Inhibition of Calcite Crystal Growth

8/13/2019 Inhibition of Calcite Crystal Growth

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Geochimicaet CosmochimicaActa, Vol. 61, No. 7, pp. 1475-1485. 1997ergamon Copyright ) 1997 ElsevierScience LtdPrinted in the USA . All rights reserved0016-7037/97 17.00 + .00P I I S 0 0 1 6 - 7 0 3 7 ( 9 7 ) 00 0 2 4 -0

I n h i b i t i o n o f c a lc i te c r y s t a l g r o w t h b y M g 2 a t 1 0 0 C a n d 1 0 0 b a r s :I n f l u e n c e o f g r o w t h r e g i m e

MAR C DELEUZE and SUSAN L. BRANTLEYDepartment of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USAReceived November 21, 1995; accepted in revised i)rm Januar), 3, 1997)

A b s t r a c t - - F o l l o w i n g S h i ra k i a n d B r a n t le y ( 1 9 9 5 ) , w h o f o u n d d i f fe r e n t g r o w t h m e c h a n i s m s f o r c a l c i tec r y s t a l g r o w t h a t 1 0 0 C a n d 1 0 0 b a r s t o t a l p r e s s u r e , w e i n v e s t i g a t e d t h e i n h i b i t i o n o f c a l c i te c r y s t a lg r o w t h b y M g 2+ u n d e r t h e s a m e c o n d i t i o n s o f t e m p e r a t u r e , p r e s s u r e , a n d c o m p o s i t i o n . T h e r e s u l t ss h o w e d t h a t t h e M g 2+ i n h i b i t i o n d e p e n d s o n t h e g r o w t h r e g i m e . F o r g r o w t h i n t h e e x p o n e n t i a l r a t er e g i m e , t h e g r o w t h o f c a l c i t e i s a l m o s t t o t a l l y i n h i b i t e d , a n d a r a g o n i t e g r o w s i n s t e a d . F o r g r o w t h i n t h el i n e a r ra t e r e g i m e , b o t h a r a g o n i t e a n d c a l c i t e p r e c i p i t a te . O n t h e o t h e r h a n d , f o r e x p e r i m e n t s b e s t d e s c r i b e db y a p a r a b o l i c g r o w t h m o d e l , M g i n h i b i t s t h e g r o w t h r a t e o f c a l c i t e , b u t n o a r a g o n i t e p r e c i p i t a t e s . T h es t u d y a l s o s h o w e d t h a t M g i n s e r t s i n t o t h e c a l c i t e s t r u c t u r e b u t n o t i n t o t h e a r a g o n i t e s t r u c t u r e t o a n ys i g n i f i c a n t e x t e n t d u r i n g g r o w t h . T h e a r a g o n i t e g r o w t h , w h i c h i s c o n t r o l l e d b y p r i m a r y n u c l e a t i o n ,s u g g e s t s t h a t n e i t h e r n u c l e a t i o n n o r g r o w t h a r e i n h i b i t e d b y M g . D i s c r e p a n t o b s e r v a t i o n s c o n c e r n i n g M gi n h i b i t i o n d o c u m e n t e d i n t h e l i te r a t u r e m a y b e e x p l a i n e d b y s u c h d i f f e r e n c e s i n t h e g r o w t h m e c h a n i s m .Copyright 1997 Elsevier Science Ltd

1 . I N T R O D U C T I O NT h e p r e c i p i t a t i o n a n d t h e d i s s o l u t i o n o f c a r b o n a t e m i n e r a l so f t e n c o n t ro l p o r o s i t y a n d p e r m e a b i l i t y o f c o m m o n l i th o l o -g i e s ( e .g . , S c h m i d t a n d M c D o n a l d , 1 9 7 9a ,b ; L u n d e g a r d a n dL a n d , 1 9 86 ; T a y l o r , 1 9 89 ; W e e d m a n e t a l. , 1 9 9 2 ) . I n th ep a s t f e w y e a r s , t h e d i s s o l u t i o n a n d t h e p r e c i p i t a t i o n o f c a l c i t e( C a C O 3 ) h a s b e e n w e l l in v e s t i g a t e d b y m a n y a u t h o r s ( e . g . ,B e r n e r a n d M o r s e , 1 9 74 ; M o r s e , 1 9 7 8 ; P l u m m e r e t a l . , 1 97 8 ;S j 6 b e r g a n d R i c k a r d , 1 9 8 4 ; B u s e n b e r g a n d P l u m m e r , 1 9 8 6 ;C o m p t o n a n d U n w i n , 1 9 90 ; G r a t z e t a l . , 1 9 9 3 ) . T h e s e s t u d -i e s h a v e s h o w n t h e i m p o r t a n c e o f p H a n d P c o 2 i n c a l -c i t e d i s s o l u t i o n a n d p r e c i p i t a t i o n , m a i n l y a t t e m p e r a t u r e so f 6 0 C .

A l t h o u g h t h e k n o w l e d g e o f c a l c i te p r e c i p i ta t i o n an d d i s s o -l u t i o n r a t e s u n d e r s u b s u r f a c e c o n d i t i o n s w o u l d h e l p t o u n d e r -s t a n d m a n y n a t u r a l p h e n o m e n a , t h e d a t a f o r c a l c i t e w a t e r -r e a c t i o n r a t e s a t h i g h t e m p e r a t u r e s a r e n o t n u m e r o u s ( e . g . ,H i r a n o a n d K i k u t a , 1 9 8 5 ; H i g u c h i e t a l . , 1 9 8 8 ; T a l m a n e ta l ., 1 9 9 0; B e c k e t a l . , 1 9 9 2 ) . F o r t h e s e r e a s o n s , S h i r a k i a n dB r a n t l e y ( 1 9 9 5 ) i n i t i a t e d a s t u d y o f c a l c i t e g r o w t h a t 1 0 0 Ca n d 1 0 0 b a r s t o t a l p r e s s u r e a n d d o c u m e n t e d d i f f e r e n t g r o w t hr e g i m e s f o r p r e c i p i t a t i o n ( i n t e r p r e t e d a s l i n e a r , p a r a b o l i c , o re x p o n e n t i a l g r o w t h r a t e m o d e l s ) . P o i n t i n g o u t t h a t l i n e a r ,p a r a b o l i c , a n d e x p o n e n t i a l g r o w t h r a t e m o d e l s c a n b e d e r i v e df o r a d s o r p t i o n , s c r e w d i s l o c a t i o n , a n d s u r f a c e n u c l e a t i o ng r o w t h m e c h a n i s m s , S h i r a k i a n d B r a n t l e y ( 1 9 9 5 ) s u g g e s t e dt h a t c a l c i t e g r o w t h a t 1 0 0 C a n d 1 0 0 b a r s o c c u r r e d a c c o r d i n gt o d i f f e r e n t m e c h a n i s m s . G r o w t h r a t e o f c a l c i t e i n a l l t h er e g i m e s o c c u r r e d r e l a t i v e l y q u i c k l y . C o n s i d e r i n g t h a t n a t u r a lr e a c t io n s t a k e p l a c e i n i m p u r e m e d i a , w e h a v e i n v e s t i g a te dc a l c i t e g r o w t h i n h i b i t i o n f o r s u r f a c e - c o n t r o l l e d p r e c i p i t a t i o nu n d e r t h o s e d i f f e re n t g r o w t h m e c h a n i s m s i n th e p r e s e n c e o fd i s s o l v e d M g . C r y s t a l l i z a ti o n o f c a r b o n a t e m i n e r a l s h a s b e e nf o u n d t o b e i n h i b i t e d b y s o m e s o l u t e s s u c h a s m a g n e s i u m ,p h o s p h a t e , h e a v y m e t a l s , a n d o r g a n i c p r o d u c t s ( e . g . , M o r s e ,1 9 8 3 ) . W e h y p o t h e s i z e d t h a t t h e p r e s e n c e o f i n h i b i t o r sm i g h t s i g n i f i c a n t l y s l o w t h e r a t e o f c a l c i t e c r y s t a l l i z a t i o n ,

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e v e n u n d e r d i a g e n e t i c c o n d i t i o n s . D u e t o t h e u b i q u i t y o fs e a w a t e r a s p o r e f l u i d a n d t h e h i g h c o n c e n t r a t i o n o f d i s -s o l v e d M g i n s e a w a t e r , w e c h o s e t o i n v e s t i g a t e t h e e f f e c to f M g 2+ o n c a l c i t e p r e c i p i t a t i o n u n d e r t h e s a m e c o n d i t i o n s( t e m p e r a t u re , p r e s s u re , c o m p o s i t i o n ) a d o p t e d b y S h i r a k i a n dBra nt ley ( 1995 ) .

