Geochemical Processes at Mineral Surfaces - American Chemical

30 Influence of Surface Area, Surface Characteristics, and Solution Composition on Feldspar Weathering Rates

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Michael Anthony Velbel Department of Geological Sciences, Michigan State University, East Lansing, MI 48824-1115 Current best estimates for natural plagioclase weathering rates are one to three orders of magnitude lower than laboratory rates. Surface characteristics which may play a role in determining rates and mechanisms of feldspar dissolution (including non-stoichiometric dissolution and parabolic kinetics) in the laboratory include adhered particles, strained surfaces, defect and dislocation outcrops, and surface layers. The narrow range of rates from experiments with and without pretreatments indicates that these surface characteristics alone cannot account for the disparity between artificial and natural rates. Either the intrinsic surface area and characteristics of natural materials are significantly different from those used in laboratory experiments, or other non-mineralogical factors must account for the disparity between laboratory- and field-determined rates of feldspar weathering. Predicting basis

the rates

studies nuclear in

wasteforms,

hydrocarbon

problems existing

on r e a c t i o n

The purpose

literature

weathering, areas

such

of

rocks. a n d made this

i t imperative

paper

and t o suggest, there

is

to review

need

of

of

landscapes and

to

permeability these

kinetics i n

to evaluate

applicability

and r a t e s

on the basis

i s urgent

pressing

of solving

o f geochemical

on t h e mechanisms

i n which

of

of porosity

and t h e i r

on t h e

laboratory

as the s t a b i l i t y

The n e c e s s i t y

the study rates

from

susceptibility

and the d i s t r i b u t i o n

reservoir

interactions

i n understanding

issues

the chemical

interactions,

data

systems.

some

importance

has rejuvenated

water-rock

water-rock

and e x t r a p o l a t i o n

and geochemical

deposition,

recent

models

i s o f paramount

environmental acid

of natural

of theoretical

to

the natural

and evaluate

the

feldspar

o f the l i t e r a t u r e for further

review,

research.

0097-6156/86/0323-0615S06.00/0 © 1986 American Chemical Society

Davis and Hayes; Geochemical Processes at Mineral Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

616

G E O C H E M I C A L P R O C E S S E S AT M I N E R A L S U R F A C E S

Weathering o f

Feldspar


F e l d s p a r s a r e the most abundant m i n e r a l s o f igneous and metamorphic r o c k s (J_>2). B e i n g the most abundant r o c k - f o r m i n g m i n e r a l s o f the e a r t h ' s c r u s t , t h e y have r e c e i v e d a p r o p o r t i o n a t e l y l a r g e s h a r e o f a t t e n t i o n from s t u d e n t s o f w e a t h e r i n g , and most o f the major a n a l y t i c a l advances and c o n c e p t u a l models which have been a p p l i e d t o o t h e r m i n e r a l groups have a l s o been a p p l i e d t o f e l d s p a r s . F e l d s p a r i s a framework s i l i c a t e i n which t e t r a h e d r a c o n t a i n i n g A l or S i a r e l i n k e d t o one a n o t h e r by s h a r e d oxygens i n a l l d i r e c t i o n s r a t h e r t h a n i n c h a i n s o r s h e e t s (3) · The p l a g i o c l a s e solid-solution series Ca Nai_ Ali Si3_ 08> x

