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Ch em ical G eology,

97 (1992) 101-112

Elsevier Science Publishers B.V., Amst erd am

[7]

O l i v i n e d i s s o l u t i o n k i n e t i c s at n e a r su r f a ce c o n d i t i o n s

101

Roy A. Wogelius ~ and John V. Walther

De pa rtme nt of Geological Sciences, Northwestern University, Evanston. 1L 60208, USA

(Rece ived March 29, 1991 ; revised and accept ed Novemb er 8, 1991 )

ABSTRACT

Wogelius, R.A. and Wal ther, J.V., 1992. Oli vine dissoluti on kinetics at near-surface conditions. Chem. Geol., 97: 101-

112.

Fluidized bed dissolution experiments have been conduc ted as a function o fp H at 25 C with fayalitic olivine (Fo6) in

HC1 solutions and wit h forsteritic olivine (Fo91) in solutions contain ing the organic ligand potassi um hydrogen phthal ate

(KHP ). At 25C the dissolution rate (R ) of fayalite as a function of pH is:

R (mo l cm -2 s -1 ) = 1.1-10 -1an+ o.69+ 3.2 10-14+ 1.2-10-J6aH+ -o.sl (1)

where an+ is the activity of H + in solution. The dissoluti on rate at 25 C of Fo6 at a given pH is a factor of 6 greater than

that of forsteritic olivine. The assumption that the rates increase on a molar basis with Fe content allows calculation of

dissolution rates of Fe-Mg solid solution olivines of inter mediate compositions. Batch-type dissolution experiments were

completed with

FO91

at 65C in solutions at pH 1.8, 6.0 and 9.8. The rate equatio n obtaine d from these experimen ts is:

R (mol cm -2 s -I ) =3. 5-1 0-1 al l+ o.5+ 1.0- 10-~3+6.3 10-17all+ -O.5 (2 )

When combi ned with previously published data for Fo91at 25 C, the 65 C exp erime nts indicate that the activa tion energy

of the olivine dissolution rea ction in ac idic, organic-free solutions is ~ 19 _+ 2.5 kcal. mol -~. Dissoluti on experi ments with

forsteritic olivine in solutions containing KHP at 25 C demo nstrate that the rate of dissolution is increased in these solu-

tions relative to rates measured in KHP-absent HC1-H20 solutions. Apparently, the increase in rate is caused by Mg

complexation at the olivine surface. Ligand- promoted dissolution is thought to occur in parallel with pr oton-promo ted

dissolution. Theref ore, the net rate, Rnet, is the sum of the two rates:

R.e (tool cm -2 s -1 ) =0. 8-

lO-12[Lp]45 RH

(3)

where [Lp] denotes the concentration of KHP; and RH+ denotes the proton-p romoted dissolution rate.

1 . In troduct ion

R e c e n t l a b o r a to r y s t u d i e s o n t h e d i s s o l u t i o n

r a te s o f si l ic a t e m i n e r a l s h a v e s u s t a i n e d G o l d -

i c h s ( 1 9 3 8 ) p r o p o s e d w e a t h e r i n g s e q u e n c e

a n d h a v e s h o w n t h a t t h is s e q u e n c e i s t h e r e su l t

o f th e c h a n g e i n t h e c h e m i c a l b o n d i n g e n v i r o n -

m e n t a m o n g s i l i c a t e m i n e r a l s ( s e e , e .g . , F u r r e r

a n d S t u m m , 1 9 8 3 , 1 9 8 6; H e l g e s o n e t a l. , 1 98 4 ;

B l u m a n d L a s a ga , 1 9 8 8; C a r r o l l- W e b b a n d

~Currently at Department of Earth Sciences, University

of Oxford, Oxford OX1 3PR, UK.

Walther, 1988; Brady and Walther, 1989; Casey

et al., 1989; and the references containe d

therein). Fe is the fifth most abundant ele-

ment in the Earth's crust and approximately

one-half is present in the ferrous state. Because

this valence state is unstable in the presence of

free oxygen in the Earth's atmosphere, when

minerals that contain ferrous iron dissolve the

Fe released into solution will oxidize:

4Fe+2+O2 +2H 20~ 4Fe +3+ 4OH - (1)

Thus, the interpretat ion of the dissolution ki-

netics of ferrous iron-bearing silicates must

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102 R .A . WOGELIUS AND J .V . WALTHER

c o n s i d e r t h e f e r ri c c o m p o n e n t i n so l u t i o n .

T h e r e f o r e , d i s s o l u t i o n p r o c e s s e s o f f e r ro u s

i r o n - b e ar i n g m i n e r a l s a r e o f t e n c o m p l i c a t e d b y

t h e p r e c i p i ta t i o n o f o x i d i z ed F e c o m p o u n d s o r

m i n e r a l s u r f a c e c o a t i n g s a n d b y t h e p o s s i b l e

o x i d a t i o n o f F e a t t h e m i n e r a l s u r f ac e . W e h a v e

u s e d f l o w - t h r o u g h r e a c t o r s y s t e m s t h a t d i s -

s o l v e f a y a l i t i c o l i v i n e a n d a d d f e r r o u s i r o n t o

t h e s o l u t i o n p h a s e a t a c o n s t a n t r a t e . S i n c e t h e

s o l u b il i ti e s o f f e rr ic ( h y d r ) o x i d e s a r e e x -

t r e m e l y l o w , w e a s s u m e t h a t t h e r a t e t h a t d i s -

s o l v e d f e r r o u s i r o n c a n o x i d i z e ( D a v i s o n a n d

S e e d , 1 9 8 3; M i l l e r o e t a l. , 1 9 8 7 ) c o n s t r a i n s t h e

p r e c i p i t a t i o n r a t e o f f e rr i c ( h y d r ) o x i d e s .

T h e r e f o re , b y k n o w i n g b o t h t h e o x i d a t i o n r a t e

a n d d i s s o l u ti o n ra te , w e c a n p r e d i c t h o w m u c h

F e c a n b e t r a n s p o r t e d o u t o f t h e f l u i d i z e d b e d

r e a c t o r s y s t e m a s d i s s o l v e d f e r r o u s i r o n b e f o r e

i t p r e c i p i t a t e s .