O v e r t h e l a s t c e n t u r y , t h e k i n e t i c i n h i b i t io n o f c a r b o n a t e -w a t e r - r e a c t i o n s b y M g 2+ h a v e b e e n w e l l s t u d i e d a t l o w t e m -p e r a t u r e . T h e p r e s e n c e o f d i s s o l v e d M g f a v o r s t h e p r e c i p i t a -t i o n o f C a C O 3 a s a r a g o n i t e i n s t e a d o f c a l c i t e f r o m s u p e r -s a t u r a t e d s e a w a t e r a n d o t h e r M g - r i c h a q u e o u s s o l u t i o n s(Lei tmer , 1910 , 1916; L ippman, 1960 , 1973; Ki tano , 1962;S i m k i s s , 1 9 6 4 ) . O t h e r w o r k e r s ( T a f t , 1 9 67 ; B i s c h o f f a n dF y f e , 1 9 6 8 ) s h o w e d t h a t t h e r e c r y s t a l l i s a t i o n o f a r a g o n i t ei n t o c a lc i t e is i n h i b i te d e v e n f o r v e r y l o w r a t io s o f M g / C a( r a t i o o f d i s s o l v e d M g t o d i s s o l v e d C a ) . T h e f ir s t d e t a i l e ds t u d y o n a r a g o n i t e a n d c a l c i t e p r e c i p i t a t i o n w a s m a d e b yB e r n e r ( 1 9 7 5 ) w h o f o u n d t h a t o n l y c a l c i t e c r y s t a l l i z a t i o n i si n h i b i t e d b y d i s s o l v e d M g . H o w e v e r , B e r n e r ( 1 97 5 ) r e p o r t e dt h a t a t M g 2+ l e v e l s l e s s t h a n a b o u t 5 % o f s e a w a t e r , M g 2d o e s n o t a p p r e c i a b l y r e t a r d t h e s e e d e d p r e c i p i t a t i o n o fc a l c i t e . W h e r e a s f o r l e v e l s u p t o 5 % , d i s s o l v e d M g i n s e a w a -t e r s e v e r e l y r e t a r d s t h e r a t e o f s e e d e d p r e c i p i t a t i o n o f c a l c i t e .K a t z ( 1 9 7 3 ) s h o w e d t h a t c a l c i t e p r e c i p i t a t i o n is n o t s t o p p e de v e n a t M g c o n c e n t r a t i o n l e v e l s a p p r o a c h i n g t h a t o f o c e a nw a t e r , c o n c l u d i n g t h a t t h e M g / C a r a t i o i s s u f f i c ie n t l y l o wi n o c e a n w a t e r t o n o t s u p p r e s s c a l c i t e p r e c i p i t a t i o n . M o r er e c e n t s t u d i e s ( R e d d y a n d W a n g , 1 9 8 0 ; M u c c i a n d M o r s e ,1 9 8 3 b ) s h o w t h a t c a l c i te g r o w t h r a t e d e c r e a s e s w i t h i n c r e a s -i n g d i s s o l v e d M g c o n c e n t r a t i o n . T h e s e r e l a t i v e l y re c e n t s t u d -i e s d i d n o t m e n t i o n a n y e v i d e n c e o f a r a g o n i t e p r e c i p i t a t i o na s s u g g e s t e d b y t h e e a r l i e r e x p e r i m e n t s . T h e r e f o r e , t h e e f f e c to f M g 2+ o n c a l c i t e c r y s t a l l i z a t i o n i s s t i l l c o n t r o v e r s i a l a t l o wt e m p e r a t u r e . D i f f e r e n c e s i n e x p e r i m e n t a l c o n d i t i o n s b e t w e e nt h e s e s t u d i e s ( s o l u t i o n c h e m i s t r y , t e m p e r a t u r e , p r e s s u r e , s a t -u r a t i o n s t a t e ) m a y e x p l a i n d i s c r e p a n c i e s : h o w e v e r , n o s y s -t e m a t i c s t u d y h a s b e e n c o m p l e t e d .


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1476 M. Deleu ze and S. L. Brantley2 . R A T E L A W S

S h i r a k i a n d B r a n t l e y ( 1 9 9 5 ) s u m m a r i z e d b o t h p h y s i c a lm o d e l s o f s u r f a c e - c o n t r o l l e d c r y s t a l l i z a t i o n a s w e l l as m e c h -a n i s t i c m o d e l s b a s e d u p o n e l e m e n t a r y r a t e l a w s . A s t h e a i mo f t h i s w o r k i s t o i n v e s t i g a t e t h e e f f e c t o f M g 2+ o n t h ec r y s t a l g r o w t h r a t e o f c a lc i t e w h e n t h e m e c h a n i s m i s s u r fa c e -c o n t r o l l e d , a n d t o c o m p a r e t h e r e s u l t s w i t h t h e M g - f r e e e x -p e r i m e n t s , w e s u m m a r i z e t h e p r e c i p i t a t i o n r a t e l a w s b a s e du p o n p h y s i c a l m o d e l s o f c r y s t a l g r o w t h u s e d b y S h i r a k i a n dB r a n t l e y ( 1 9 9 5 ) t o m o d e l th e p r e c i p i t a ti o n o f c a l ci t e a t1 0 0 C a n d 1 0 0 b a rs t o t a l p r e s s u r e ( T a b l e 1 ) . T h e s e l a w s ,d e r i v e d f o r m o d e l s o f s u r f a c e - c o n t r o l l e d c r y s t a l l i z a t i o n l i m -i t e d b y a d s o r p t i o n , s p i r a l g r o w t h a t s c r e w d i s l o c a t i o n s , o r b yt w o - d i m e n s i o n a l s u r f a c e n u c l e a t i o n o n c r y s t a l s u r f a c e s h a v eb e e n s u m m a r i z e d b y N i e l s en ( 1 9 8 3 ) a n d M u l l i n ( 1 9 9 3 ) .

I n e a c h o f t h e t h r e e r a t e m o d e l s s u m m a r i z e d i n T a b l e 1 ,R p pt i s t h e g r o w t h r a t e ( t o o l c m - 2 s - ~ ) , k i s t h e r a t e c o n -s t an t ( t o o l c m - 2 s ~ ), A G i s t h e d r i v i n g f o r c e o f c r y s t a l l i-z a t i o n , R i s t h e g a s c o n s t a n t , T i s t h e a b s o l u t e t e m p e r a t u r e ,a n d f 2 i s th e s a t u r a t i o n s t a t e exp AG/RT). A l t h o u g h S h i -r a k i a n d B r a n t l e y ( 1 9 9 5 ) d i d n o t p r o v e a ct u a l p h y s i c a l m e c h -a n i s m s ( e . g . , a d s o r p t i o n v s . s c r e w d i s l o c a t i o n g r o w t h ) , t h e yd i d o b s e r v e d i f f e r e n t r e g i m e s o f P c o 2 a n d A G w h e r e t h er a t e l a w s i n T a b l e 1 a p p l i e d . A s i n d i c a t e d i n T a b l e 1 , t h et h r e e m o d e l s p r e d i c t a l i n e a r d e p e n d e n c e ( a d s o r p t i o n ) , ap a r a b o l i c d e p e n d e n c e ( s c r e w d i s l o c a t i o n g r o w t h ) , a n d a ne x p o n e n t i a l d e p e n d e n c e o n A G ( s u r f a c e n u c l e a t i o n ) .

N u m e r o u s a u t h o r s h a v e u s e d t h e s e ra t e m o d e l s t o d e s c r i b et h e p r e c i p i t a t io n o f v a r i o u s c r y s t a ls : e . g . , q u a r t z ( R i m s t i d ta n d B a r n e s , 1 9 8 0 ) , s i l v e r c h l o r i d e ( D a v i e s a n d Jo n e s ,1 9 5 5 ) , k a o l i n i t e , a n d g i b b s i t e ( N a g y e t a l ., 1 9 9 0 , 1 9 91 ; N a g ya n d L a s a g a , 1 9 9 2 ) . B l u m a n d L a s a g a ( 1 9 8 7 ) u s e d M o n t eC a r l o c a l c u l a t i o n s t o s h o w t h a t f o r t h e s p i r a l g r o w t h - l i m i t e dm e c h a n i s m , n = 2 - 3 d e s c r ib e s g r o w t h c o n t ro l l e d b y s c r ewd i s l o c a t i o n s . G r a t z e t a l . ( 1 9 9 3 ) h a v e c a u t i o n e d , h o w e v e r ,t h a t m a n y o f th e a s s u m p t i o n s i m p l i e d i n a n a l y t i c a l t h e o r i e so f c r y s ta l g r o w t h o f c a l c i t e m a y b e u n t r u e , s u g g e s t i n g t h a ts i m p l e a tt r ib u t i o n o f m e c h a n i s m b a s e d u p o n t h e f o r m o f t h er a t e la w m a y l e a d t o e r ro r . T h e r e f o r e , o n e o f o u r g o a l s i nt h i s i n v e s t i g a t i o n w a s t o d e t e r m i n e w h e t h e r c a l c i t e g r o w t hi n t h e t h r e e g r o w t h r e g i m e s ( l i n e a r , p a r a b o l i c , e x p o n e n t i a l )s h o w e d d i f f e r e n c e s i n t h e e x t e n t o r e f f e c t o f M g 2 i n h i b i t i o n ,a s w o u l d b e e x p e c t e d i f t h e th r e e r e g i m e s r e p r e s e n t e d d i f f e r -e n t m e c h a n i s m s . F o r e x a m p l e , i f M g 2+ a d s o r p t i o n o n t h ec a l c i t e s u r f a c e i n h i b i t e d n u c l e a t io n , a s s u g g e s t e d b y B i s c h o f fa n d F y f e ( 1 9 6 8 ) , w e w o u l d p r e d i c t t h a t t h e ra t e o f c a lc i t eg r o w t h i n t h e e x p o n e n t ia l g r o w t h r e g i m e w o u l d b e d e c r e a s e di f s u c h g r o w t h r e f l e c t s s u r f a c e n u c l e a t i o n . I n c o n t r a s t , i ft h e p a r a b o l i c g r o w t h r e g i m e c o r r e s p o n d s t o s p ir a l g r o w t h a t

Fig. 1. Scanning e lec tron photomicrograph of ca lc i te seed crystal .

s c r e w d i s l o c a t i o n s , t h e n n o n u c l e a t i o n i s r e q u i r e d , a n d M g 2+a d s o r p t i o n m i g h t h a v e o n l y a l i m i t e d e f f e ct . F i n a l l y , i f M g 2~d e h y d r a t i o n o n t h e c a l c i t e s u r f a c e i s s l o w e r t h a n C a 2+ d e h y -d r a t io n , t h e n M g m i g h t a l s o i n h i b i t c a l c i t e g r o w t h i n t h ea d s o r p t i o n r e g i m e .