x

+ x

x

where 0 < χ < 1

i s the commonest r o c k - f o r m i n g s e r i e s ( 2 ) . Plagioclase feldspars v a r y w i d e l y i n t h e i r s u s c e p t i b i l i t y t o w e a t h e r i n g ; G o l d i c h (k) observed that c a l c i c p l a g i o c l a s e ( a n o r t h i t e ) i s extremely s u s c e p t i b l e whereas s u c c e s s i v e l y more s o d i c p l a g i o c l a s e s a r e progressively l e s s weatherable. T h i s c o m p o s i t i o n a l e f f e c t on n a t u r a l w e a t h e r i n g o f p l a g i o c l a s e has been c o n f i r m e d by numerous other observational studies. A l k a l i f e l d s p a r s { ( K , N a ) A l S i 3 0 8 ) form a c o n t i n u o u s s o l i d s o l u t i o n s e r i e s a t h i g h t e m p e r a t u r e s , but tend t o e x s o l v e upon c o o l i n g i n t o a p o t a s s i u m - r i c h phase and a s o d i u m - r i c h p h a s e , r e s u l t i n g i n p e r t h i t i c intergrowths (2). S o d i c p l a g i o c l a s e s and p o t a s s i u m - r i c h f e l d s p a r s t e n d t o be s t u d i e d t o g e t h e r , i n p a r t because o f s i m i l a r i t i e s i n s t r u c t u r e , c o m p o s i t i o n , o c c u r r e n c e , and w e a t h e r a b i l i t y ( e . g . , _4). Parabolic

Kinetics

" P a r a b o l i c k i n e t i c s " r e f e r s t o the o b s e r v a t i o n t h a t the c o n c e n t r a t i o n o f a s p e c i e s r e l e a s e d t o an aqueous s o l u t i o n by a l t e r a t i o n o f a p r i m a r y m i n e r a l p l o t s as a l i n e a r f u n c t i o n o f the square r o o t o f t i m e . F i g u r e 1 (data from 5) i l l u s t r a t e s t h i s . Numerous workers ( e . g . , 5-11) have o b s e r v e d t h i s b e h a v i o r . The i m p o r t a n c e o f " p a r a b o l i c k i n e t i c s " i n l a b o r a t o r y s t u d i e s o f m i n e r a l d i s s o l u t i o n has v a r i e d as i n t e r p r e t a t i o n s o f the u n d e r l y i n g r a t e - c o n t r o l l i n g mechanism have c h a n g e d . Much o f t h e r e s e a r c h on s i l i c a t e m i n e r a l w e a t h e r i n g undertaken i n the p a s t decade o r so s e r v e d t o t e s t v a r i o u s h y p o t h e s e s f o r the o r i g i n o f p a r a b o l i c kinetics. At l e a s t f o u r d i f f e r e n t e x p l a n a t i o n s have been proposed t o a c c o u n t f o r p a r a b o l i c k i n e t i c s . The o l d e s t and b e s t e s t a b l i s h e d i s the " p r o t e c t i v e - s u r f a c e - l a y e r " h y p o t h e s i s . C o r r e n s and von E n g l e h a r d t (6) proposed t h a t d i f f u s i o n o f d i s s o l v e d p r o d u c t s t h r o u g h a s u r f a c e l a y e r which t h i c k e n s w i t h time e x p l a i n s t h e observed p a r a b o l i c b e h a v i o r . G a r r e l s (_12>13) proposed t h a t t h i s p r o t e c t i v e s u r f a c e c o n s i s t s o f hydrogen f e l d s p a r , f e l d s p a r i n which hydrogen had r e p l a c e d a l k a l i and a l k a l i n e e a r t h c a t i o n s . Wollast (5) s u g g e s t e d t h a t i t c o n s i s t s o f a secondary aluminous or alumino-silicate precipitate. In e i t h e r c a s e , a p r o t e c t i v e s u r f a c e l a y e r e x p l a i n s p a r a b o l i c k i n e t i c s as f o l l o w s : I f the c o n c e n t r a t i o n o f any d i s s o l v e d p r o d u c t a t t h e boundary between t h e f r e s h f e l d s p a r


30.

