Silica Apparent Solubilities and Rates of Dissolution and Precipitation

21

S i l i c a A p p a r e n t S o l u b i l i t i e s a n d Rates of D i s s o l u t i o n and

Precipitation

For ca. 25 C o m m o n Minerals in Seawater

at

1°-2°C,

pH

7.5-8.5

Downloaded by CORNELL UNIV on October 7, 2016 | http://pubs.acs.org Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch021

DAVID C. HURD, CHARLES FRALEY, and JAMES K. FUGATE Hawaii Institute of Geophysics, University of Hawaii, Honolulu, HI 96822 T h i s paper i s b a s i c a l l y an outgrowth o f an ear l i e r p a p e r (JL) w h e r e i n t h e a u t h o r c o n s i d e r e d t h e p o s s i b l e e f f e c t s o f g l a c i a l w e a t h e r i n g on t h e s i l i c a budget of A n t a r c t i c w a t e r s . I n t h a t s t u d y a number o f f i n e l y g r o u n d r o c k s , c o n s i d e r e d t o be t y p i c a l o f t h o s e now b e i n g g l a c i a l l y e r o d e d f r o m t h e A n t a r c t i c c o n t i nent a n d d e p o s i t e d n e a r b y , were a l l o w e d t o r e a c t w i t h 1-2°C seawater having d i s s o l v e d s i l i c a c o n c e n t r a t i o n s t y p i c a l o f A n t a r c t i c s u r f a c e and bottom w a t e r s . I t was f o u n d t h a t a number o f r o c k s e i t h e r a d s o r b e d s i l i c a from s o l u t i o n o r d i s s o l v e d so s l o w l y as t o p r o v i d e n e g l i g i b l e i n p u t t o the s i l i c a budget. This f i n d i n g was somewhat d i f f e r e n t f r o m t h o s e o f e a r l i e r i n v e s t i g a t o r s ( 2 ^ 3_) who h a d p o s t u l a t e d t h a t t h e s e same r o c k s m i g h t w e l l be a s i g n i f i c a n t s o u r c e o f d i s s o l v e d s i l i c a t o the w o r l d oceans and comparable t o t h e a n n u a l i n p u t by r i v e r s . C o n s t r u c t i v e c r i t i c i s m of H u r d 's m a n u s c r i p t by C. V. C l e m e n c y a n d P. E . C a l k i n S t a t e U n i v . N.Y., B u f f a l o , p e r s o n a l c o m m u n i c a t i o n 1 9 7 7 ) s u g g e s t e d the need f o r s t u d y i n g i n d i v i d u a l m i n e r a l s as w e l l as the above r o c k s o b s e r v e d by Hurd. The f o l l o w i n g study i s , i n p a r t , a response t o that c r i t i c i s m . We h a v e c h o s e n t o l o o k a t t h e d a t a f r o m a p a r t i c u l a r point of view: how much r e a c t i o n w o u l d o c c u r d u r i n g a 0-6 week p e r i o d f o r a g i v e n m i n e r a l a t v a r i o u s d i s s o l v e d s i l i c a c o n c e n t r a t i o n s . We w a n t e d t o t r y t o b e g i n t o u n d e r s t a n d t h e f o l l o w i n g q u e s t i o n s by such an approach : 1. W o u l d a n y o f t h e 25 common s i l i c a - c o n t a i n i n g m i n e r a l s we w e r e s t u d y i n g r e l e a s e o r p r e c i p i t a t e d i s s o l v e d s i l i c a over the range of d i s s o l v e d s i l i c a v a l u e s commonly e n c o u n t e r e d i n open o c e a n w a t e r s o r i n s e d i m e n t p o r e w a t e r s , a n d i f s o a t what r a t e s a n d magnitudes would these r e a c t i o n s occur?