3 . E X P E R I M E N T A LThe fo l lowing e xpe r im e nts r e produc e e xa c t ly the c ondi t ions use dby Shiraki and Brantley (1995) .

3 1 M a t e r i a l sReagent grade CaCO3 (Fisher Scientif ic and Co. ; Fig. 1) with aspecific surface area o f 0.175 _+ 0.011 m 2. g-~ (m easu red b y 2 poin tKr gas B ET ) w as used as seed crystal for prec ipi ta t ion experiments .Note tha t some of the seed crysta l exhibi ted rounded corners , pre-sumably due to powder production processes .NaO H solut ions for preparing as nutr ient were f irs t bubbled withCO2-Nz gas mixtures (see Tab le 2) for a t leas t 24 h. The chemicalcomp osit ions o f solutions before the addit ion of M g z were thesame as those used by Shiraki and Brantley (1995) (Table 2) .Th e saturation states of the solutions w ere adjusted so as to beundersaturated with respect to calcit e at roo m temperature and super-sa tura ted a t high tempera ture . Solut ions C2 and D , however , weresligh tly supersaturated with respec t to calcite at roo m temperature.For these solutions , i f the CaC12 and the M gCI2 were added beforethe bubbling, we ob served af ter 24 h the prec ipi ta t ion of ca lc i te inthe nutr ient solution reservoir . Therefore , CaC12 and M gCI2 wereadded jus t before the run to avoid any c a lc i te prec ipi tat ion in thenutrient solution reservoir. T he concentration of Ca 2 and M g 2 inthe nutrient solut ions were analyzed during each run and no chang ein composit ion was observed during the length of the experiments .Since the CaC12 and the M gCI2 were added individually for eachexperimen t, sm all differences in C a 2 and Mg 2+ concentra t ions areinevitable . Therefore , Table 2 l is ts the average concentra tions foreach solution. Ho we ver, the difference s are less than _+5 .

T a b l e I : Growthm e c h a n i s m a n d c o r r e s p o n d i n g r a t e laws.M e c h a n i s m A G d e p e n d e n c e R a t e l a w s

AGdsorption Li .. . Rpp t= k(ex F(~ )-1) (1)Spiral-growth t s c r e w

d i s l o c a t i o nParabolic AG nRppt= k(e xI( ~)- l) (2)

=Aexn(-k ~ / (3)Rppt '\ (AG RT)j

3 2 R u n sCrysta l growth w as performed in a continuously s t ir red tank reac-tor (300 mL v olum e, a l l wetted par ts o f Ti , Fig. 2) . T he solut ionwas injec ted using a high pressure f low -mon itor ing pump, m ixedcom plete ly throughout the vesse l by a magnetica l ly dr iven impeller,and a l lowed to f low out of the system through a back-pressure regu-la tor . Eff luent solut ions w ere sampled pe r iodica l ly for Ca, Na, Mg,and pH analysis. T he seed crystals w ere suspended in the vesse l byst ir r ing the solut ion with an impeller a t 1200 rpm. Under these

conditions, Shiraki and Brantley (19 95 ) argued that calcite precipita-urface-nucleation Exponential


3/11

I n h i b i t io n o f c a l c i t e c r y s t a l g r o w t h b y M g 2+ 1 4 7 7

Growth modelSolutions BpH 7.31Na 3.13

m MC a2+ 2.05m MMg * 0m MCI- 4.10m MPCO2 0.01TIC 3.43m MAlk 3.09m M

Mg/Ca 0

Tab le 2 : Average compostion of nutrient solutions.Parabolic Exponential Linear

B' B C2 C2' C2 D D' D7.31 7.35 7.12 7.15 7.14 6.81 6.72 6.743.00 3.21 6.45 5.91 6.10 9.90 9.60 9.272.40 2.05 3.10 3.07 3.11 3.01 2.94 2.850.74 1.38 0 0.97 1.90 0 0.98 1.796.28 6.8 6 6.20 8.09 10.02 6.02 7.83 9.280.01 0.01 0.03 0.03 0.03 0,1 0.1 0.I3.40 3.40 7.37 7.47 7.29 12 .92 11 .24 11.713.06 3.09 6.30 6.44 6.27 9.59 7.90 8.301/3 2/3 0 I/3 2/3 0 1/3 2/3

t i o n w a s s u r f a c e - c o n t r o l l e d . S o l u t i o n s w e r e p u m p e d t h r o u g h t h e r e-a c t o r a t r o o m t e m p e r a t u r e f o r m o r e t h a n o n e h o u r b e f o r e t h e h e a t e rw a s t u r n e d o n . T h e r u n t e m p e r a t u r e w a s q u i c k l y o b t a i n e d a n d as t e a d y - s t a t e o f C a c o n c e n t r a t i o n r e a c h e d . A s c a l c i te s h o w s a r e t r o -g r a d e s o l u b i li t y w i t h t e m p e r a t u r e , t h e s u p e r s a t u r a t i o n a t r u n t e m p e r a -t u r e c a u s e d p r e c i p i ta t i o n o n t h e s e e d c r y s t a l. H o w e v e r , i n s o m ee x p e r i m e n t s ( n o t e d i n r e s u l t s s e c t io n ) , e v i d e n c e o f p r i m a r y n u c l e -a t i o n ( M u l l i n , 1 9 9 3 ) , d e f i n e d a s s p o n t a n e o u s n u c l e a t i o n w i t h o u ts e e d , w a s i d e n t i f i e d b e c a u s e w e s a w s i t e s o f n u c l e a t i o n o n t h e w e t t e dp a r t s o f t h e re a c t o r . S a t u r a t i o n s t a t e w a s v a r i e d b y c h a n g i n g t h ea m o u n t o f o r i g i n a l s e e d c r y s t a l a n d b y d e c r e a s i n g t h e f l ow r a t ed u r i n g t h e e x p e r im e n t . T h e r e a c t or w a s c l e a n e d b e t w e e n e a c h e x p e r i -m e n t b y w a s h i n g i n d i lu t e H C I .

F i g u r e 3 s h o w s a n e x a m p l e o f t h e c h a n g e i n C a c o n c e n t r a ti o n i nt h e e f f l u e n t s o lu t i o n a s a f u n c t i o n o f t i m e i n th e D ' s e r i e s. A s t h es o l u t i o n w a s p u m p e d t h r o u g h t h e r e a c t o r a t r o o m t e m p e r a t u r e f o rm o r e t h a n o n e h o u r b e f o r e t a k i n g t h e f i r s t s a m p l e , t h e e a r l i e s t m e a -s u r e m e n t s s h o w t h e C a c o n c e n t r a t i o n o f t h e n u t r ie n t s o l u t io n . A f t e rt h e r e a c t o r w a s h e a t e d , t h e s t e a d y s t a t e v a l u e o f C a c o n c e n t r a t i o nw a s q u i c k l y o b t a i n e d . I n t h i s e x p e r i m e n t , t h e f l u i d r e s i d e n c e t i m ei n t h e r e a c t o r w a s a p p r o x i m a t e l y 0 . 9 h f o r a f l o w r a t e v a l u e o f 9 . 51 0 5 L ' s t .3 . 3 . S o l u t i o n A n a l y s i s

N a + c o n c e n t r a t i o n s i n a l l s o l u t io n s w e r e m e a s u r e d b y A A S( a t o m i c a b s o r p t i o n s p e c t r o s c o p y ) w h e r e a s C a a n d M g c o n c e n t r a -t i on s w e r e m e a s u r e d b y I C P ( I n d u c t i v e l y C o u p l e d P l a s m a s p e c tr o -

F i g . 2 . E x p e r i m e n t a l a p p a r a t u s , 1 r e a c t i o n v e s s e l , 2: i m p e l l o r a n ds h a f t , 3 : m ag n e t i c s t i r r e r , 4 : s o l u t i o n i n l e t p o r t , 5 : f i l te r ( s o l u t i o no u t l e t ) , 6 : c o n t r o l t h e r m o c o u p l e , 7 : p r e s s u r e g a u g e , 8 : m a n t l e h e a t e r ,9 : t e m p e r a t u r e c o n t r o l le r , 1 0: t e m p e r a t u r e a n d s t i r r i n g s p e e d c o n t r o l -l e r, 1 1: n u t r ie n t s o l u t i o n r e s e r v o i r, 1 2 : C O 2 - N 2 g a s m i x t u r e , 1 3:b e a k e r w i t h w a t e r , 1 4 : h i g h p r e s s u r e f l o w - m o n i t o r i n g p u m p , 1 5 : N 2g a s ( f o r t o t a l p r e s s u r e c o n t r o l ) , 1 6: b a c k p r e s s u r e r e g u l a t o r , 1 7:co o l i n g b a t h , 1 8 : s am p l e b o t t le .

p h o t o m e t r y ) . T h e p H o f n u t r ie n t s o lu t i o n s w a s m e a s u r e d u s i n g ac o m b i n a t i o n g l a s s e l e c tr o d e b e f o r e a n d d u r i n g t h e r u n . T h e e r r o rr a n g e s a r e _ + 2 % f o r A A S a n d I C P a n d _ + 0 .0 3 p H u n i t s f o r p Hm e a s u r e m e n t s .