VELBEL

and

the

Feldspar

protective

equilibrium and

Weathering

the

or

the

surface

layer

fixed

by

either

rate

transformation

of

the

concentration

solution,

Fick's

of

at

first

the

law

that

product

of

gradient


of

the

of

fresh the

well.

with

equations

his

vs.

with

of

to

presence

of

a

or

the

particles

adhering In

material

with

strained

or

this

than

Appendix

A).

readily;

as

destroyed,

the the

destruction by

Holdren particles initial

be

stage

rate

tapers

mineral.

and

grinding

to

fact,

be

"a"

represents

freshly

or

pretreated

in

Curve

"b"

shows

ultrasonically material

as

shows

any

of

in

results ground way.

of

acetone

material

on

Figure Curve

but

"c"

of

of

etc.,

(e.g.,

11, more

are rate

of

layer

is

preparation (7)

and

for

Nickel

dissolution layers.

that

adhered

fine

pseudo-parabolic Figure

experimental

which

has

2

runs.

not

been

was

covered

electron ten as

much

reduced the

had

been

adhered

adhered

remove

removal

represents

albite.

which

much v e r y

to

greatly

of

much f i n e

that

that

the

albite microns)

albite,

remove

2 that

in

the

the

to

linear

surface

attempts

(~0.5-10 microns)

high-dissolution-rate.

to

of

solution

surface

Kiang

three

revealed

withstood

into

clearly

of

made u p

respect

the

that to

layer

abundance

initial

"washed"

removed,

is

material

"unwashed"

to

outer

particles

for

is due

particles,

goes

albite,

this

(submicron)

Note

the

hyperfine

experiments.

of

(submicron

been

particles

high

is

surface

with

and

Scanning

SEM p h o t o s had

Huang

of

layer

mineral

dissolution

possible.

ultrasonically.

the

the

matter

adhered

on

precipitates,

kinetics

large

to

disturbed

particulate

had

off

that

dissolution

cleaned

the

silica

model

reaction

disturbed

such

that

particles

the

dissolved

fine

fine

responsible

the

revealed

fine

of

demonstrated

(VV)

Curve

adhered

The

experiments;

laboratory

photomicrographs

closely

outer

therefore

and/or

from

with

that

outer the

edges

layer

(modified washed

the

energy;

layer

suggested

VO

out

derived

strained

mineral

Berner

of

to

disturbed

bulk

in

or

bulk

attributed

could,

(5) found

quantities

undisturbed

disturbed

and

surface

transport

model

the

stable

others)

from

the

parabolic of

less

(8)

layer

"front"

products

secondary

be

dissolution

might

to

disturbed

crushing

(among

large

exposed

layer

laboratory rates

for

stage

distorted

of

this

bonds,

the

the

and

for

of

surface

protective

a mass-transfer

proposed

dissolution

of

the

Wollast

data

to

results.

case,

the

The

ambient

feldspar

material

from

thickening layers

higher-than-normal

broken

of

phenomenon,

electrostatically

either

As

the

time.

determined

presence

disturbed

dissolution

rate

calculated

structurally

mineral.

mineral,

the

proportional

hydrogen

across

with

high-dissolution-rate

mineral,

of

concentration

layer.

the

constructed

explanation

the

created

the

this

curves

(JJJ,JJ5)

the

cause

of

is

the

precipitation of

and

model

time

favorable

second

early,

by

J

D and

surface

decreases

progressively

similar The

time,

experimentally

Helgeson

basis

or

that

(1)

products

coefficient

encroachment

layer

concentration matched

dissolved

feldspar,

is

= -D(dc/dx)

concentration gradient

surface

diffusion

of

surface

thermodynamic

primary

diffusion

protective

by

decreases

the

the

the

either

solution, layer

flux

diffusion

across

thickens, into

the the

is

outer

J states

617

Rates

of

fine

it the

the

coarser

initial

dissolution


curve


618

GEOCHEMICAL PROCESSES AT MINERAL SURFACES

Figure dissolution

2

Effect

kinetics

o f sample (modified

pretreatment from

on f e l d s p a r

Ref. 11).

See t e x t f o r

discussion.


30.