0-8412-0479-9/79/47-093-413$08.25/0

© 1979 American Chemical Society

Jenne; Chemical Modeling in Aqueous Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


414

C H E M I C A L M O D E L I N G IN

AQUEOUS SYSTEMS

2. I f any o f t h e a b o v e m i n e r a l s w e r e t o be t h e o n l y m i n e r a l i n j e c t e d i n t o the b u l k of ocean w a t e r s , c o u l d i t m e a s u r a b l y a f f e c t the o v e r a l l s i l i c a budget? I f so, to what e x t e n t ? 3. I f we w e r e t o l o o k a t a l l o f t h e m i n e r a l s u n d e r t h e i r c o n d i t i o n s o f maximum d i s s o l u t i o n r a t e ( i . e . at near-zero d i s s o l v e d s i l i c a c o n c e n t r a t i o n s a n d / o r f r o m f r e s h l y g r o u n d m a t e r i a l ) c a n we c r e a t e w h a t w i l l be a n a l o g o u s t o a c h e m i c a l weathering s e q u e n c e f o r t h e s e m i n e r a l s i n s e a w a t e r a t low t e m p e r atures? I f we c a n , how d o e s t h i s a r r a n g e m e n t r e l a t e to those c u r r e n t l y i n e x i s t e n c e f o r f r e s h w a t e r s ? 4. I s t h e r e any s i m p l e way t o r e l a t e m i n e r a l s t r u c t u r e t o s o l u b i l i t i e s and r e l e a s e r a t e s ? Will t h i s r a n k i n g a l l o w us t o s u g g e s t t h e o r d e r o f i m p o r tance of these m i n e r a l s w i t h r e s p e c t to i n p u t or r e m o v a l of s i l i c a t o the s i l i c a budget? The r e s u l t s and d i s c u s s i o n s o f o u r e x p e r i m e n t s i n v o l v i n g t h e a b o v e q u e s t i o n s w i l l be c o n s i d e r e d i n two s e c t i o n s a l o n g w i t h a H a w a i i I n s t i t u t e o f G e o p h y s ics data r e p o r t . The f i r s t s e c t i o n , d e a l i n g w i t h o u r p r o p o s e d method f o r c o n s i d e r i n g the p r o b l e m of e s t i mating a l u m i n o - s i l i c a t e s o l u b i l i t i e s i n seawater at 1-2°C, pH 7.6-8.3 and o f a m e t h o d f o r e s t i m a t i n g p a r t i c l e d i s s o l u t i o n r a t e s , w i l l be d i s c u s s e d and c r i t i c i z e d i n t h i s paper. The s e c o n d s e c t i o n , d e a l i n g w i t h the a p p l i c a t i o n of these c a l c u l a t i o n s t o the s i l i c a c y c l e i n the oceans w i l l appear i n the near future. The d a t a r e p o r t w h i c h w i l l be a c o m p i l a t i o n o f t h e d i s s o l v e d s i l i c a and pH m e a s u r e m e n t s , and X - r a y d i f f r a c t i o n and e l e m e n t a l a n a l y s e s o f e a c h m i n e r a l s a m p l e , as w e l l as a number o f t h e f i r s t - o r d e r f l u x and d i s s o l u t i o n r a t e c a l c u l a t i o n s w i l l be a v a i l a b l e by S p r i n g 1979. A c o p y may be o b t a i n e d by w r i t i n g t o : P u b l i c a t i o n s O f f i c e , H a w a i i I n s t , of G e o p h y s i c s , 2525 C o r r e a Road, H o n o l u l u , H a w a i i 96822. Me t h o d s M i n e r a l s a m p l e s w e r e i n i t i a l l y b r o k e n up u s i n g a Diamond^ r o c k c r u s h e r . U s i n g a n a g a t e m o r t a r and p e s t l e t h e c o a r s e s a n d and s m a l l e r s i z e d p a r t i c l e s w e r e f u r t h e r g r o u n d t o a f i n e p o w d e r , w h i c h was then s c r e e n e d t h r o u g h ,a 160 mesh s i e v e . The r e s u l t i n g s a m p l e p a r t i c l e s w e r e a l l l e s s t h a n 100 m i c r o n s i n d iame t e r . T h e n , 2.00 ± 0.02 grams o f a g i v e n p o w d e r e d sample w e r e w e i g h e d o u t and t r a n s f e r r e d t o a c l e a n , l a b e l e d , polyethylene bottle. Approximately 75 ± 3 cm por t i o n s of 1-3°C, 33 ± 2 % s a l i n i t y , f i l t e r e d s e a w a t e r , 0



21.

HURD E T

AL.