F o r n u t r i e n t s o l u t i o n s , t h e T o t a l I n o r g a n i c C a r b o n ( T I C ) w a sc a l c u l a t e d u s i n g t h e m e a s u r e d p H a n d P co ~ v a l u e o f t h e b u b b l e dg a s . A l k a li n i ty ( A l k ) w a s t h e n c a l c u la t e d u s i n g t h e m e a s u r e d p Ha n d t h e T I C v a l u e b y (M o r e l , 1 9 8 3 )

A I k - T I C / ( 1 0 P u 1 0 ~' - 35 + 1 ) ( 1 )F o r a f e w r u n s , a l k a l i n it y w a s m e a s u r e d u s i n g a s t a n d a r d t i t ra t i ont o p H - 4 .5 i n o rd e r t o v e r i fy t h e ca l cu l a t i o n . T h e r e s u l t s o b t a i n eds h o w e d g o o d a g r e e m e n t (_ + 3 % ) w i t h t h e v a l u e c a l c u l a t e d f r o m t h ep H a n d t h e T I C .F o r e f f l u e n t s o l u t i o n s , a s t h e a l k a l i n i t y c a n b e c a l c u l a t e d b y

A I k = [ N a + ] + 2 l C a 2 + ] + 2 [ M g 2+] IC 1 ] ( 2 )t h e a l k a l i n i t y v a l u e f o r t h e e f f l u e n t s o l u t i o n w a s c a l c u l a t e d f r o mm e a s u r e d C a 2+ a n d M g 2+ , a n d b y a s s u m i n g N a ' a n d C I c o n c e n t r a -t i o n d i d n o t c h a n g e d u r i n g t h e r u n . T h e T I C w a s c a l c u l a t e db y T I C ~ , i, ~ I -A C a - A M g , w h e r e A C a a n d A M g a r e th e d e c r e a s e o fC a 2+ a n d M g 2 - c o n c e n t r a t i o n , r e s p e c t i v e l y , d u r i n g t h e r u n . T h e p Hv a l u e o f e f f l u e n t s o l u t i o n s w a s c a l c u l a t e d f r o m t h e T I C a n d t h ea l k a l in i t y , b e c a u s e t h e d e g a s s i n g o b s e r v e d i n t h e s a m p l e t u b i n g c o n -n e c t e d t o t h e b a c k p r e s s u r e r e g u l a t o r d i d n o t a l lo w u s e o f m e a s u r e dp H v a l u e s f o r t h e e f f l u e n t s o l u t i o n s .3 . 4. R u n P r o d u c t A n a l y s i s3 4 1 Magnesium analysis

I n o r d e r t o c a l c u l a t e t h e w t % M g 2~ i n c a lc i t e o v e r g r o w t h , s m a l la m o u n t s o f r u n p r o d u c t w e r e d i s s o l v e d in a k n o w n v o l u m e o f H N O ~ -

2 . 82 . 62 . 4

: ~ 2 . 2~ 2

1 . 81 . 61 . 4

o r i g i na l s eed : 98 . 1 m gF l o w r a t e 1 : 1 . 4 1 E - 4 I .s -1

\ F l o w r a t e 2 : 8 . 7 1 E - 5 I .s - 1\

_ _ _ JO 50 100 150 200

T i m e r a n )F i g . 3 . C a c o n c e n t r a t i o n i n e f f l u e n t s o l u t io n a s a f u n c t i o n o f t i m ef o r s o l u t i o n D ' . M a s s o f o r i g i n a l s e e d c r y s ta l a n d f l o w r a t e s a r ei n d i ca t ed .


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1478 M. Deleuze and S. L. BrantleyT a b l e 3 : Thermodynamicdata used.

R e a c t i o n s log Kat 25C, log K at 100C,1 bar 100 barsH 2 C 0 3 = H + + H e 0 3 -6.35 -6.38

H +H C O 3 = + C O ~ - -10,33 -10.|1C a H C O ~ = C a + + H C O 3 -1.ll -1.00

Ca 2C a C O = + C O ~ - - 3 . 2 2 - 4 . 1 7C a 2 +C a C 0 3 s ) = + C O ~ - -8.48 -9.17

1 t 2 0 = O H - + H + -13.99 -12.22c o 2 1 g + a 2 o = a 2 c ow i t h -1.47 -1.91

H 2 C O 3 = C 0 2 a q ) + H 2 C O 3 a qHenry's aw constant, KH

H20 solution and were analyzed by ICP. The amount of calciumcarbonate precipitated in a given run was calculated from the de-crease of Ca 2+ concentra tion and the durati on of the exper iment.Since the stirring speed was 1200 rpm for all experiments, we as-sumed an homo geneous distribution of initial seed and overgrowthmaterial in the small amount of powder analyzed. Consequently, themeasured values of Mg concentration were normalized and presentedas the calculated wt% of Mg 2+ (Table 4) in overgr owth material.When the amount of precipitated material was sufficient, we checkedthe calculation by analyzing two samples from the same run. The% of Mg 2+ was a lways foun d to be the sam e within the err or range.

3 . 5 . X - R a y D i f f r a c t i o n M e a s u r e m e n t sThe run products were also analyzed by X-ray diffraction. In someexperiments, both calcite and aragonite precipitation took place. In

order to calculate the wt% of each phase, the areas under the mostintense X-ray diffraction peaks of each product were measured. Tonormalize the results, we then determined XRD patterns for knownquantities of mixed calcite and aragonite (ground together), m easuredunder the same conditions. The empirical relation allowed calculation

of the wt% of each phase in our run products, within an acceptableaccuracy (_+5%) over a range from 5 to 95% aragonite content.Since the amount of overgrowth material can be calculated fromthe decrease of Ca concentration and the duration of the experiment,the maximum theoretical wt% of aragonite can also be calculatedassuming all the CaCO3 precipitation was aragonite. These results(listed in Table 4) can be compared to the measured wt% of arago-nite in overgrowth material.3 . 6 . S a t u r a t i o n S t a t e

SOLMI NEQ. 88 (Khar aka et al., 1988) was used to calculate thespeciation of solutions. The saturation index f~ was calculated fromthe product of the ion activities of calcium and carbonate ions(ac~ -~+,a c o ~ ) and the solubility product of calcite (K~p):

OCa2 a c 0 2~), - - - 3 )~pHoweve r, Shiraki and Brantley ( 1995 ) noted that speciation calcu-

lations using the original SOLMINEQ.88 database showed that solu-tions in some runs were under-saturated with respect to calcite,although experimental observation indicated the solutions were atequilibrium. Talman et al. ([990) observed a similar inconsistencyin their calcite dissolution experiments at 100, 150, and 210C, andthey concluded that they needed to change the thermodynamic con-stants of the SOLMI NEQ. 88 database: they used log K, = - 1 forthe association constant of CaHCO3+(aq) at 100C. This change inthe database predicted f~ values close to unity for solutions whichreached equilibrium. All other thermodynamic constants were as-sumed to be calculated correctly by SOLMINEQ.88. Table 3 liststhe thermodynamic constants used for speciation of the carbonatesystem at 25C and 100C.

Errors in saturation index were estimated in the same way asShiraki and Brantley ( 1995 ), assuming the largest possible measure-ment errors. This approach yields a saturation state error of +12, 9,and 10% for B, C, and D series, respectively.3.7. Growth R a t e

As the solution flows through the reactor at a constant rate, theprecipitation rate can be determined using the decrease in Ca 2+

LinearM o d e l

P a r a b o l i cModel

E x p o n e n t i a lModel

T a b l e 4 : X-ray diffraction and ICP analysis results.

wt%a r a g o n i t e

m e a s u r e d i nprecipitate 2

wt%Molality Solution a r a g o n i t eratio compositi on assumingMg/Ca onlyaragonitep r e c i p i t a t i o n I

511/3 sol D' 60572/3 sol D 57421/3 sol B' 44352/3 sol B 35961/3 sol C2' 91942/3 sol C2 92

D e c r e a s e inwt % c a l c u l a t e d [Mg2 ] inc a l c i t e wt% Mg in effluent

m e a s u r e d i n p r e c i p i ta t e d s o l u t i o n sprecipitate 3 material ( )7 44 1.6 36 54 1.2 330 27 1.3 3

22 35 1.4 30 42 1.3 50 4 4 1 .4 50 35 2 6.50 35 1.9 6.586 10 0.13 282 9 0.06 295 0 0.13 087 5 0,06 0

1 100 mass total precipitat e/(mass precipitate + m ass seed)2 100 m ass aragonite precipitated /(mass precipitate + mass seed)3 100 mas s calcite precip itated/( mass precipitate + mas s seed)4 See text


5/11

Inhibition of calcite crystal grow th by M g 2+ 1479concentra t ion between inle t and outle t . When a constant C a 2 c o n -c e n t r a t i o n is reached in the effluent, materia l b a lance in te rms o f Cayie lds

C~.,~ - c~. .o,)QR p p = ( 4 )A ,wh ere Cc,,m and Cc,,ou~are the nutrient and the effluent concentrationso f C a 2 , respectively, in moles L ~, Q is the flow rate in L s ~, Afis the final surface area in cm 2, and Rpp is the precipitation rate of theCaCO3 component of ca lc i te in m oles cm -2 s-~ . In some experiments ,incorporation of M g 2+ into calcite overgrowth material was observed.Thus, to calculate the precipitation rate of all carbonate, Eq n. 4 shouldcontain a te rm to account fo r the MgC O3 component . Ho wever , aswe discuss later, this Mg 2+ uptake is ver y small (2% , Table 4 ) andthus, this term is very small. Consequently, Eqn. 4 expresses the rateof growth of the CaCO 3 comp onent in the prec ipi tated carbonate, aswell as the rate of growth of total carbonate within error.The f ina l surface area is ca lcula ted using the re la t ion

AI = 0.034 mppt q - 0 . 1 7 9 ( 5 )m i m s c e d

where m p p t m i m ~ e e d s the ratio of calcite mass precipitated during arun with respect to the mass of ini t ia l seed used. This re la t ion wasobta ined by Shiraki and B rantley ( 1995 ) by measu rement o f surfacearea run products during grow th of ca lc i te under the same tempera-ture and pressure conditions.