P r e v i o u s s t u d i e s h a v e d i s c u s s e d t h e r o l e t h a t

o r g a n i c a c i d s p l a y i n t h e s u r f a c e c o n t r o l l e d d i s -

s o l u t i o n r e a c t i o n . I n g e n e r a l, b i d e n t a t e o r g a n i c

l i g a n d s t e n d t o i n c r e a s e d i s s o l u t i o n r a te s b y

c o m p l e x i n g w i t h m e t a l i o n s a t t h e m i n e r a l s u r -

f ac e a n d w e a k e n i n g t h e m e t a l - o x y g e n b o n d s o f

t h e s o l i d b y b o n d p o l a r i z a t i o n ( s e e, e .g ., H u a n g

a n d K e l le r , 1 9 7 0 ; H u a n g a n d K i a n g , 1 9 72 ; A n -

t w e i l e r a n d D r e v e r , 1 9 83 ; F u r r e r a n d S t u m m ,

1 98 3 , 1 9 86 ; Z u t i c a n d S t u m m , 1 9 84 ; M a s t a n d

D r e v e r , 1 9 8 7 ) . S p e c i fi c al ly , b o t h G r a n d s t a f f

( 1 9 8 6 ) a n d W o g e l iu s a n d W a l t h e r ( 1 99 1 )

h a v e s h o w n t h a t p o t a s s i u m a c i d p h t h a l a te i n-

c r e a s e s t h e d i s s o l u t i o n r a t e o f f o r s t e r it i c o l iv -

i n e . W e p r e s e n t f u r t h e r e x p e r i m e n t s d e t a i l i n g

t h i s e f f e c t .

F i n a ll y , to b e a b l e to u n d e r s t a n d d i s s o l u t i o n

k i n e t ic s a t h i g h e r t e m p e r a t u r e s , w e p r e s e n t e x -

p e r i m e n t s a t 6 5 C . U s i n g t h e s e d a t a t o g e t h e r

w i t h p r e v i o u s l y r e p o r t e d 2 5 C d a t a ( W o g e l i u s

a n d W a l t h e r, 1 99 1 ) w e e v a l u a t e t h e t e m p e r a -

t u r e d e p e n d e n c e o f t h e o l i v i ne d i s s o l u t i o n

r e a c t i o n .

2 Experimen tal methods

O l i v in e s o f t w o d i f fe r e n t c o m p o s i t i o n s w e r e

u s e d i n t h e s e e x p e r i m e n t s : F o 9 1 ( f r o m S a n

C a r l o s, A r i z o n a , U . S . A . ) ; a n d F o6 ( f r o m t h e

F o r s y t h e I ro n M i n e , Q u 6 b ec , C a n a d a ) . B e f o re

t h e o l i v i n e s w e r e p r e p a r e d f o r d i s s o l u ti o n ,

p o r t i o n s o f al l m i n e r a l s a m p l e s w e r e a n a l y z e d

b y e l e c t r o n m i c r o p r o b e w a v e l e n g t h - d is p e r s i v e

s p e c t ro m e t r y ( W D S ) t e c h n i q u e . T a b l e 1 s h ow s

t h e m e a s u r e d c o m p o s i t i o n s o f t h e m i n e r a l s

u s e d i n t h e s e e x p e r i m e n t s . N o e v i d e n c e o f

z o n i n g o r c o n t a m i n a t i n g p h a s e s w i t h i n g r a in

b o u n d a r i e s w a s f o u n d , a l t h o u g h t h e F o 6 w a s

i n t e r g r o w n w i t h d i o p s i d e .

A l l e x p e r i m e n t a l s a m p l e s w e r e i n i t i a l l y

c r u s h e d t o m m s i ze g r a in s i n a s t ai n l e s s -s t e e l

o r a g a t e m o r t a r . F o r F O 6 a b i n o c u l a r m i c r o -

s c o p e a n d v a c u u m n e e d l e w e r e u s e d a ft e r in i -

t i a l c r u s h i n g t o s e p a r a t e o u t F o 6 f r o m t h e i n -

t e r g r o w n d i o p s i d e . A f t e r p r e l i m i n a r y c r u s h i n g

a n d s e p a r a t i o n , u n w a s h e d c r y s ta l s o f b o t h

c o m p o s i t i o n s w e re g r o u n d t o a f i ne p o w d e r i n

a n a g a t e m o r t a r a n d e x a m i n e d b y X - r a y d if -

f r a c t i o n ( X R D ) f r o m 11 t o 6 4 ( 2 0 ) . N o

c o n t a m i n a t i o n w a s d i s c o v e r e d b y t h i s t e c h -

n i q u e , i n d i c a t i n g t h a t t h e s t a r ti n g m a t e r i a l w a s

a t le a st 9 5 % p u r e . A r a n d o m s a m p l i n g o f t h e

F o 6 t o b e u s e d i n t h e s e e x p e r i m e n t s w a s a l s o

T A B L E 1

C h a r a c t e r i z a ti o n o f e x p e r i m e n t a l m a t e r i a l

O l i v i n e o x i d e w e i g h t p e r c e n t a g e s

O x i d e F o 9 L F o 6

M g O 4 9 . 2 4 2 . 4 5

F e O 9 . 2 1 6 8 . 0 6

S i O 2 4 0 . 7 2 2 9 . 8 0

N i O 0 . 3 9 0 . 0 0

M n O n . a. 0 . 7 0

T o t a l 9 9 . 5 4 1 0 1 . 1 0

n . a . = n o t a n a l y z e d .

S i e v e s i ze s a n d B E T s u r f a c e a re a s

S i e v e s i ze B E T

( / ~ m ) ( c m 2 g - 1

2 5 0 < d < 4 2 0 3 0 7 . 0

1 4 9 < d < 2 5 0 5 9 8 . 0

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1 0 4 R.A. WO GELIUS AND J.V. WALTHER

- 1 1 1

I [ 9 I i i

1 2 f

d

- 1 4

F o y o l i t e

D i s s o l u t i o n R o t e

0 )

0 , _ I , I , I , ~

1 2 3 4 5 6

p H

F i g . I . F a y a l i t i c o l i v i n e d i s s o l u t i o n r a t e s a s a f u n c t i o n o

p H i n t h e a c i d i c r e g io n . Circles a r e d i s s o l u t i o n r a t e s c a l -

c u l a t e d f r o m M g r e l e a s e , triangles a r e r a t e s f r o m S i r e -

lease , squaresa r e r a t e s f r o m F e r e l e a s e . Hexagons a r e r a t e s

c a l c u l a t e d f r o m S i e v e r a n d W o o d f o r d ' s ( 1 9 79 ) F e r e le a s e

d a t a , inverted triangles a r e r a t e s f r o m t h e i r S i r e l e a s e d a t a .

Solid symbols a r e f o r ra t e s u n a f f e c t e d b y p r e c i p i t a t i o n o r

o x i d a t i o n o f s u r fa c e F e , open symbols a r e f o r r a t e s a f -

f e c t e d b y e i t h e r o f t h o s e p r o c e s s e s . Superimposed symbols

i n d i c a t e s t o i c h i o m e t r i c d i s s o l u t i o n ( s e e te x t ) .