Silica and

Silica-Containing Minerals

415

whose d i s s o l v e d s i l i c a c o n c e n t r a t i o n had b e e n a d j u s t e d t o t h e d e s i r e d v a l u e , w e r e a d d e d t o e a c h o f t h e sam ples. The s a m p l e b o t t l e s w e r e t h e n p l a c e d on a s h a k e r t a b l e , s h a k e n a t 100- 150 s h a k e s / m i r i j and k e p t a t 1-3°C f o r t h e d u r a t i o n of t h e e x p e r i m e n t . A f t e r the a p p r o p r i a t e s a m p l i n g i n t e r v a l had p a s s e d , t h e s a m p l e s w e r e r e m o v e d f r o m t h e s h a k e r t a b l e and c e n t r i f u g e d a t 10002250 rpm ( 3 0 0 - 1 0 0 0 g) f o r 4-15 m i n u t e s d e p e n d i n g on the sample. The c l e a r s u p e r n a t a n t was p o u r e d o f f i n t o c l e a n p l a s t i c b o t t l e s , f i l t e r e d t h r o u g h 0.45 micron M i l l i p o r e @ or Gelman@ f i l t e r s and a n a l y z e d f o r d i s s o l v e d s i l i c a a c c o r d i n g t o t h e method d e s c r i b e d by S t r i c k l a n d and P a r s o n s ( 4 ) . F r e s h , c o l d 75 cm^ por t i o n s of the c o r r e s p o n d i n g s e a w a t e r s o l u t i o n s were a d d e d t o t h e s a m p l e s and s h a k i n g was r e s u m e d . S i l i c a s t a n d a r d s o l u t i o n s w e r e p r e p a r e d by f u s i n g a w e i g h e d amount o f p o w d e r e d q u a r t z w i t h Na£C03 i n a platinum c r u c i b l e , f o l l o w i n g a procedure d e s c r i b e d i n M a x w e l l (_5 ) . The r e s u l t i n g f u s e d m a t e r i a l was dis s o l v e d i n d i s t i l l e d - d e i o n i z e d w a t e r and d i l u t e d t o a c o n c e n t r a t i o n o f e i t h e r 10 o r 2 0 χ 10"^ m o l a r S i 0 2 « A c o u p l e o f p e l l e t s o f NaOH w e r e a d d e d t o t h e s o l u t i o n to m a i n t a i n b a s i c i t y . The s t a n d a r d s o l u t i o n s u s e d i n the a n a l y s e s w e r e d i l u t i o n s o f t h e s e S1O2 s o l u t i o n s , u s i n g 0.70 Ν NaC1 as t h e d i l u t i n g l i q u i d . A s e t of s t a n d a r d s was done w i t h e a c h s e t of a n a l y s e s , and t h e p r e c a u t i o n s o f F a n n i n g and P i l s o n (_6) a d h e r e d t o , as w e l l as t h o s e of S t r i c k l a n d and P a r s o n s (^). Those s o l u t i o n s w h i c h w e r e b u f f e r e d (pH r a n g e 7.6-7.9) had t h e pH a d j u s t e d by t h e a d d i t i o n of e n o u g h s o d i u m b i c a r b o n a t e t o make t h e s o l u t i o n 10 mM. Those s o l u t i o n s w h i c h were u n b u f f e r e d g e n e r a l l y r e m a i n e d i n t h e pH r a n g e 8 . 3 - 8 . 5 . The pH m e a s u r e m e n t s w e r e made a f t e r c e n t r i f u g a t i o n but b e f o r e f i l t r a t i o n of each sample. S p e c i f i c s u r f a c e a r e a s were g e n e r a l l y measured a t t h e b e g i n n i n g and end o f e a c h r u n . The one e x c e p t i o n to t h i s was t h e e x p e r i m e n t a l s e r i e s pH 8 . 3 - 8 . 5 , c a . 100 μΜ S i ( O H ) ^ w h e r e i n t h e s e a w a t e r s o l u t i o n s w e r e d i v i d e d r o u g h l y i n h a l f a f t e r s i x weeks. One o f t h e p o r t i o n s was c o n t i n u e d i n t h e e x p e r i m e n t u s i n g 37 t o 38 cm^ p o r t i o n s o f t h e s p i k e d s e a w a t e r . The o t h e r h a l f o f t h e s a m p l e was c e n t r i f u g e d , and t h e s o l i d w a s h e d s e v e r a l t i m e s w i t h d i s t i l l e d w a t e r and d r i e d . T h i s l a t t e r s a m p l e was t h e n u s e d t o d e t e r m i n e the "half-way" point s p e c i f i c surface area. Difficulties a t t e n d a n t w i t h t h i s procedure are d e s c r i b e d i n the results section.