4 . R E S U L T SR e s u l t s o f e x p e r i m e n t s a r e l is t e d in T a b l e 5 . T a b l e 4 s h o w s

a s u m m a r y o f th e X - R a y d i f fr a c ti o n m e a s u r e m e n t s a n d t h eI C P a n a l y s i s o b t a i n e d f o r d i f f e re n t g r o w t h r e g i m e s . S i n c eS h i r a k i a n d B r a n t l e y ( 1 9 9 5 ) s h o w e d t h a t d i f f e r e n t g r o w t hm o d e l s a r e a p p l i c a b l e t o d i f f e r e n t r e g i m e s o f n u t r ie n t s o l u -t i o n c o m p o s i t i o n , w e i n v e s t i g a t e d t h e e f f e c t o f M g 2+ o n t h ec a l c i t e c r y s t a l g r o w t h i n e a c h r e g i m e . A s w e d i s c u s s l a t e r ,w i t h i n e a c h g r o w t h r e g i m e , t h e M g 2+ e f f e c t is d i f f e re n t . T h i se f f e c t i s e i t h e r m a n i f e s t e d a s a c h a n g e i n p r e c i p i t a t i o n r a t eo f c a l c i t e o r as g r o w t h o f a r a g o n i t e i n s t e a d o f c a l ci t e . T oc o m p a r e t h e g r o w t h r a te o f M g a n d M g - f r e e e x p e r im e n t s ,w e p l o t t e d t h e p r e c i p i t a t i o n r a t e o f C a C O 3 u s i n g t h e s a m em o d e l s e s t a b l i s h e d b y S h i r a k i a n d B r a n t l e y ( 1 9 9 5 ) . N o a t -t e m p t i s m a d e t o i d e n t i t y t h e a r a g o n i t e g r o w t h m e c h a n i s m .

4 .1 . L i n e a r G r o w t h E x p e r i m e n t sF o r c a l c i t e c ry s t a l g r o w t h i n s o l u t i o n D , S h i r a k i a n d

B r a n t l e y ( 1 9 9 5 ) f o u n d t h a t t h e d a t a c a n b e fi t b y t h e l i n e a rr e a c t i o n r a t e e q u a t i o n

R p p t = 1 0 S , 6 4 0 . 0 7 ( ~ ] _ _ 1 1 0 9 0 . 1 0 ( 6 )w h e r e R p p i s th e p r e c i p i t a t i o n r a t e o f c a l c it e ( m o l c m z s - ~ )a n d ~2 i s t h e s a tu r a t i o n s t a t e ( = e x p ( A G / R T ) ) . S u c h a l i n e a rm o d e l c a n b e d e r i v e d f o r a n a d s o r p t i o n g r o w t h m o d e l ( N i e l -s e n , 1 9 8 3 ) .

T h e p r e s e n c e o f M g 2+ d u r i n g e x p e r i m e n t s r u n w i t h n u t r i -e n t s o l u t i o n D l e a d s t o t h e g r o w t h o f b o t h c a l c i t e a n d a r a g o -n i t e ( T a b l e 4 ) . F u r t h e r m o r e , t h e w t % o f a r a g o n i t e p r e c ip i -t a t ed i n c r ea s e s w i t h in c r e a s i n g M g c o n c e n t r a t i o n ( ~ 6 % f o rM g / C a = 1 / 3 a n d ~ 2 5 % f o r M g / C a = 2 / 3 ) . T h e S E Mo b s e r v a t i o n s ( F i g . 4 a ) s h o w t h a t t h e a r a g o n i t e c a n g r o w o nc a l c i t e s e e d c r y s t a l , b u t t h a t t h e m a j o r p a r t o f t h e p r e c i p i t a -t i o n t a k e s p l a c e b y p r i m a r y n u c l e a t i o n ( M u l l i n , 1 9 9 3 ) , d e -f i n e d a s s p o n t a n e o u s n u c l e a t i o n w i t h o u t s e e d . T h e e x p e r i -m e n t a l o b s e r v a t i o n s a l s o c o n f i r m e d n u c l e a t i o n o n a f e w s i t e s

o n t h e w a l l o f th e r e a c to r . T h e p r e c i p i t a te d a r a g o n i t e c a n b ee a s i l y i d e n t if i e d b y S E M ( F i g . 4 a ) , b e c a u s e c r y s t a l s ar en e e d l e l i k e .

I n t h e s e e x p e r i m e n t s ( T a b l e 4 ) , t h e M g > a n a l y s i s o fr u n p r o d u c t s c o n t a i n b e t w e e n 1 .2 a n d 1 .6 w t % M g 2 . T h i si n d i c a t e s a M g u p t a k e i n t o t h e c r y s t a l l i n e s t r u c t u r e d u r i n gg r o w t h . S E M p h o t o m i c r o g r a p h s o f c a l c i t e a l s o r e v e a l m i c r o -f r a c t u r e s ( F i g . 4 a a n d b ) . N o f r a c t u r e s w e r e o b s e r v e d i na r a g o n i t e. S h i r a k i a n d B r a n t l e y ( 1 9 9 5 ) d i d n o t o b s e r v e s u c hm i c r o f r a c t u r e s i n c a l c i t e g r o w t h i n t h e i r M g - f r e e e x p e r i -m e n t s . M i c r o f r a c t u r e s s i m i l a r t o t h o s e d e p i c t e d i n F i g . 4 a , bh a v e b e e n p r e v i o u s l y d o c u m e n t e d b y B e r n e r ( 1 97 5 ) o n m a g -n e s i a n c a l c i t e p r e c i p i t e d o n c a l c i t e s e e d c r y s t a l . T h e I C Pa n a l y s i s o f e f f l u e n t s o l u t i o n s s h o w s a m a x i m u m d e c r e a s eo f 3 % i n M g > c o n c e n t r a t i o n b e t w e e n t h e i n le t an d o u t l e ts o l u t i o n s in t h e s e e x p e r i m e n t s , w h i c h s u g g e s t s M g u p t a k ei n t o o v e r g r o w t h m a t e r i a l . N e v e r t h e l e s s , t h e I C P e r r o r ( _+2 % )d o e s n o t a l l o w p r e c i s e e v a l u a t i o n o f t h e s e r e s u l t s .

I n F i g . 5 , w e p l o t t h e g r o w t h r a t e o f c a l c i t e a g a i n s t t h es a t u r at i o n s t a t e ( w i t h r e s p e c t t o c a l c it e ) f o r s o l u t i o n s D , D ' ,a n d D . T h e r a te i s d e c r e a s e d b y t h e p r e s e n c e o f M g ( M g /C a = 1 / 3 o r M g / C a = 2 / 3 ) . H o w e v e r , w h e n t h e M g >c o n c e n t r a t i o n i n c r e a s e s f r o m M g / C a = 1 / 3 to 2 / 3 , t h e p r e -c i p i t a t i o n r a t e a p p e a r s t o i n c r e a s e s l i g h t l y . T h i s o b s e r v a t i o ni s p r o b a b l y n o t s i g n i f ic a n t h o w e v e r . B e c a u s e a r a g o n i t e p r e -c i p i t at e s , t h e p r e c ip i t a t i o n r a t e s h o u l d a c t u a l l y b e n o r m a l i z e db y t h e s u r f a c e a r e a ( E q n . 4 ) o f b o t h c a l c i t e a n d a r a g o n i t er a t h e r t h a n j u s t c a l c i t e ; t h e r e f o r e , a n e r r o r o c c u r s d u e t o t h eg r e a t d i f f e r e n c e b e t w e e n c a l c i t e a n d a r a g o n i t e s u r f a c e a r e a( 0 . 1 7 5 m x g - t f o r c a l c i t e , S h i r a k i a n d B r a n t l e y ( 1 9 9 5 ) a n da p p r o x i m a t e l y 1 .7 m 2 g ' f o r a r a g o n i t e , W a l t e r , p e t s . c o m -m u n . ) . W h e n t h e w t % o f p r e c i p i t a t e d a r a g o n i t e i n c re a s e s ,m o r e c a r b o n a t e s u r f a c e a r e a is a v a i l a b l e f o r p r e c i p i t at i o n a n dt h e r a t e a p p e a r s t o b e h i g h e r . I n t h e s e r u n s . w e d i d n o tm e a s u r e t h e f i n a l s u r f a c e a r e a b e c a u s e t h e s m a l l q u a n t i t y o fo v e r g r o w t h m a t e r i a l ( 0 . 1 3 g m a x i m u m ) d i d n o t a l l o w p r e c is em e a s u r e m e n t b y B . E . T , a n d b e c a u s e s o m e o f t h is m a t e r i alw a s n e e d e d f o r I C P a n a l y s i s . A s s h o w n i n F i g . 5 , t h e m e a -s u r e d r a te f o r b o t h s o l u t i o n s D ' a n d D a r e l in e a r w i t h r e -s p e c t to ~ , a s o b s e r v e d f o r s i m i l a r s o l u t i o n s w i t h M g / C a= 0 ( S h i r a k i a n d B r a n t l e y , 1 9 9 5 ) .