Sold line

r e p r e s e n t s t h e f a y a l i t e r a t e l a w d i s c u s s e d i n t h e t e x t ,

dashed

line

a p p r o x i m a t e s t h e f a y a l i te d i s s o l u ti o n r a t e l a w

m i n i m u m .

s p h e r e , a l o n g w i t h d a t a f r o m t h e b a t c h e x p e r i -

m e n t s o f S ie v e r a n d W o o d f o r d ( 1 9 7 9 ) f o r F o 4 .

T h e l o g a r i t h m o f t h e m e a s u r e d r a te , n o r m a l -

i z e d p er 4 g- at . o f o x y g e n ( m o l e o f o l i v i n e ) p e r

c m 2 p e r s e c o n d , i s s h o w n a s a f u n c t i o n o f p H .

C i r c l e s a r e r a t e s c a l c u l a t e d f o r o u r e x p e r i -

m e n t s f r o m M g r e l e a s e , t r i a n g l e s f r o m S i r e -

l e a s e , a n d s q u a r e s f r o m F e r e l e a s e . H e x a g o n s

a r e r a t e s c a l c u l a t e d f r o m F e r e l e a s e a n d i n -

v e r t e d t r i a n g l e s a r e f r o m S i r e l e a s e f o r S i e v e r

a n d W o o d f o r d s e x p e r i m e n t s a t p H 4 . 5. O u r

e x p e r i m e n t s w e r e c o m p l e t e d i n s i m p l e H C 1 -

H 2 0 s o l u t io n s , t h o s e o f S i ev e r a n d W o o d f o r d

u s e d a c e ti c a c i d - l i t h i u m a c et a te m i x t u r e s o f 0 . 2

M c o n c e n t r a t i o n . S o l i d s y m b o l s f o r o u r f lu i -

d i z e d b e d d a t a r e p r e s e n t r a t e s m e a s u r e d d u r -

i n g an i n i ti a l p e r i o d o f d i s s o l u t i o n o f < 5 0 0 h r

a n d s o l i d s y m b o l s f o r t h e S i e v er a n d W o o d -

f o r d d a t a a r e f o r d i s s o l u t i o n d u r i n g a n i n i t i a l

6 5 0 - h r p e r i o d i n a d e o x y g e n a t e d a t m o s p h e r e .

F o r o u r e x p e r i m e n t s t h e o p e n s y m b o l s r e p r e -

s e n t m i n i m u m r at es m e a s u r e d a f te r a t le a st 5 0 0

h r o f r e a c ti o n a n d f o r S ie v e r a n d W o o d f o r d s

b a t c h d a t a t h e o p e n s y m b o l s g i v e t h e r a t e c a l -

c u l a t e d a f t e r 1 5 0 h r o f l in e a r r e l e a s e i n a n e x -

p e r i m e n t co m p l e t ed w i t h P o 2 = 0 . 2 a t m . S u-

p e r i m p o s e d s y m b o l s f r o m th e s a m e e x p e r i m e n t

i n d i c a t e s t o i c h i o m e t r i c d i s s o l u t i o n . N o t e t h a t

d i s s o l u t i o n i s s t o i c h i o m e t r i c i n a ll e x p e r i -

m e n t s e x c e p t t h e b a tc h e x p e r i m e n t a t r e d u c e d

P o 2 - T h e s o l i d l i n e w a s o b t a i n e d b y a r e g r e s -

s i o n t h r o u g h a l l t h e s o l i d s y m b o l s e x c e p t f o r

t h e r a t e c a lc u l a t e d f r o m S i r el e a se i n t h e S i e v e r

a n d W o o d f o r d e x p e r i m e n t . T h e d a s h e d l in e

r e p r e s e n t s o u r e s t im a t e o f t h e d i s s o l u t i o n r a te

- 1 0 . . . . . . . r ~ T

- 1 1 - 12) H 3 IIIIIII~i-;I

0 , 4 - 1 4 i i i

i o s o o 1 o o o 1 s o o 2

E - 1 0

l

--~ - 1 1 . . i l _ r _ . i L i a _ i 4 1 1 1 _ l l .

- 1 2 i 1

-I 3 I 6) pH 2

- 1 4 i i I I

0 1 O 0 2 0 0 3 0 0 4 0 0 5 0 0

I ~ - 1 0

- 1 1

- 1 2 r - ~ l ~ @ l - l r i g O

- 1 3 . L - 2 0) p H 2 0 0 0 O @ I i

1 4 1

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0

t i m e h r s )

F i g . 2 . F l u i d i z e d b e d e x p e r i m e n t r e s u lt s a s a f u n c t i o n o

t i m e f o r F o 6 d i s s o l u t i o n i n H C 1 .

Circles

a r e d i s s o l u t i o n

r a t e s b a s e d o n M g r e le a s e ( e x p e r i m e n t 20 o n l y ) , triangles

a r e r a t e s b a s e d o n S i re l e a s e , squares a r e r a te s b a s e d o n F e

r e l ea s e . W h e n d i s s o l u t i o n i s s t o i c h i o m e t r i c symbols a r e

superimposed

( s e e t e x t ) .

IIorizontal lines

a r e b e s t - f i t

s t e a d y - s ta t e d i s s o l u t i o n r a t e s f o r r e a c t i o n t i m e s o f < 5 0 0

hr . Solid lines a r e c a l c u l a t e d f r o m M g r e l e a s e , long-dashed

lines a r e c a l c u l a t e d f r o m S i r e le a s e , a n d short-dashed lines

a r e c a l c u l a t e d f r o m F e r e l e a s e . Numbers in parentheses

c o r r e s p o n d to e x p e r i m e n t n u m b e r : ( 1 2 ) = e x p e r i m e n t 12

a t p H 3 , n o t e d e c r e a s e i n r a t e a f t e r ~ 60 0 h r o f r e a c t i o n

w i t h m i n i m u m a t l o g R = - 1 3 .5 ; ( 1 6 ) = e x p e r i m e n t 1 6 a t

p H 2 , n o t e s t o i c h i o m e t r i c r e l e a s e a n d s t e a d y - s t a t e d i s s o -

l u t i o n m a i n t a i n e d u p t o ~ 4 50 h r; a n d ( 2 0 ) = e x p e r i m e n t

20 a l s o a t p H 2 , r a t e i s t h e s a m e a s e x p e r i m e n t 16 f o r

5 0 0 h r ., t h e n d e c r e a s e s b y t w o o r d e r s o f m a g n i t u d e .