416

C H E M I C A L M O D E L I N G IN

AQUEOUS SYSTEMS


Results Release Rate Curves. F i g u r e s l a , b and c show a p o r t i o n o f t h e raw d a t a i n v o l v i n g d i s s o l u t i o n and p r e c i p i t a t i o n o f s i l i c a i n pH 8 . 3 - 8 . 5 , 1-3°C seawater s o l u t i o n s f o r t h e m i n e r a l s o l i v i n e , a l b i t e and k a o l i n i t e r e s p e c t i v e l y . T h e s e f i g u r e s show t h e d i f f e r e n c e between the i n i t i a l l y a d j u s t e d v a l u e of d i s s o l v e d s i l i c a i n t h e e x t r a c t i n g s o l u t i o n and w h a t e v e r t h e v a l u e was a t t h e t i m e o f s o l u t i o n c h a n g e . This par t i c u l a r approach of u s i n g a s e r i e s of seawater e x t r a c t i o n s a l l h a v i n g t h e same i n i t i a l d i s s o l v e d s i l i c a c o n c e n t r a t i o n f o r a g i v e n e x p e r i m e n t was an a t t e m p t t o i m i t a t e the s i t u a t i o n of a g i v e n m i n e r a l suspended i n o p e n o c e a n s e a w a t e r , w h i c h has a c e r t a i n d i s s o l v e d s i l i c a value. I n t h a t t h e r e i s so l i t t l e s u s p e n d e d s i l i c a t e or a l u m i n o - s i l i c a t e m a t e r i a l per u n i t volume i n t h e d e e p o c e a n s (^7, and r e f e r e n c e s t h e r e i n ) , t h e d i s s o l u t i o n or p r e c i p i t a t i o n of such m a t e r i a l w o u l d n o t i m m e d i a t e l y h a v e a s i g n i f i c a n t e f f e c t on t h e w a t e r s u r r o u n d i n g i t . I t i s o b v i o u s t o us t h a t o u r e x p e r i ments i m p e r f e c t l y m i m i c t h e d e s i r e d s e t o f c o n d i t i o n s . A g r a p h of i n s t a n t a n e o u s c o n c e n t r a t i o n o f d i s s o l v e d s i l i c a i n the e x t r a c t i n g s o l u t i o n v e r s u s time would show a s e r i e s o f ramps a b o v e t h e i n i t i a l d i s s o l v e d s i l i c a v a l u e f o r d i s s o l u t i o n and b e l o w i t f o r p r e c i p itation. The c o n c e n t r a t i o n o f d i s s o l v e d s i l i c a i n s o l u t i o n i s always c h a n g i n g but i s p e r i o d i c a l l y r e t u r n e d t o t h e same r e f e r e n c e p o i n t a t t h e t i m e o f s o l u t i o n c h a n g e t o see i f t h e p r o c e s s o f d i s s o l u t i o n or p r e c i p i t a t i o n of d i s s o l v e d s i l i c a w i l l c o n t i n u e a t that reference value. In F i g u r e l a i t appears that the change from net d i s s o l u t i o n to net p r e c i p i t a t i o n o c c u r s somewhere b e t w e e n 200 and 475 μΚ d i s s o l v e d s i l i c a i n t h e pH r a n g e 8 . 3 - 8 . 5 . In F i g u r e l b , the c h a n g e o v e r o c c u r s n e a r 50 y^M f o r a l b i t e and i n F i g u r e 1 c , b e t w e e n 2 and 50 μΜ f o r k a o l i n i t e . The t y p e o f i n f o r m a t i o n t o be g a i n e d f r o m t h e a b o v e f i g u r e s i s u s e f u l b u t l i m i t e d u n l e s s we know t h e amount o f a v a i l a b l e r e a c t i v e s u r f a c e f o r each m i n e r a l per u n i t volume of e x t r a c t i n g s o l u t i o n . Surface Area Data. T a b l e I summarizes the s u r f a c e a r e a d a t a f o r t h e r o c k s and m i n e r a l s s t u d i e d . There are t h r e e groups of s u r f a c e a r e a v a l u e s . Those i n the column l a b e l e d " i n i t i a l " a r e those v a l u e s o b t a i n e d f r o m d r y - g r o u n d s a m p l e s w h i c h had no c o n t a c t w i t h s e a w a t e r o r any o t h e r f l u i d . Those v a l u e s i n the 5 t h and 6 t h c o l u m n s r e f e r t o numbers o b t a i n e d f r o m t h e pH 8 . 3 - 8 . 5 , c a . 100 μΜ d i s s o l v e d s i l i c a e x p e r i m e n t