4 .2 . P a r a b o l ic G r o w t h E x p e r i m e n t sF o r s o l u t i o n B ( w i t h o u t M g 2 ~ ) , S h i r a k i a n d B r a n t l e y

( 1 9 9 5 ) f o u n d t h a t th e g r o w t h m e c h a n i s m i s b e s t d e s c r i b e db y a p a r a b o l i c r a t e e q u a t i o n

Rpp~ = 10 9.00+0.1~(~ l) l~ =: l ~ (7 )S u c h a p a r a b o l i c g r o w t h m o d e l c a n b e d e r i v e d f o r c r y s ta l l i-

z a t i o n r a t e - l i m i t e d b y g r o w t h a t s c r e w d i s l o c a t i o n s a t l o ws u p e r s a t u r a t i o n ( B u r t o n e t al . , 1 95 1 ) . T h e r e s u l t s o f o u r e x p e r -i m e n t s u s i n g t h e s a m e s o l u t i o n s b u t w i t h d i s s o l v e d M g p r e s e n ta r e sh o w n i n F i g . 6 . F o r M g / C a = 1 / 3 , t h e g r o w t h r a t e o fc a l c i te i s n o t a ff e c t ed b y d i s s o l v e d m a g n e s i u m , w h e r e a s f o rM g / C a = 2 / 3 , t h e p r e c i p i ta t i o n r at e s h o w s a s ig n i f i c an t d e -c r e a s e . I n t h e s e e x p e r i m e n t s , t h e X - r a y d i f f r a c t i o n m e a s u r e -m e n t s ( T a b l e 4 ) a n d S E M o b s e r v a t i o n s ( F i g . 4 e ) s h o w n oa r a g o n i t e . T h e I C P a n a l y s i s o f r u n p r o d u c t s s h o w s a s l i g h td i f f e r e n c e b e t w e e n t h e t w o r u n s i n t e r m s o f t h e a m o u n t o fM g 2+ i n c o r p o r a t e d d u r i n g g r o w t h : 1 .9 w t % M g 2~ i n c o r p o -


6/11

1 4 8 0 M. D e l e uze a nd S . L . B ra nt ley

T a b l e 5 . C h e m i s t r y o f e f f lu e n t s o l u t i o n s a n d c a l c u l a t e d r a t e s.Flow Ca Mg pH* pH* TIC* Al ka l i n i t y * R ~LISol. r a te m M m M 25C 100C m M m M mol. cm -'-s ~ 100C

D' 1.41E-4 1.62 0.946 6.44 6.46 10,27 5.64 6.00E-09 0.50D' 1.41E-4 1.8 1 0.948 6.42 6.44 10.01 5.42 5.60E-09 0.52D' 1.41E-4 1.64 0.951 6.43 6.45 9.99 5.43 5.30E-09 0.41D' 8.71E-5 1.58 0.946 6.42 6.44 10.23 5.56 3,30E-09 0.36D' 8.71E-5 1.73 0.948 6.39 6.41 9.98 5.25 3.24E-09 0.31D' 8.71E-5 1.55 0.951 6.40 6.42 9.90 5.26 2.85E-09 0.21D' 4.60E-5 1.64 0.948 6.37 6.39 9.89 5.07 1.55E-09 0.16D' 4.60E-5 1.49 0.951 6.39 6.41 9.84 5.14 1.25E-09 0.12D 1.41E-4 1.51 1,761 6.44 6.45 10.68 5.88 6.67E-09 0.38D 1.41E-4 1.54 1.740 6.43 6.44 10.37 5.63 5.40 E-09 0.31D 1.12E-4 1.46 1.7 61 6.43 6.44 1(I.63 5.78 4.50E-09 0.30D 1.04E-4 1.52 1.740 6.42 6.44 10.34 5.58 3.60E-09 0.28D 8.33E-5 1.42 1.7 61 6.42 6.43 10.59 5.69 3.10E-09 0.21D 7.53E-5 1. 47 1.740 6.41 6.42 10.29 5.49 2.45E-09 0.19B' 1.41E-4 1.97 0.699 6.78 6.78 2.96 2.15 3.80E-09 0.59B' 1.41E-4 1.97 0.700 6.77 6.77 2.90 2.09 2.62E-09 0.52B' 8.71E-5 1. 95 0.699 6.76 6.76 2.94 2.1 I 2.22E-09 0.48B' 8.71E-5 1.94 /).700 6.74 6.75 2.87 2.03 1.56E-09 0.37B' 4.60E-5 1.9 1 0.699 6.73 6.74 2.91 2.05 1.11E 09 0.34B' 4,60E-5 1.92 0.700 6.72 6.73 2.85 1.99 7.80E-10 0.28B 1.42E-4 1,66 1.216 6.85 6,85 3.42 2.60 2.60E-09 0.82B 1.42E-4 1,64 1.292 6.83 6,83 3.39 2.56 2.30E-09 0.69B 9.86E-5 1.64 1.216 6.83 6.83 3.39 2.56 1.80E-09 0.69B 9.82E-5 1. 62 1.292 6.81 6.81 3.37 2.51 1.55E-09 0.59B 1.41E-4 1. 73 1.249 6.83 6.83 3.(/3 2.28 1.30E-119 0.60B 6,11E -5 1.58 1.216 6.78 6.78 3.33 2.44 1.17E-09 (I.43B 5.39E-5 1. 57 1.292 6.78 6.78 3.32 2.42 9.90E- I 0 /).38B 9.88E-5 1.69 1.249 6.79 6.79 2.98 2.19 9.66E-10 0.39B 6.14E-5 1. 67 1.249 6.78 6.78 2.96 2.15 6.11E-10 0.31C2' 1.41E-4 1.98 0.949 6.64 6.65 6.20 4.10 1.15E-9 0,87C2' 1.40E-4 1.89 0.907 6.63 6.64 6.41 4.22 1.31E-9 0.82C2' 1.42E-4 1. 87 0.956 6.62 6.63 6.36 4.13 7.80E- 10 0.72C2' 1.12E-4 1. 83 0.907 6.61 6.62 6.35 4.10 9.00E-10 0.64C2' 1.12E-4 1. 83 0.956 6.60 6.61 6.32 4.04 4.5E-10 0.58C2' 9.03E-5 1.87 0.949 6.59 6.60 6.09 3.87 5.45E-10 0.51C2' 8.35E-5 1.79 0.956 6.58 6.59 6.28 3.96 2.3E- I 0 0.45C2' 8.29E-5 1.76 0.907 6.58 6.59 6.28 3.95 6.35E-10 0.43C2' 4.60E-5 1.77 0.949 6.55 6.56 5.99 3.67 2.45E-10 0.26C2 1.41E-4 1.87 1.90 6.58 6.59 5.97 3.77 1.19E-9 (/.53C2 1.13E-4 1.83 1.90 6.57 6.58 5.93 3.70 7.05E- I 0 0.44C2 1.41E-4 1.84 1.99 6.56 6.57 6.21 3.83 1.23E-9 /).43C2 1.41E-4 1.84 1.90 6.56 6.57 5.91 3.65 I. 19E-9 0.40C2 1.12E-4 1.82 1.99 6.55 6.56 6.20 3.80 7.49E-10 0.38C2 6.77E-5 1.79 1.90 6.55 6.56 5.90 3.62 3.76E-10 0.33C2 1.12E-4 1.81 1.90 6.54 6.55 5.87 3.58 6.85 E-10 0.29C2 8.28E-5 1.76 1.99 6.52 6.53 6.14 3.68 4.70E-10 0.25C2 8.36E-5 1.75 1.90 6.52 6.53 5.82 3.47 4.49 E-10 0.17

* Calculated as describe d in text .

r a te d f o r M g / C a = 2 / 3 , s o l B a n d 1 .3 w t % tb r M g / C a= 1 / 3 , s o l B '. T h e I C P s o l u t i o n a n a l y s i s a l s o s h o w s a s m a l ld i f f e r e n c e b e t w e e n t h e d e c r e a s e o f M g 2+ c o n c e n t r a t i o n d u r i n gt h e r un : 5 % f o r M g / C a = 1 / 3 a n d 6 . 5 % f o r M g / C a = 2 / 3 .E v e n i f th e s e d i f f e r e n c e s a r e s m a l l w i t h r e s p e c t to m e a s u r e -m e n t e r ro r, w e a r g u e t h a t , in r e l a t io n w i t h t h e S E M o b s e r v a -t i o n , th e y a r e s ig n i f i c a n t . I n d e e d , t h e r u n p r o d u c t s s h o w m o r em i c r o f r a c t u r e s w h e n t h e M g 2+ c o n c e n t r a t i o n i s h i g h e r ( F i g .4 c - 4 e ) . T h i s o b s e r v a t i o n s u g g e s t s t h at t h e M g i n s er t s i n t ot h e c a l c i te s t r u c t u re a n d p r o d u c e s g r o w t h d e f e c t s .

4 .3 . E x p o n e n t i a l G r o w t h E x p e r i m e n t sS h i r a k i a n d B r a n t l e y ( 1 9 9 5 ) f i t p r e c i p i t a t i o n r a t e d a t a fo r

c a l c i t e g r o w t h i n s o l u t i o n C 2 f o r ~ - 1 > 0 . 6 t o t h e f o l l o w -i n g e x p o n e n t i a l m o d e l ( F i g . 7 ) :

2 . 3 6 0 . 2 1Rpp = 10 7,280,49 ex p 8)A G / R T )

S u c h a n e x p o n e n t i a l r a t e m o d e l c a n b e d e r i v e d f o r a s u r -f a c e - n u c l e a t io n m e c h a n i s m w i t h m o n o n u c l e a r o r p o l y n u -c l ea r g r o w t h ( N i e l s e n , 1 9 8 3 ) . I n c o n tr a st , n e a r e q u i l i b r i u m( i . e . , $2 - 1 < 0 . 6 ) , t h e p r e c i p i t a t i o n i s c o n t r o l l e d b y s p i r a lg r o w t h a s s e e n in s o l u t i o n B ( F i g . 7 ) . I n a t t e m p t s t o m e a s u r ec a l c it e p r e c i p i ta t io n f r o m s o l u t i o n C 2 ( T a b l e 5 ) w i t h d i s-s o l v e d M g , w e o b s e r v e d o u r r e s u l ts t o b e i r r e p r o d u c i b l e .

