a t swamp i n f l u e n c e d t h e c l a y mineral assemblage. K a o l i n i t e i s e n r i c h e d towards the margins of the depositional basin; an a u t h i g e n i c o r i g i n i n a c i d i c f r e s h w a t e r c o n d i t i o n s i s p r o p o s e d f o r much o f t h i s c l a y . Illite/mica is p r i m a r i l y d e t r i t a l , as i n d i c a t e d by the presence o f the high-temperature polytype. An i n c r e a s e i n i l l i t e / m i c a t o w a r d s t h e center of the basin r e f l e c t s the g r e a t e r s t a b i l i t y of t h i s c l a y i n a marine-influenced environment. The l e v e l o f m e t a m o r p h i s m o f t h e c o a l ( r a n k ) may e x p l a i n t h e p a u c i t y o f e x p a n d a b l e c l a y s .


3. RIMMER AND DAVIS

Inorganic Composition of Lower Kittanning Coal

Acknowledgments The authors acknowledge U.S. Department of Energy Contract Number DE-AC22-80PC30013 for financial support of the research presented here. Additional support was provided by the Pennsylvania State University Cooperative Program in Coal Research and by the Phillips Petroleum Foundation.


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MINERAL MATTER AND ASH IN COAL

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RECEIVED August 1, 1985


Mineral Matter and Ash in Coal - American Chemical Society

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