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O L I V I N E D I S S O L U T I O N K I N E T I C S A T N E A R - S U R F A C E C O N D I T I O N S 1 0 5

in the pH range 2-6 for long-term experiments

conduct ed in conta ct with atmospheri c Po2.

Fluidized bed rate measurements are plot-

ted vs. time in Fig. 2 with circles, triangles and

squares cor responding to rates calculated from

Mg, Si and Fe release, respectively. The lines

represent the best-fit steady-state olivine dis-

solution rate calculated from Mg release (solid

line), Fe release (short d ashes) and Si release

(long dashes). For both experiment 12 (pH 3 )

and experiment 20 pH 2) a period of steady-

state dissolution persists of ~ 500 hr followed

by a period during which the rate decreases

significantly. Experiment 16, also at pH 2,

shows a period of 500 hr of steady-state stoi-

chiometric dissolution at essentially the same

initial rate meas ured in experimen t 20.

3.2. Interpretation

Solubility calculations done using the

E Q 3 N R

computer code (Wolery, 1983 ) for the experi-

mental fluids that are in equilibrium with at-

mospheric Po2 show that they are all supersa-

turated with at least one ferric iron-bearing

solid phase such as hematite or goethite. Thus

we must conside r the kinetics of Fe oxidation

and precipitation in these experiments. The

rate of oxidation of ferrous iron to ferric in so-

lution has been studied by Davison and Seed

(1983), and Millero et al. (1987). Davison

and Seed ( 1983 ) obta ined the following equa-

tion for the rate of oxidation (Rox) of Fe in

dilute solutions at 25 C:

Rox ( mol l -1 s- l) =

3.33-10 ~ [Fe( II) ]Po2 (O H- )2 (2)

where [ Fe (II) ] is the mola rity of ferrous iron;

Po2 is the partial pressure of oxygen in atmo-

spheres; and ( O H - ) is the activity of hydrox-

ide in solution. Similar experiments by Millero

et al. (1987) confirm that the depend ence of

the oxidation rate on the square of the hydrox-

ide activity is valid in dilute solutions to pH-

rate values as low as 5 and may be applicable

at even lower pH. The rate of addition of

Fe (II) to our experime ntal fluids from fayalite

dissolution is at least two orders of magnitude

faster than the rate at which the Fe can be oxi-

dized and then presuma bly precipitated out of

solution in our fluidized bed apparatus. For

example, at pH 3, the dissolution rate of fay-

alite is 10-12 mol

c m - 2 S - 1 .

In our experimen-

tal set-up, this corresponds to a steady-state Fe

concentr ation of 10 -4 mol 1-~. The residence

time of fluid in the reactor is ~ 5- 104 s. To be

conservative in the following application of the

oxidation rate law, we assume that the mini-

mu m rate of oxidation is obtained at pH 5 and,

therefore, use a hydroxide activity of 10-9 M

in this calculation. Given the above oxidation

rate law with an Fe concen tra tion of 10-4 M at

atmospheri c partial pressure of 02 (0.2 atm. )

the rate of Fe oxidation for solutions with

pH ~< 5 in the fluidized bed is calculat ed to be

7-10-12 mol 1-~ s -~. For the residence time

given, the calcula ted oxidation rate could con-

vert 3.5.10 -7 mol 1-1 Fe(I I) to Fe(I II ) before

the Fe leaves the system, only 0.35% of the to-

tal Fe in solution. I f the oxidation rate con tin-

ues to decrease as a function of ( OH -) 2 as pH

decreases below 5, then the rate of Fe oxida-

tion at pH 3 will be even slower and the per-

cent of Fe oxidized while in the re actor system

will be even smaller. Therefore, simple oxida-

tion resulting in precipitation of a ferric iron

phase as the fayalite dissolves is too slow a re-

action to decrease the observed dissolution rate

by the amounts observed in our experiments.

Presumably, this is why the rates are unaf-

fected for several hundred hours in the flui-

dized bed reactor; the amount of Fe oxidized

and precipitated out of solution is small com-

pared to the total Fe concentration. Appar-

ently, after an extended period of time, a sig-

nificant amount of Fe on the mineral surface

becomes oxidized or enough ferric-oxide or

-hydroxide has precipitated on the surface to

decrease the number of rapidly reacting sur-

face sites and thereby decrease the dissolution

rate. Clearly, from the data p rese nted in Fig. 2,

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106 R.A. WOGELIUS AND J.V. WALTHER

the rates established in the short term begin to

decrease after 500 hr. White and Yee's ( 1985 )

experiments on Fe-bearing silicates showed

that aqueous Fe 3+ is reduced by ferrous iron

exposed at silicate surfaces. In their long-term

experiments with hornblende, reduction at the

mineral surface of Fe 3+ in solution is rapid and

the release of unoxidized Fe 2+ from the min-

eral surface to the solut ion at low pH slows after

~ 500 hr. When the dissolved Fe 3+ is reduc ed

at the silicate surface, a ferric ion is created in

the coordination environment of the silicate

surface. Presumably in our experiments sur-

face Fe also oxidizes and thus over time a dif-

ferent surface enviro nment is presented for in-

teraction with the fluid and the fundamental

dissolution reaction of protonation. The oxi-

dized surface apparently hydrolyzes more

slowly and thus gives a decreased dissolution

rate. Siever and Woodford's experime nt at pH

4.5 with Po := 0. 2 atm. rapidly (t < 150 hr)

gave rates similar to the decreased rates mea-

sured in our exper iments after 500 hr at lower

pH and at the same Po2- Their deoxygenated

experiment gave a long-term Fe release rate

that is consistent with our < 500 hr measured

rates. The lower than stoichiometric Si release

in their deoxygenated experi ment probably in-

dicates precipitation of a silicate phase since

they completed this e xperiment in a fluid su-

persaturated with several SiO2 polymorphs.

Because the Siever and Woodford oxygenated

experiment shows similar rates with a differ-

ent experimental set-up, we conclude that these

results indicate a change in the rate cont rolling

reaction with time. It is interesting to note that

throughout the acidic pH range for times of

< 1200 hr the dissolution rates calculated from

Mg, Si and Fe release decrease to about the

same value. These nearly pH-i ndepe ndent re-

lease rates probably represent the minimum

dissolution rate for oxidized fayalitic olivine

surfaces in aqueous fluids at 25 C.