I n c r y s t a ll i z a ti o n e x p e r i m e n t s w i t h s o l u t io n C 2 a n d M g /C a = 1 / 3 o r 2 / 3 , X R D a n a l y s is o f th e r u n r e v e a l e d h i g hc o n c e n t r a t i o n s o f p r e c i p i t a t e d a r a g o n it e . T h e X - R a y d i f f r a c -t io n m e a s u r e m e n t s h o w s t h a t t h e w t % o f a r a g o n it e i s a l m o s te q u a l t o t h e t h e o r e t i c a l v a l u e c a l c u l a t e d a s s u m i n g t h a t o n l ya r a g o n i t e c r y s t a l l iz a t i o n o c c u r r e d ( T a b l e 4 ) . T h e f r a c t io n o fa r a g o n it e p r e s e n t ( b a s e d o n X R D a n a l y s i s ) a l s o i n d i c a te s ad i f f e r e n c e b e t w e e n s o l u t i o n s C 2 ' a n d C 2 : w h e n t h e M g 2+c o n c e n t r a t i o n i n c r e a s e s , t h e w t % o f p r e c i p i t a t e d a r a g o n i t ei n c r e a se s a n d a p p r o a c h e s t h e t h e or e t ic a l v a l u e ( T a b l e 4 ) .I C P a n a l y s i s o f ru n p r o d u c ts s h o w ( T a b l e 4 ) a n a v e ra g e o f0 . 1 % M g 2 + i n th e p r e c i p i t a t e d p r o d u c t . W e a l s o o b s e r v e d


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Inhibition of calcite crystal growth by Mg 2+ 1481that the aragonite crystallization took place almost entirelyon the reactor (walls and components, rather than on theseed) and that this effect increased with increasing Mg con-centration. As seen before for solutions D' and D , primarynucleation thus occurred during the experiment with C2'and C2 , while for solution C2, surface nucleation controlledthe growth. Because only aragonite crystallization tookplace, a plot of precipitation rate of calcite vs. saturationstate of calcite is irrelevant. However, SEM observationsreveal some information about the growth. Figure 4f showscalcite and aragonite crystals, and Fig. 4g shows growth ofaragonite on calcite. In these photos, calcite crystals do notshow any effect from the precipitation, in particular for theMg/Ca = 2/3 experiments where we observed smooth sur-faces without a lot of fractures (Fig. 4g, 4h ). Also, in agree-ment with the low wt% of Mg observed in the run product(0.1%), we observed little fracturing in the calcite crystals,suggesting that almos t no Mg 2+ is inserted in the calcitestructure, but rather growth is almost totally stopped. Fur-thermore, there is little evidence that the Mg 2+ inserts intothe aragonite structure. In support of this, the ICP solutionanalysis shows a decrease in Mg 2+ concent ration of about2% tbr Mg/Ca = 1/3 and no significant decrease for Mg/Ca = 2/3.

5. DISCUSSION5 1 I n f lu e n c e o f M e c h a n i s m

Summarizing our results, we observed: (1) When Mg 2+is added to solutions where calcite grows by a linear model,both calcite and aragonite precipitate from the solution, andthe wt% of aragonite in the precipitate increases with in-creasing Mg 2+ concentration. (2 ) When Mg 2+ is added to asolution in which calcite grows according to a parabolicmodel fo r Mg-free conditions, a sufficient amount of Mg 2+(Mg/Ca = 2/3) reduces the precipitation rate of calcite andno aragonite precipitates. At lower concentrations of Mg 2+(M g/ Ca = 1/3), the rate is not affected. (3) When Mg 2+is added to a solution in which calcite grows according toan exponential model, the calcite growth is almost totallyinhibited in favor of aragonite crystallization.

Differences in the effect of Mg 2 on growth in each ofthe three regimes is consistent with different mechanismsof growth. These observations also yield implications withrespect to the Mg 2~ inhibition mechanism, as discussed inthe following. However, the arguments summarized beloware qualitative in nature, reflecting the lack of firm observa-tional evidence of mechanism and rate-limiting step.5 2 E x p o n e n t i a l G r o w t h M o d e l

The orthorhombic structure of aragonite shows a strongpreference for di valent cations the size of Ca 2+ or largerto insert into the structure (Speer, 1983). These structuralarguments are in agreement with the observed lack of incor-poration of Mg > in aragonite for solutions C2' and C2(Table 4) . The very low wt% of Mg 2+ (0.1 %) in the runproducts tbr these experiments also confirms the (near -)ab -sence of calcite growth, since Mg 2+ can be incorporated incalcite during growth.

Previous workers ( Bischof f and Fyfe, 1968; Berner, 1975;

Mucci and Morse, 1983) suggested that Mg :+ inhibits therate of calcite nucleation. This model would suggest thatgrowth rate-limited by surface nucleation should be stronglyinhibit ed by the presence of Mg 2+ . If growth is rate-limit edby surface nucleation for those experiments in the exponen-tial growth regime (Nielsen, 1983), then our observationthat calcite growth is almost totally inhibited in favor ofaragonite growth in this regime is in agreement with theseearlier authors.

The process of inhibition is likely related to the smallersize, the higher charge density, and the resultant strongerhydration of Mg ~+ in comparison to Ca 2+. As the dehydra-tion of reactants on the surface of a growing crystal is oftena rate-contro lling step in crystallization processes (Nancolla sand Purdie, 1964), a hydrated Mg 2+ ion adsorbed on acalcite active growth site will remain for a relatively longtime without dehydration and incorporation into the structure(in comparison with a Ca 2+ ion). This slow dehydrationcould prevent nucleation of calcite and lead to the crystalliza-tion of aragonite. A version of this model was first suggestedby Bischoff (196 8): as Mg 2+ inhibits calcite growth byblocking the nucleation, aragonite, which precipitates morerapidly, is kineticall y stabilized. In other words, since calcitenucleation does not occur, the solution, which is still super-saturated with respect to calcium carbonate, cannot stay ina highly unstable state, and, therefore, aragonite precipitates.

We also observe that for the experiments in the exponen-tial growth regime, aragonite crystallized preferentially onthe reactor walls, showing that, under these conditions, Mg 2+inhibits neither nucleation nor growth of aragonite. The lackof an inhibito ry effect by Mg 2+ on aragonite growth andnuclea tion can be explained by the observation that Mg 2+ isadsorbed at the surface of calcite to a much larger extentthan at the surface of aragonite (Degroot an d Duyvis, 1966),and also, because the Mg 2+ is not incorporated into thearagonite structure.5 3 L i n e a r G r o w t h M o d e l

For growth which is linear with respect to AG, growthmay be rate-limited by adsorption on the surface structureand especially on the active site (Nancollas and Purdie,1964). We observed that in calcite growth in the linearregime, Mg 2+ does not totally inhib it the calcite growth, butboth calcite and aragonite crystallization takes place. Thiseffect depends on the Mg 2+ concentration: the wt% of pre-cipitated aragonite increases with increasing Mg 2+ concen-tration, suggesting that inhibition of calcite precipitation in-creases with Mg 2+ concentra tion.

A hypothesis can be advanced to explain the observedresults. As argued previously, the process of inhi bition mayreflect the stronger hydration of Mg 2+ in comparison toCa 2+, since the dehydration of r eactants on the surface o fthe growing crystal may be the rate-controlli ng step (Nancol -las and Purdie, 1964). In this linear growth regime, if theprocess is controlled by adsorption, and if the dehydrationof Mg 2+ is s lower than that o f Ca 2+ , then the Mg 2+ adsorbedon the calcite structure may lead to inhibition of calcitegrowth, and, tbr the same reason as before, to aragonitecrystallization. If inhibition depends on the density of cationsadsorbed, then for low concentrations of Mg 2+ the probabil-


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1482 M. Deleuze and S. L. Brantley

Fig. 4. SEM photomicrographs of run products. (a) Solution D' Mg/Ca = 1/3; (b) Solution D Mg/Ca = 2/3;(c) Solution B' Mg/Ca = 1/3; (d) Solution B' Mg/Ca = 1/3; (e) Solution B Mg/Ca - 2/3; (f ) Solution C2'Mg/Ca = 1/3 ; (g) and (h) Solution C2 Mg/Ca - 2/3.

ity of occupyin g all the active sites is small, but this occupa-tion will increase with increas ing Mg z+ concentration. Fur-thermore, as calcite growth is not stopped by Mg > adsorp-tion, the Mg 2+ can be incorporated i nto the calcite duringprecipitation. The insertion of Mg should produce somegrowth defects which could lead to the formation of micro-fractures duri ng the growth. Finall y, the effect o f Mg 2+ issmaller in this growth regime than in the surface-nucleationgrowth regime because calcite nucleation still occurs (andis not rate-limiting), while growth of the calcite is inhibited.5 4 P a r a b o li c G r o w t h M o d e l

For precipitation in the regime of parabolic growth at lowMg concentrat ions (M g/ Ca = ~,(~),Mg 2+ is inserted into thecalcite structure, but the precipitation rate is not affected.Thus, the data can be fit by the same law (Eqn. 7) used byShiraki and Brantl ey ( 1995 ). This parabolic rate law is simi-lar to the law attributed to growth at screw dislocations(Burton et al., 1951). For higher concentrations (Mg /Ca= 2/ 3) , the precipitation rate shows a decrease, more Mg 2+is incorporated into the precipitated calcite, and, the data aredescribed by R p p t = 1 0 - 9 5 1 + 1 4 ( ~ - 1 ) 1 3 3 + 0 1 5 . For both

concentra tions of Mg 2+, aragonite crystals do not grow fromthe solution.