VELBEL

for

Feldspar

ground

particles

and had

HF-H2SO4.

of

Holdren

behavior

observed

and

by

third

in

the

Berner

in

through

particles The

feldspar

removed

albite

linear. diffusion

proposed

for

non-linear

that

behavior The

fourth

explanation

three

than

products.

in

any

pH a n d

affinity

and

discussed

due

to

not

to

dissolution

the

kinetics

mineral

of

is

that

surface

secondary

non-linear

concentration

for

non-linear

kinetics

the

intrinsic

property

of

solution

minerals vs.

time

kinetics

reverse

detail

by

or

reaction

Helgeson

the

solid can

either

rate.

the

or

apparent

the

and

or

influence of

of

chemical

phenomena

(Vf)

the

solution

reactants

effect

These

Murphy

from

the

produce

through

through

and

differs

composition of

composition

equilibria,

CO2

additional

of

of

the

released silica,

silica

they

are

Usually, silica,

studies

was

often

correspond

to

observed

have

been

Helgeson

and

and

alkaline as

only

the

for

solution

and

suggested in

earths.

Such

that

with

the

concept

of

are

feldspar

silica

preferential kind

of

which

do

excess

not

of

This

and

aluminum

relative

were

to

retention,

was

of

Often,

(J_9).

in

phase

elements earths,

analyzed,

abundant.

solid

some

the

elements.

non-stoichiometric dissolution,

consistent

of

alkaline

released

is

dissolution.

proportions

least

some

few

elements

the

are

the

retained

experiments

dissolution

all

in

of

earths

aluminum i s

a

alkalis,

during

analyses

stoichiometry

non-stoichiometry

manifested

analyzed although

released

alkaline

dissolved

dissolution

non-stoichiometric

Where m u l t i p l e

to

bulk

preferentially

alkalis

be

report

released the

vs.

is,

analyzed.

alkalis and

laboratory

above

that

a l u m i n u m may

few

of

cited

feldspar;

and

only

aspect

stoichiometric

studies

by

feldspar,

be

parabolic

precipitation of

concerns

the

in

question

being

was

but

(J_8).

One Many

studies

is

"parabolic"

the

it

dissolution

changing

the

that

Changing

parabolic

others

much

(J_6).

previous

true

for

that

preparation.

from

accounts

the particles

sample

be

but

solution

occurs; adhered

layer,

explanation

linearly,

fine

with

fine

suggested

released

adhered

grains

stage

of

surface

during

may

a l l

the

dissolution

protective

generated

which

initial

(JN)

products

rather

from

treating

previous

a

619

absence

dissolved from


washed been

Rates

No p s e u d o - p a r a b o l i c

dissolution

fine

Weathering

long

thought

residual

to

surface

layer. By

the

accepted of

mid-1970»s

as

the

feldspars,

almost

a l l

minerals

the

but

of

magnesium

non-carbonate As

(8).

shown

and by

protective-surface-layer difficulties,

which

trained

optical

their

minerals

(see

observations weathering and this

their of

literature surface

and The

on

surface-

to the

chemistry

1980 s. f

seek

apparent

kinetics

Studies

evidence

of

as

upon

however,

the

numerous petrographers silicate

electron-microscopic of

k i n e t i c models

interface-)

dominate

only of

rock-forming

have

(see

the of

below).

feldspar surface

reaction

for

controlled

protective-surface-layer

investigation of

(or

to

widely not

ultimately,

(2^,22),

microscopes

acceptance

linear

the

Holdren

was

dissolution and,

(20)

proved

newfound

based

associated

and

increasingly

wider

surface

the

non-sulfur-bearing

electron

to

in

silicates

hypothesis

became

below).