Jenne; Chemical Modeling in Aqueous Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1979. Time in Weeks Jf

Figure l(a, b, c). Difference in concentration at time of solution change, μΜ Si(OH) vs. time in weeks. Note that each data point represents a solution change. Where two points are in paren theses, a single point had been taken at a two-week interval and the value halved for ready comparison with previous one-week sampling intervals. Note also the order of magnitude range in concentration differences between olivine and albite, even though the total areas available for reaction are comparable in both cases.

Time in Weeks



I 2

0. 29

Quartz (1 HF) Quartz (2 HF) Apua Pt. Lava Olivine Hyaloclastite Nepheline Albite Reticulite "Serpentine" Hornblende Anorthite Tremolite K-feldspar Diopside Quartz Epidote Orthoclase Kyanite Obsidian Hypersthene Chlorite Biotite Kaolinite Muscovite Bentonite (N.E.W.) 11lite Montmorillonite Bentonite ( C . R . C . ) Montmorillonite

0. 31 0. 43 0. 56 0. 61 0. 62 0. 63 0. 70 0. 72 0. 72 0. 73 0. 75 0. 75 0. 86 0. 88 0. 90 0. 94 0. 98 1. 1 4. 0 5. 5 8. 6 9. 4 33 34 65. 7 68. 4 75. 6

Initial

Mineral

-

-

0. 73 0.72 0.60 0.84 0.61 0.87 0.70 0.68 0.67 0.66 0.96 1.06 4.0 5.9 8.0 8.4 10.4 20 55 33

-

10.0 8.9 31 53 43

8.3

0.68 0.63 0.97 1.2 0.84 1.1 0.61 1.2 0.69 0.53 0.88 0.84 0.94 1.3 3.3

0. 77

.29 .27 0.53 -1.1

-

0.32 0.38 0.56 0.45 0.43

Ca. 1-5 μΜ (buffered)

Ca. 1-5 μΜ (unbuffered)

--

25 8.1

-

_

6.4

-

-

-

-

-

.29

(buff)

Ca. 50 μΜ (unbuffered)

-

27

7.7 25

2.3 4.0 9.5 9.3

1.44 0.72 0.90 1.08 1.13

0.93

1.20 1.20

0.99 0.66

2.7

-

_ _

-

-

-

-

-

_

-

46 43

36

-

0.82 2.5 12.7 8.2

0.88 0.50 0.52 0.74 0.47

0.59

0.63 0.51

0.39 0.38

0.22 0.53

_

Ca. 100 μΜ 6 wks 12 wks

_

-

8.4 8.2 8.7 34 33 54

_

0.97 0.89 0.81 1.16 0.81 1.23 0.77 0.61 0.84 0.55 0.97 1.1 0.80

.31 .28 0.49 1.03 0.64 0.68 0.49

Ca. 100 μΜ (buffered)

1.28(400)

_

-

9.0 --

-_

1.54

_

_

_

-

69.7(400) 44.3(400)/40.6(800)

-

16.7(400)/10.7(800)

_ _ _

3.4(400)

_ _ _ _ _

_

-

0.75

_

1.25(400)

_ _ _ _ _ _

0.63(800)

_ _

Ca. 400/800 -

-

_

0.67*

_ _ _ _

1.17 0.58

_

_

Ca. 200 μΜ (unbuffered)

S p e c i f i c s u r f a c e areas of m i n e r a l samples i n m /gm before ( i n i t i a l ) and a f t e r (others) 6-12 weeks r e a c t i o n w i t h 1-2°C, pH 7.6-8.5 seawater. The d i s s o l v e d s i l i c a c o n c e n t r a t i o n s below i n d i c a t e the nominal value i n the e x t r a c t i n g s o l u t i o n s .

TABLE


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ο

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00


21.

HURD E T A L .