A possible explanation of these observations is found inthe BCF theory for the spiral growth mechanism (Burton etal., 1951 ). A crystal possessing a screw d islocation containsa self-perpetuating step, and thus has no need for two-dimen-sional nucleation. For growth by such a mechanism, it issometimes assumed that the surface diffusion of crystallizingspecies is the rate-controlling step (Nancollas and Purdie,1964). On the other hand, Gratz et al. (1993) suggested thatsurface diffusion is not rate-limiting for step advance ingrowth of oxides and hydroxides in aqueous systems. Otherrate-limiting steps might include volume diffusion in aboundary layer or a dehydration of adsorbed molecules atthe surface. The dehydration of a Mg 2+ ion adsorbed on thesurface, because of its high hydration energy, should be moredifficult and s lower than Ca 2+. For low concentrations ofMg the inhibitory effect may be expected to be negligible,and calcite crystallization should not be inhibited. However,at higher Mg 2+ concentrations, the probabilit y of interactionbetween Mg 2+ and active sites is higher (even though diffu-sion on the surface is still slow) and the growth rate maybe decreased. Thus, the inhibitory effect may be observed


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Inhibi t ion of ca lc i te crysta l growth by Mg - 1483

Fig. 4. (Continued)

t o b e d e p e n d e n t u p o n M g ~-+ c o n c e n t r a t i o n , a s o b s e r v e d . F u r -t h e r m o r e , a s t h e c r y s t a l c o n t a i n s a s e l f - p e r p e t u a t i n g s t e pw h i c h a l l o w s g r o w t h o n a l o t o f a c t i v e s i te s , a n d a s i n t h isc a s e , t h e p r e c i p i ta t i o n r a te o f c a l c i t e i s l o w b u t n o t n e g l i g i b l e( l o w e r t h a n t h e r at e o f t h e a d s o r p t i o n - c o n t r o l l e d e x p e r i -m e n t s ) , a r a g o n i t e c r y s t a l l iz a t i o n d o e s n o t t a k e p l a c e b e c a u s et h e r e i s n o k i n e t i c s t a b i l i z a t i o n .

5 5 I m p l i c a t i o n o f T h e s e R e s u l t sA l t h o u g h w e h a v e s h o w n n o d i r e c t e v i d e n c e t h a t t h e th r e e

g r o w t h r e g i m e s ( l i n e a r , p a r a b o l i c , e x p o n e n t i a l ) c o r r e s p o n dt o t h e a d s o r p t i o n , s c r e w d i s l o c a t i o n , o r s u r f a c e n u c l e a t i o nm e c h a n i s m s , t h e r e l a t i v e e f f e c t s o f M g -~- i n h i b i t i o n o b s e r v e da r e at l e a st c o n s i s t e n t w i t h t h r e e m e c h a n i s m s o f g r o w t h . I n

1 . 5 1 0 - 9 Sol D M g/C a=0 J ~ -Sh irakl and Brantley ( ~ So l D ' Mg/C a= l /3

_~ ~ ~ ~ ~ ~o 10.10~ I5 . o

0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 . 6f l - 1

Fig. 5. Calcite crystal growth rate plotted vs. ~-1 for solution D;D ' and D ( l inear grow th) . Satura t ion s tate (f~) ca lcula ted withrespect to ca lc i te us ing data of SOLMINEQ.88. Solut ion composi-tions are indicated.

4 . 0 1 0 - 1

A3 . 0 1 0 1 0, w .N

Eo 0 - 1 o_ ~ 2 0 1o0 1o. 0 1n-

S o l B M g /C a = 0 ~S h i r a k i a n d B r a n t l e y ( 1 9 9 5 ) S o l B ' M g /C 8 = 1 /3

0 . 6 0.8~ - 1Fig. 6. Calcite crystal growth rate plotted vs. ~-1 tor solution B,B ' , and B (parabolic grow th) . Saturat ion s ta te (~ ) ca lculated w ithrespect to ca lc i te us ing data of SOLMINEQ.88. Solut ion composi-tions are indicated.

0 0 . 2 0 . 4


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1484 M. Deleuze and S. L. Brantley2 . 0 1 0 9

1 . 5 1 0 9

Eu~ 1 . 0 1 0 - 9oE

~ 5 . 0 1 0 1

[2 Sol B Mg/ Ca=OS hi rak l and Brant l ey 1 9 9 5 )0 S o l C 2 M g / C a = OS hi rak i and Brant l ey 1 9 9 5 )

0 . 2 0 . 4 0 . 6 0 . 8f l - 11 1 . 2

Fig. 7. Calcite crystal growth rate plotted vs. ft-1 for solution Band C2. Saturation state (f~) calculated with respect to calcite usingdata of SOLM1NEQ.88. Solution compositions are indicated.

the exponential growth regime, if calcite growth occurs bya surface n ucleat ion mechanism, the observat ion that Mg 2+almost totally inhib its calcite precipitation is cons istent witha model whereby adsorption of Mg 2+ inhibits 2D-nucleation.Indeed, adsorption of Mg 2+ in solutions of low Mg/Ca hasno effect on calcite growth where nucleation is assumedunnecessary: in the regime of parabolic (screw dislocation )growth. However, at higher values of Mg/Ca, even growthin this regime is inhibited, although the rate law of growthis still consistent with spiral growth at screw dislocations.

Previous studies (see Introduction) based on natural me-dia conditions have shown several discrepancies. Althoughour conditions (100C and 100 bars total pressure, dilutesolutions) are very different from seawater under ambientconditions, our results may help to explain such discrepanc-ies. If Mg 2+ inhib ition depends on growth mechanism, dis-crepancies may be due to different growth regimes resultingin either an inhibition (partial or total) of calcite growth infavor of aragonite (Leitmer, 1910, 1916; Lippman, 1960,1973; Kitano, 1962; Simkiss, 1964; Taft, 1967; Bischoff andFyfe, 1968) or an inh ibiti on of calcite precipitation withoutaragonite formation (Katz, 1973; Beruer, 1975; Reddy andWang, 1980; Mucci and Morse, 1983b). For example,Berner (1975) reported spontaneous aragonite precipitationat the end of long runs for experiments investigating crystal-lization of magnesian calcite. The results of Berner (1975)might suggest a parabolic growth regime as he observed alow inhibiti on effect for low Mg 2+ concent ration (5 ofseawater) bu t a strong inhibitio n effect from solutions withMg 2+ concentrati on equiva lent to seawater. Nevertheless,the presence of aragonite crystallization only at the end oflong runs cannot be explained. Thus, several discrepanciesinvo lving the effect of Mg on calcite growth and nucleat ion,over a range of temperature and pressure, remain to be inves-tigated.

6. CONCLUSIONThe study of inhibition of calcite crystal growth by Mg 2.

at 100C and 100 bars total pressure for surface-controlledexperiments shows a strong influence of growth regime. Thepresence of low quantities of Mg (Mg/ Ca = 1/3, 2/3) leadsto Mg 2+ concentration-dependent phenomena: ( l) Total

calcite growth inhibi tion in favor of aragonite crystallizationin the exponential growth regime; (2) Partial calcite growthinhibition in favor of aragonite crystallization in the lineargrowth regime; (3) A decrease of the calcite growth ratewithout aragonite crystallization in the parabolic growth re-gime for Mg/Ca = 2/3; (4) No effect on calcite growthrate and no aragonite crystallization in the parabolic growthregime for Mg/Ca -- 1/3. Furthermore, these experimentsalso show that Mg > is incorporated in the calcite structurebut not in the aragonite structure during nucleation andgrowth.

The differences are consistent with different growth mech-anisms operating within each regime. Since the strongestinhib ition effect is observed for the growth regime wherenucleation is hypothesized to control the rate of crystalliza-tion, Mg 2+ may i nhib it calcite nucleation. This inhibi tionmay be related to the high hydration energy of Mg 2+ and tothe calcite structure which allows incorporation of Mg 2+ .On the other hand, neither nucleation nor growth of aragoniteare affected by the presence of Mg 2+ ions because aragonitepreferentially incorporates cations of ionic radii equal to orgreater than that of Ca 2+, exclud ing small ions such asMg 2+. These observations suggest that the presence of dis-solved Mg in pore solutions under diagenetic conditions maysignificantly affect carbonate precipitation reactions in thesubsurface. Depending upon the solution composition,calcite precipitation may be unaffected, may be slowed, ormay be replaced by aragonite precipitation.A c k n o w l e d g m e n t s - - T h e authors would like to thank the DGA/DRET for supporting M. Deleuze at Penn State, D. Voigt for helpand discussion, H. Gong for the ICP measurements, C. Perry andM. Angelone for the SEM work, and R. Shiraki. Acknowledgmentis made to the donor of the Petroleum Research Foundation, adminis-tered by the American Chemical Society, for partial support of thisresearch.Edi torial handl ing. J. Tossell

R E F E R E N E S

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Inhibition of Calcite Crystal Growth

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