factor

Berner

led

challenge

avenues

diffusion-inhibiting

rate-controlling

As

feldspar

reactions

a

result

hypothesis,

of

two

weathering morphology

mechanisms

and

besides


that


620


p r o v i d e d by d i s s o l u t i o n s t u d i e s (see b e l o w ) , and f u r t h e r l a b o r a t o r y s t u d i e s o f d i s s o l u t i o n k i n e t i c s have been u n d e r t a k e n t o r e s o l v e a p p a r e n t a m b i g u i t i e s between e a r l i e r l a b o r a t o r y s t u d i e s and the newer s u r f a c e morphology and s u r f a c e c h e m i s t r y r e s u l t s d i s c u s s e d below. Chou and W o l l a s t (23>24) performed e x p e r i m e n t s on f e l d s p a r d i s s o l u t i o n u s i n g a f l u i d i z e d - b e d r e a c t o r , which a l l o w e d them t o v a r y the c o m p o s i t i o n o f s o l u t i o n s d u r i n g the c o u r s e o f the experiment (for i n s t a n c e , a f t e r the l i n e a r , s t e a d y - s t a t e stage o f the r e a c t i o n had been a t t a i n e d ) and m o n i t o r the e f f e c t s o f the s o l u t i o n p e r t u r b a t i o n s on t h e d i s s o l u t i o n k i n e t i c s . They a l s o a n a l y z e d t h e i r s o l u t i o n s f o r Na, A l , and S i , r a t h e r than f o r any one element a l o n e , a l l o w i n g them t o e v a l u a t e n o t o n l y the b e h a v i o r o f i n d i v i d u a l elements w i t h t i m e , but a l s o the s t o i c h i o m e t r i c o r n o n - s t o i c h i o m e t r i c b e h a v i o r o f the r e a c t i o n s . The e x p e r i m e n t a l r e s u l t s p r e s e n t e d by Chou and W o l l a s t (23,24) show n o n - 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 i n e a r l y s t a g e s o f the r e a c t i o n , r e s u l t i n g i n the f o r m a t i o n o f a r e s i d u a l s u r f a c e l a y e r d e t e r m i n e d by m a t e r i a l b a l a n c e t o be " a few t e n s o f Angstroms t h i c k " , t h e c o m p o s i t i o n o f which i s s t r o n g l y pH-dependent. A c c o r d i n g t o Chou and W o l l a s t , d i f f u s i o n i s r a t e - l i m i t i n g w i t h r e s p e c t t o a l k a l i r e l e a s e a t t h i s s t a g e o f the r e a c t i o n . L i n e a r k i n e t i c s and 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 p r e v a i l i n l a t e r s t a g e s o f the r e a c t i o n (at any g i v e n pH), s u g g e s t i n g t h a t the l a y e r r e a c h e s a q u a s i - s t e a d y - s t a t e t h i c k n e s s m a i n t a i n e d by d e s t r u c t i o n o f t h e o u t e r s u r f a c e o f the r e s i d u a l l a y e r a t the same r a t e a t which f r e s h f e l d s p a r i s t r a n s f o r m e d a t the i n t e r f a c e between i t and the residual layer. More i n t e r e s t i n g , however, i s t h e i r o b s e r v a t i o n t h a t i n s t a n t a n e o u s m o d i f i c a t i o n o f s o l u t i o n pH r e s u l t s i n a new e p i s o d e o f i n i t i a l l y n o n - l i n e a r k i n e t i c s , and r e l e a s e o f elements i n d i f f e r e n t p r o p o r t i o n s t h a n a t o t h e r p H ' s , i m p l y i n g the f o r m a t i o n o f a r e s i d u a l s u r f a c e l a y e r w i t h a d i f f e r e n t (pH-dependent) composition. They argue t h a t o n l y i n t h e f i r s t p o r t i o n o f any e x p e r i m e n t a l run c o u l d p a r a b o l i c b e h a v i o r o f s o l u t e s be e x p l a i n e d by f i n e p a r t i c l e s : p a r a b o l i c k i n e t i c s o b s e r v e d upon c h a n g i n g t h e s o l u t i o n pH must be due t o the f o r m a t i o n o f a new s u r f a c e l a y e r , the f o r m a t i o n o f w h i c h , t h e y a r g u e , must be d i f f u s i o n - c o n t r o l l e d . Chou and W o l l a s t (23) s u g g e s t e d t h a t t h e i r own r e s u l t s c l e a r l y i n d i c a t e d the formation o f a sodium-depleted s u r f a c e l a y e r . W o l l a s t and Chou (25) a l s o a p p l y and c r i t i q u e H e l g e s o n and o t h e r s ' (18) f o r m u l a t i o n o f a c t i v a t e d complex t h e o r y , and make s u g g e s t i o n s for future refinements to f a c i l i t a t e i t s a p p l i c a t i o n to feldspar weathering s t u d i e s . The r e c e n t work o f H o l d r e n and Speyer (26,27) has s u g g e s t e d t h a t a t l e a s t some s t u d i e s a r e s t r o n g l y a f f e c t e d by the abundance and d i s t r i b u t i o n o f d e f e c t s and d i s l o c a t i o n o u t c r o p s i n ways t h a t are not simply r e l a t e d to surface a r e a . Pétrographie