Silica and Silica-Containing Minerals

419

wherein: the v a l u e s i n the 5 t h column r e f e r t o t h e h a l f o f t h e s u s p e n s i o n w h i c h was p o u r e d o f f a f t e r s i x weeks, and t h e 6 t h column r e f e r s t o the r e m a i n i n g s o l i d whose a r e a was d e t e r m i n e d a f t e r a n a d d i t i o n a l 4-6 w e e k s . The r e m a i n i n g numbers r e f e r t o s a m p l e s w h i c h had r e a c t e d w i t h the seawater s o l u t i o n s f o r a t l e a s t s i x weeks, and w h i c h were washed b r i e f l y w i t h d i s t i l l e d w a t e r , d r i e d and a n a l y z e d . T h e r e a r e many t r o u b l i n g i n c o n s i s t e n c i e s i n t h e numbers i n T a b l e I . The o b s e r v a n t r e a d e r w i l l n o t e t h a t i n many c a s e s , the a v e r a g e o f the columns 5 and 6 d a t a f o r a g i v e n m i n e r a l may be v e r y c l o s e t o t h e c o l u m n 1 v a l u e , e v e n t h o u g h t h e numbers i n c o l u m n s 2 a n d 3 a r e q u i t e d i f f e r e n t from each o t h e r . This prompted an experiment which produced the d a t a i n T a b l e I I . I n t h i s e x p e r i m e n t , two grams o f f r e s h l y g r o u n d m i n e r a l w e r e p l a c e d i n a s m a l l p l a s t i c b o t t l e w i t h 75 cm^ o f s e a w a t e r ; t h e b o t t l e a n d c o n t e n t s v i g o r o u s l y s h a k e n f o r c a . 60 seconds and a p p r o x i m a t e l y h a l f o f the s u s p e n s i o n poured o f f as r a p i d l y as p o s s i b l e . Then t h e s p e c i f i c s u r f a c e a r e a o f e a c h p o r t i o n was d e t e r m i n e d . The d a t a i n Table I I suggest that there i s a great d e a l of inhomogeneity i n f r e s h l y ground samples h a v i n g spe c i f i c s u r f a c e a r e a s o f c a . < 1 m^/gm. L a r g e r s u r f a c e a r e a s ( i . e . c a . > 20 m^/gm) a r e s e e m i n g l y much l e s s a f f e c t e d probably because of t h e longer s e t t l i n g times of t h e s m a l l e r p a r t i c l e s . These r e s u l t s suggest t h a t u n l e s s a g i v e n sample has been p r e v i o u s l y s i z e s o r t e d , s a m p l e s o f t h e s u s p e n s i o n s h o u l d n o t be p e r i o d i c a l l y removed by p o u r i n g o f f s m a l l volumes b e c a u s e the s u r f a c e a r e a o f s o l i d r v o l u m e o f s o l u t i o n may c h a n g e i n a n u n p r e d i c t a b l e manner. Y e t a n o t h e r d i f f i c u l t y a r i s e s when t h e d a t a i n c o l u m n s 2-4, 7 a n d 8 a r e c o m p a r e d w i t h c o l u m n 1 a n d t h e a v e r a g e s o f c o l u m n s 5 a n d 6. D i f f e r e n c e s among t h e f o r m e r c o l u m n s o f 100% a r e common a l t h o u g h a num b e r o f t h e m i n e r a l s show f a i r l y s m a l l s c a t t e r . This s u g g e s t s t h a t t h e v a r i a b i l i t y f r o m one s a m p l e t o t h e n e x t f o r t h e same m i n e r a l i s i m p o r t a n t , e s p e c i a l l y i n v i e w o f t h e f a c t t h a t no a t t e m p t t o s i z e s e p a r a t e t h e f i n e s t p a r t i c l e s f r o m o u r f r e s h l y g r o u n d s a m p l e s was made. We a l s o do n o t a p p e a r t o h a v e e n o u g h i n f o r m a t i o n t o c o n c l u s i v e l y s a y w h e t h e r t h e r e s h o u l d be c o n s i s t e n t increases or decreases i ns p e c i f i c surface a r e a d e p e n d i n g on w h e t h e r d i s s o l u t i o n o r p r e c i p i t a t i o n of d i s s o l v e d s i l i c a o c c u r s . I ti s l i k e l y that inters a m p l e s p e c i f i c s u r f a c e a r e a v a r i a b i l i t y swamps most o f t h e e f f e c t s o f c h e m i c a l r e a c t i o n d u r i n g o u r sam pling periods.


420

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Silica Apparent Solubilities and Rates of Dissolution and Precipitation

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