The rate of fayalite dissolution in a deoxy-

genated atmosphere at a given pH in the pH

interval 2-7 and at 25C is six times greater

than that of forsterite (Wogelius and Walther,

1991 ). For forsterite in the pH range 2-12 we

have:

Rvo mol cm - 2 s- ~ =

9.07.10-12an+ o54+ 5.25- 10-15

+2.33 10-1vaH+ -o3J (3)

whereas for fayalite at acidic to neutra l pH we

can write:

Rva mol c m- 2 s- l ) =

1.1- 10-1an+ 069+3.22 10 -14 (4)

If the pH dependence of dissolution rates is

similar for all silicates in basic solutions (Brady

and Walther, 1989), then we can extend the

fayalite dissolution rate law out to pH 12 by

adding this depe nden ce at high pH to obtain:

Rva

(m ol cm -2 s - ~ ) = 1.1-10-~a~+ 69

+ 3.22 .10- 14+ 1.2.10-16all+ 03 (5)

We can interpolate between eq. 3 and eq. 5 to

calculate rates for olivines of intermediate

composition along the Fo-Fa solid solution

join by assuming that the rate can be obtained

by summing the end- membe r rates on l-mol

fraction basis:

R o l m o l c m - Z s - l ) = X v a R v a + X v o R F o (6)

where Xva and Xvo denote the fayalite and for-

sterite mole fraction of the olivine, respec-

tively. The total amount of Fe oxidized per unit

time, as given in eqn. 2, is a function o f the

fluid volume. In order to calculate the relative

rate of dissolution to oxidation the solid sur-

face area to fluid volume ratio

S / V )

must be

known. For stoichiometric dissolution the rate

of Fe release is:

Ro = 2XvaRo, S / V) (7)

The relative rate of Fe released to Fe oxidized

will then simply be

R J R o ~ .

A positive relative

rate indicat es that in a flow-through system the

concentr ation of Fe( II ) in solution will equal

a steady-state level and thus allow transport of

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OLIVINE DISSOLUTION KINETICS AT NEAR-SURFACE CONDITIONS 107

n -

U

n , ,

2 )

0

14

12

10

8

6

2

0

2

I I I I I I

I I [ I I I 4

2 3 4 5 6 7 8

pH

Fig. 3. Comparison of Fe release rate from olivine disso-

lution,

Rd,

to rate of Fe oxidation, Rox, in dilute fluids

(Davison and Seed, 1983).

Solid circles

give the relative

rate calculated for our fluidized bed experiments,

solid

hexagon

gives the relative rate calculated for Siever and

Woodford's ( 1979 ) Po~ = 0.2 atm. experiment. The solid

curvejoining the symbols represents a solid surface area

to fluid volume ratio S/V) contour of 104 cm 21 -~.

Fe away from the reacting olivine surface. For

our laboratory experiments the parameters

necessary to complete relative rate calcula-

tions are well chara cteriz ed; the surface area to

volume rate of the system is on the order of 10 4

cm 2 1 ~, the Fe co ncentr ations range between

10 -6

and

10 -4 M,

and we assu me a Po2 o f 0.2

atm.

The results of relative rate calculations for

the fayalite dissolution experiments are pre-

sented in Fig. 3 as the log of the relative rate

vs. pH. We show the relative rates arrayed

along the 104-cm2-1-1 S~ V contour. The solid

circles are our expe riments at pH's 2 and 3, the

hexagon is for Siever and Wo odfor d's data. As

this diagram clearly shows, the fluidized bed

experiments were completed in an environ-

ment where oxidation kinetics are sluggish

compared to dissolution. Only above pH 6

would Fe oxidation become approximately

equal to the rate of addit ion of Fe( II) to

solution.

4 F o r s t e r i t e d i s s o l u t i o n a t 6 5 C

Our previous discussion focussed on how

adding Fe to the olivine solid solution effects

the rate. Now, we turn to some preli minary ex-

periments that explore how the dissolution rate

of forsteritic olivine varies as a function of

temperature.

4.1. Results

FOgl was

dissolved at 65C by batch tech-

nique in buffere d pH 1.8 and 9.8 solutions and

in an unbuffered pH 6.0 solution. Changes in

concentration over t ime for these experiments

are plotted in Fig. 4 and listed along with the

7

E

1 2 0

1 0 0

80

60

20

i r i

o

5 4

o i L d L - - - - r - - - - ~ - - - 4

0

0 - - 5 1 0 1 5 2 0 2 5 3 0 O - - 5 1 0 1 5 2 0 2 5

t i m e h r )

I I I I

2O

0 i t I . I I I

0 5 10 15 20 25

f l m e h r )

Fig. 4.

FOgl

batch experiments at 65C showing concentration increasing linearly as a function of time.

Circles

are Mg

concentration;

triangles

are Si concentration.

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1 08

T A B L E 2

Fo91

b a t c h d i s s o l u t i o n e x p e r i m e n t s a t

6 5 C

( r a t e s i n m o l

c m - 2 s - ~ )

T i m e ( 1 0 - S m o l l - ~ )

( h r . )

[ M g ] [ S i ]

E x p e r i m e n t

14

u n b u f f e r e d , p H 6 :

0 . 5 0 . 5 2 0 . 0 0

1 .0 0 . 7 0 0 .1 8

3 .9 1 .75 0 .42

25.1 16.9 1.49

l og r at e - 1 2 . 1 0 - 1 2 . 1 3

E x p e r i m e n t

15

K C 1 - HC 1 b u f f e r , p H 1 .8 :

1 .2 5 .62 3 .52

2.8 14 .3 8 .40

3 .8 20.3 12.5

20 .1 1 04 .0 7 0 .2

log ra te - 10.35 - 10.29

E x p e r i m e n t 1 6, N a O H - N a - b o r a t e b u f fe r , p H 9 .8 :

1 .2 0 . 1 6 0 . 00

3 .8 1 .57 0 .00

20 .1 1 3 .6 0 . 00

log ra te - 11 .38

I

0~

0 4

I

E

o

E

p

0~

o

- 9

- 1 1

- 1 3

- 1 5

J

F o r s f e r T f e D i s s o l u t i o n R G t e s

6 5

C . . '

,,,,,,

. Q . - , = 0 s

I I l ~ l l l ~ l

2 4 6 8 1 12

pH

Fig . 5 .

Fo91

d i s s o l u t i o n r a t e s a t 6 5 C c o m p a r e d t o t h e

2 5 C d a t a .

S y m b o l s

s a m e a s F ig . 2 a b o v e .

S o l i d l i n e

g i v e s

2 5 C r a t e l a w w i t h s l o p e o f 0 . 31 i n b a s ic r e g i o n ,

d a s h e d

l ine

g i v e s 6 5 C r a t e l a w w i t h s l o p e o f 0 . 5 i n b a s i c r e g i o n .

c a l c u l a t e d d i s s o l u t i o n r a t e s i n T a b l e 2 . N o S i

w a s d e t e c t e d in e x p e r i m e n t 16 ( p H 9 .8 ), w h i c h

s u g g e s ts p r e c i p i t a t i o n o f a s i l ic a t e p h a s e . B e -

c a u s e th i s p h a s e m a y c o n t a i n M g a s w e ll , t h e

r a te c a l c u l a t e d fr o m M g r e le a s e is a m i n i m u m .

R . A . W O G E L I U S A N D J .V . W A L T H E R

C a l c u l a t i o n s w i t h E Q 3 N R a s s u m i n g S i i s p r e s -

e n t a t c o n c e n t r a t i o n s j u st b e l o w t h e l o w e r l i m i t

o f d e t e c t i o n o f th e D C P s h o w t h a t t h i s f lu i d is

s a t u r a t e d w i t h a n t i g o r i te a n d b r u c i te . F l u i d p H

g i v e n i n T a b l e 2 f o r t h e s e e x p e r i m e n t s w a s

m e a s u r e d a t 2 5 C f o r e a c h r u n a n d c a l c u l a te d

a t 6 5 C w i t h E Q 3 N R .

4 2 Comparisons

T h e r a t e s g i v e n i n T a b l e 2 a r e p r e s e n t e d i n

F i g . 5 a s a f u n c t i o n o f p H . C i r c l e s a r e r a t e s c a l-

c u l a t e d f r o m M g r e le a s e , t r ia n g l e s a r e r a t e s c a l-

c u l a t e d f r o m S i r e l e a s e . T h e l o w e r s o l i d l i n e

r e p r e s e n ts t h e r a t e l aw d e r i v e d f r o m m e a s u r e -

m e n t s a t 2 5 C f o r F o 9 1 a n d F o m o ( W o g e l i u s

a n d W a l t h e r , 1 99 1 ) . T h e d a s h e d c u r v e r e p r e -

s e n t s t h e r a t e a s a f u n c t i o n o f p H c o n s t r u c t e d

f r o m o u r 6 5 C d a t a a n d t h e l og r a t e v s. p H d e -

p e n d e n c e o f 0.5 o f B r a d y a n d W a l t h e r ( 1 98 9 )

f o r s i l ic a t e d i s s o l u t i o n i n b a s i c - p H s o l u t i o n s a t

6 5 o C . T h e r e f o r e , w e w r i t e t h e d i s s o l u t i o n r a t e

l a w f o r f o r s t e r i t i c o l i v i n e a t 6 5 C a s:

R (m o l cm -2 s - 1 = 3 .5 - 1 0 - 10a ll+ O.5

+ 1.0 10-13..{_ 6.3 10 -17 all+

- 0 . 5 ( 8 )

B e t w e e n 2 5 a n d 6 5 C t h e p H d e p e n d e n c e o f

t h e r a t e u n d e r a c i d i c c o n d i t i o n s d o e s n o t a p -

p e a r t o c h a n g e s i g n i fi c a n tl y j u d g i n g f r o m t h e

p H 2 a n d 6 e x p e r i m e n t s . T h i s is c o n s i s t e n t w i t h

s i m i l a r f i n d i n g s b y S ch o t t e t a l. ( 1 98 1 ) f o r M g -

b e a r i n g p y r o x e n e s b e t w e e n 2 0 a n d 5 0 C .

W e c a n u s e t h e s e d a t a t o e s t i m a t e a n a c t i v a -

t i o n e n e r g y , E a, a t c o n s t a n t p H f o r t h e o l i v i n e

d i s s o l u t i o n r e a c t i o n t h r o u g h t h e A r r h e n i u s

r e l a t i o n s h i p :

2 . 3 0 3 R ( l o g R ~ - l o g R 2 )

E a ( k c a l. m o l - 1 =

7 2 - L - - T 1 - I

(9)

w h e r e R is th e g a s c o n s t a n t i n c a l. m o l - ~ K - ~;

R i is th e d i s s o l u t i o n r a t e a t t e m p e r a t u r e T i i n

k e l v i n s; a n d E a is th e a c t i v a t i o n e n e r g y f o r t h e

d i s s o l u t i o n r e a c t i o n . A n a c t i v a t i o n e n e r g y o f

1 9 + 2. 5 k c a l . m o l - ~ i s c a l c u l a t e d f o r t h e f o r -

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O L I V I N E D I S S O L U T I O N K I N E T I C S A T N E A R -S U R FA C E C O N D I T I O N S [ 0 9

s t e r i t i c o l i v i n e d i s s o l u t i o n r e a c t i o n i n s o l u -

t i o n s w i t h a c i d i c to n e a r - n e u t r a l p H .

5. Forsterite dissolution in potassium hydrogen

p h t h a l a t e K H P )

5 1 R e s u l t s

F o g l w a s d i s s o l v e d i n t h e f l u i d i z e d b e d r e -

a c t o r i n t h e p r e s e n c e o f t h e o r g a n i c l i g a n d p o -

t a s s i u m h y d r o g e n p h t h a l a t e ( K H P ) a t 1 0 - 2.3

M . T a b l e 3 s h o w s t h e c a l c u l a t e d p r o t o n - p r o -

m o t e d a n d o r g a n i c l i g a n d - p r o m o t e d d i s s o l u -

t i o n r a t e s o b t a i n e d f r o m t h e m e a s u r e d t o t a l

d i s s o l u t i o n r a t e f o r f o r s t e r i t i c o l i v i n e a l o n g

TABLE 3

Fo9] KH P-pro mote d dissolution rates at 25 C (rates in 10-~3 mol

CITI-2 S I )

l og p H M e a s u r e d r a te H + r a te K H P - p r o m o t e d

[ KH P ] rate

M g S i F e M g S i F e

- 1. 3 3.0 5.13 3.23 3.02 2.23 2.9 1.0 0.79

- 1.3 4.0 3.78 1.88 1.68 0.68 3.1 1.2 1.00

- 1 . 3 5.0 1.83 1.93 n.d. 0.23 1.6 1.7 n.d.

- 2. 3 4.2 1.74 1. 31 n.d. 0.54 1.2 0.81 n.d.

n . d . = n o da t a .

- 1 2

0 3

t N

I

o - 1 3

-6

E

d

~ -1 ,*

o

I , [ ~ I L I

1 2 3 4

- l o g K H P )

F i g. 6. K H P - p r o m o t e d o l i v i n e d i s s o l u t i o n r a te s d e r i v e d

f r o m G r a n d s t a f f ( 1 9 8 6 ) o p e n s y m b o l s ) a t p H 4 . 5 c o m -

p a r e d t o o u r d a ta a t p H 5 f i l led sym bols ) . Circ les a r e r a t e s

c a l c u l a t e d f r o m M g r e l e a s e ,

triangles

a r e r a t e s c a l c u l a t e d

f r o m S i r e le a s e . N o t e l i n e a r d e p e n d e n c e o f lo g r a t e o n l o g

o r g a n i c l i g a n d c o n c e n t r a t i o n .

w i t h r e s u l t s a t 1 0 -1 .3 M f r o m W o g e l i u s a n d

W a l t h e r ( 1 99 1 ). G r a n d s t a f f ( 1 9 8 6 ) a l s o c o m -

p l e t e d e x p e r i m e n t s o n o l i v i n e d i s s o l u t i o n i n

s o l u t i o n s c o n t a i n i n g K H P . T h e a b s o l u t e v a l-

u e s o f G r a n d s t a f f ' s d a t a d i f f e r f r o m o u r s d u e

t o d i f f e r e n c e s in c a l c u l a t e d s u r f a c e a r e a s ( s e e

M u r p h y , 1 9 85 ) . T o c o m p a r e d a t a s et s w e h a v e

d e c r e a s e d h i s s u r f a c e a r e a s b y a f a c t o r o f 2 0

w h i c h i n c r e a s e s h i s d i s s o l u t i o n r a t e s b y a f a c -

t o r o f 2 0 . Fi g. 6 c o m p a r e s G r a n d s t a f f ' s a d -

j u s t e d r e s u l ts ( o p e n s y m b o l s ) a s a f u n c t i o n o f

t h e n e g a t i v e l og o f t h e K H P m o l a r i t y o f t h e s o -

l u t i o n w i t h s o m e o f o u r d a t a ( f il l e d s y m b o l s )

a t - l o g ( K H P ) = l . 3 . C ir cl es a n d t ri an g le s

r e p r e s e n t M g r e l e a s e a n d S i r e l e a s e , r e s p e c -

t i v e l y . G r a n d s t a f f ' s d a t a i n t h i s f i g u r e a r e a ll a t

p H 4 .5 w h i le o u r m e a s u r e m e n t w a s m a d e a t p H

5 ; h o w ev e r , t h e r e s u l t s s t i l l ag r ee c l o s e l y .

R e g r e s s io n a n a l y si s o f G r a n d s t a f f ' s d a t a f r o m

b e t w e e n l 0 - 4 a n d 1 0 -x M K H P g i ve s a l og r a te

vs . - l o g ( K H P ) d e p e n d e n c e o f - 0 . 4 5 a s

s h o w n b y t h e s o l i d l i n e in F i g . 6. S i n c e t h e p H

o f o u r e x p e r i m e n t s is e q u a l to 5 .0 r a t h e r t h a n

4 . 5 w e d i d n o t u s e o u r d a t a i n t h e r e g r e s s i o n

c a l c u l a t io n . I f t h e l i g a n d - p r o m o t e d a n d p r o -

t o n - p r o m o t e d d i s s o l u t i o n r e a c t i o n s a r e p a r a l -

l el , t h e n t h e o v e r a l l r a t e c a n b e w r i t t e n a s a s u m

o f th e o r g a n i c f r e e r a t e l a w a n d t h e o r g a n i c -

p r o m o t e d r a t e l aw . T h e r e f o r e , t h e r a te l a w fo r

f o r s t e ri t ic o l iv i n e d i s s o l u t i o n w i t h K H P p r e s -

e n t i n s o l u t i o n a t m o l a r i t i e s a t ~ 1 0 - x is:

R F o (

t oo l c m -

2 S - 1 ) =

0 . 8 - 1 0 - 1 2 [ L p ] 0.45

+ 9 . 0 7 - 1 0 - 1 2 a l l +

0 . 5 4 +

5.25 10 - 15

+ 2 . 3 3 1 0 - J 7 a H +

--0.31

(lO)

w h e r e

[ L p ]

d e n o t e s t h e m o l a r c o n c e n t r a t i o n

o f p h t h a l at e . I n F ig . 7 w e c o m b i n e G r a n d -

s t a ff ' s r a te v s. c o n c e n t r a t i o n o f K H P d e p e n -

d e n c e w i t h t h e r e m a i n d e r o f o u r d a t a t o p r es -

e n t t h e f o r s t e r i t e d i s s o l u t i o n r a t e l a w a s a

f u n c t i o n o f p H c o n t o u r e d f o r K H P m o l a li ty .

T h e s e a r e o u r K H P r e su l ts , w i t h t h e s o l id s y m -

b o l s a t l o g ( K H P ) = - 1 .3 a n d t h e o p e n s y m -

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l l 0 R.A. WO GELIU S AND J.V. WALTHER

I

- 1 2

E

o

- 1 3

E

E

- 1 4

I I I I

K P

_ _ _ i _ ~ _ ~ ~ 1 1

2 3 4 5 6

1 0 - 1 . 3

1 0 - z - 3

1 0 - 3 - 3

1 0 - 4 3

pH

F ig . 7 . D e p e n d e n c e o f o l i v i n e d i s s o l u t i o n r a t e l aw o n m o -

l a ri ty o f K H P a n d s o l u t io n p H .

Ope n circle

a n d

open tr i

angle

g i v e t h e r a t e s c a l c u l a t e d f r o m M g r e l e a se a n d S i re -

l e a s e, r e s p e c t i v e l y , i n a 1 0 - 2 .3 M s o l u t i o n o f K H P .

S o l i d

sym bols

a r e d a t a a t 1 0 - ~3 M K H P ( s e e T a b l e 4 ) . T h e

solid

l ine

i s t h e r a t e l a w f o r f o r s t e r i t i c o l i v i n e a t 2 5 C w i t h o u t

o r g a n i c s i n s o l u t i o n , t h e

d a s h e d l in e s

g i v e t h e r a t e s f o r s o -

l u t io n s w i t h t h e i n d i c a t e d a m o u n t s o f K H P i n so l u ti o n .

bols at log(KHP)=-2.3. Again, circles and

triangles are for Mg and Si release, respec-

tively. Note that the rate law derived from Fig.

6 is consis tent with our exper iments at 10- ~.3

and 10-23 MKHP.

The 0.45 dependence of the log of the rate

on the log of the conc entrati on of KHP is sim-

ilar to the 0.6 slope measure d for olivine in as-

corbic acid and the slope of ~0.5 5 for olivine

as a funct ion of all+ in acidic solutions at 25 C

(Wogelius and Walther, 1991; Blum and Las-

aga, 1988, respectively). Our ligand-promoted

mechanism, which we suggest occurs by the

complexation of the organic compo und with a

single Mg exposed at the solid surface, predicts

a 0.5 dependenc e for the log of the dissolution

rate on the log of the ligand activity.

6 C o n c l u s i o n s

Measured dissolution rates of fayalitic oliv-

ine in experiments unaffe cted by oxidation or

precipitation are approximately a factor of 6

greater than those meas ured for forsterite at the

same pH. Fayalite does show a depende nce on

the activity of H + similar to that of forsterite

at acid pH, suggesting that the mec hani sm of

dissolution may be similar for both end-m em-

ber compositions even though the magnitudes

of the rates differ. In contrast to our laboratory

experiments, at the neutral to basic pH levels

(5-9) and low mineral surface area to fluid

volume ratios of most natural surface aquatic

systems, the dissolution of olivines with high

fayalite content, while rapid in relation to most

other silicates, will not add ferrous iron to so-

lution fas ter than the rate at which tha t Fe will

be oxidized at present atmospheric Po2. Even

at pH 5, surface area to volume relationships

would need to be greater t han 1000 cm 2 of ol-

ivine per liter of fluid in order for the kinetic

balance to shift toward transport instead of

immediate oxidation and precipitation. This

balance shifts further towards the immobili-

zation of Fe if in situ oxidation of Fe(II) oc-

curs at the olivine surface and decreases the

rate of the hydrolysis reaction, as apparently is

the case. Exper iments indicate that even at pH

as low as 2, where the initial dissolution rate is

orders of magnitude faster than the oxidation

rate, the precipitation of secondary Fe(III)-

bearing phases (or in situ oxidation of surface

Fe) may eventually interfere with the dissolu-

tion reaction. Thus in most near-surface

aqueous fluids Fe transport away from olivine

before precipitation as an (hy dr) oxid e will be

minimal. This is consistent with the ubiqui-

tous Fe-oxide phases found along cracks and

grain boundaries o f most natural olivines that

have been in contact with aqueous fluids. Our

results with fayalitic olivine, when compared

to previous results from forsteritic olivine,

show that increasing the Fe content of olivine

increases the dissolution rate of olivine. Be-

cause the detachment rate of the octahedral

cations apparently controls the dissolution rate

of silicates in solutions with acidic pH (Brady

and Walther, 1989), these measurement s sug-

gest similar dependencies may exist for other

Fe-bearing silicate solid solutions. By analogy,

if substituting Fe for Mg in the octahedral site

increases the dissolution rate for olivine, the

same result may hold true for other Fe-Mg

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11/12

OLIVINE DISSOLUTION KINETICS AT NEAR-SURFACE CONDIT IONS 1 ] l

s o l i d s o l u t i o n s s u c h a s e n s t a t i t e - f e r r o s i l i t e a n d

d i o p s i d e - h e d e n b e r g i t e .

A d i s s o l u t i o n r a t e d e p e n d e n c e f o r f o r s t e r i t i c

o l i v i n e o n t h e c o n c e n t r a t i o n o f s e ve r a l o r g a n ic

l ig a n d s t o b e t w e e n t h e 0 . 4 a n d 0 . 6 p o w e r s u p -

p o r t s t h e i d e a t h a t t h e i n c r e a s e o f t h e r a te o c -

c u r s b y s u r fa c e M g c o m p l e x a t i o n . A b s o l u t e

v a l u e s o f c h e la t e d r a t es m u s t b e d e t e r m i n e d b y

e x p e r i m e n t s f o r s p e c i f i c l i g a n d s , b u t w e p r e -

d i c t t h e f u n c t i o n a l d e p e n d e n c e o n t h e s u r f a c e

a c t i v i t y o f t h e l i g a n d s h o u l d b e u n i v e r s a l t o o r-

g a n i c s t h a t i n c r e a s e t h e r a te b y 1 : 1 c o m p l e x i n g

w i t h M g .

A s e x p e c t e d , b a t c h e x p e r i m e n t s a t 6 5 C o n

f o r s t e r i t i c o l i v i n e d e m o n s t r a t e t h a t t h e o l i v i n e

d i s s o l u t i o n r e a c t i o n i s s t r o n g l y t e m p e r a t u r e

d e p e n d e n t . A c t i v a t i o n e n e r g i e s 1 9 k c a l.

m o l - l ) f o r f o r s t er i ti c o l i v i n e d i s s o l u t i o n i n

a c i d i c a n d n e u t ra l s o l u t i o n s c o m p u t e d b y c o m -

p a r i n g o u r 6 5 C d a t a t o p r e v i o u s l y p u b l i s h e d

l o w e r - t e m p e r a t u r e d a t a a r e c o n s i s t e n t w i t h a

s u r f a c e - c o n t r o l l e d m e c h a n i s m . F i n a l l y , b e -

c a u s e t h e d i s s o l u t i o n r a t e s m e a s u r e d f o r o l i v -

i n e a r e a m o n g t h e h i g h e s t r a t e s m e a s u r e d f o r

c o m m o n s i li c a te s , o u r r e su l ts w i t h o r g a n i c

a c i d s a n d f a y a l i t i c c o m p o s i t i o n s s u g g e s t t h a t

o l i v in e d i s s o l u t i o n k i n et i cs m a y d o m i n a t e b o t h

t h e M g a n d F e m a s s t r a n s f er i n t h e w e a t h e r i n g

o f n a t u r a l s y s t e m s t h a t c o n t a i n a b u n d a n t

a m o u n t s o f o l i v in e . E x a m p l e s o f s u c h s y s te m s

i n c l u d e b a s a l ti c f l o w s , t u f f a n d a s h e m p l a c e -

m e n t s ; m a f i c a n d u l t r a m a f i c p l u t o n s ; a n d m e -

t a m o r p h o s e d d o l o m i t i c l i m e s t o n e s.

A c k n o w l e d g e m e n t s

T h i s w o r k w a s s u p p o r t e d i n p a r t b y N S F

g r an t s E A R - 8 7 - 1 9 4 5 7 , E A R - 9 1 - 0 3 4 5 8 , a n d b y

a N o r t h w e s t e r n U n i v e r s i t y R e s e a r c h F e l l o w -

s h ip . T h e a u t h o r s w o u l d l ik e t o t h a n k P a t B r a d y

f o r h e l p w i t h t h e l a b o r a t o r y w o r k a n d f o r n u -

m e r o u s t h e o r e t i c a l a n d p r a c t ic a l d i s c u s s i o n s ;

E l a i n e S t r e e t s a n d A r g o n n e N a t i o n a l L a b o r a -

t o ry f o r t h e B E T m e a s u r e m e n t s ; R i c k K r a m e r

f o r a s s is t a n ce w i t h t h e S E M w o r k ; B e rn i e W o o d

for s u p p l y i n g t h e

Fo91;

and a careful r e v i e w e r

for comme nts that impro ved the manuscript.

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