Observations

W i l s o n (28) n o t e d the p r e s e n c e o f e t c h p i t s ( c r y s t a l l o g r a p h i c a l l y c o n t r o l l e d voids or features of negative r e l i e f , or "negative c r y s t a l s " ) on some s o i l f e l d s p a r s , and reviewed s i m i l a r o b s e r v a t i o n s from e a r l i e r s t u d i e s . Some examples o f e t c h p i t s on n a t u r a l l y weathered f e l d s p a r s are shown i n F i g u r e 3· Etch p i t s



30.

VELBEL

Feldspar

Figure surfaces Incipient

3

in

from S.

Scanning various of

etch

naturally

Pooley

and

of

Rates

electron

stages

formation

development prismatic

Weathering

of

shallow etch

feldspar

the

a)

almond-shaped

pits

weathered

photomicrographs

weathering,

prismatic into

621

pits,

d) All

etch

Extensive

interiors.

materials.

Fresh

of

feldspar

surface, pits,

c)

b) Moderate

penetration

Photographs

photomicrographs

b-d by

author.


of are Alan


622

reflect

the presence

defects, because

they

enhanced Until the

tautological. with

an etchant

control

should

released

layer

possess

the

local

rate

strictly

reaction

layer,

dissolution

dissolution (or,

smooth,

rate

i s a function

speaking,

(e.g.,

31_) :

e.g., in

R is

equilibrium

the

ambient

solution

sites

o f enhanced

etc.)

local

allowing The

surfaces attack

feldspar altered for

surfaces

surface-reaction

observation;

feldspar

hydrous

they

feldspar

surface

preparation.

cm /sec, 2

upon

through

principle

even

diffusion

resulted

controlled

a

cm^/sec)

pits

than

etch

of

against

pits

corners,

decrease,

to catch up. attacks

mineral

site-selective which

would inferred

the

workers

and a r t i f i c i a l l y microscopic h a s become

possible

another

have

also

weathered

examination

a routine

during

(D i n e q u a t i o n

of

test

sample

could

that

The c o a t i n g weathered

feldspars

coefficient

a crystalline

i f

i t were should

be r a t e

i s slower

observed

from the

than

by B e r n e r behaves

t o be

have

of

2 0

solid. i n

provided the

a

10~

limiting u

10~^ ,

that

surface

and Holdren much more

of approximately

solid

sufficiently

a crystalline

then

of

coatings

storage and

propose,

through

layer

and

kind

clay

and a r e detached

1) o n t h e o r d e r

much g r e a t e r

i n a rate

that

discontinuous,

"micro-mudcracks"

porous

important

(2J[,^2) o b s e r v e d

as i t s proponents

diffusion like

surface

(28) c o r r e c t l y

and e l e c t r o n

for etch

around

and

should

pits)

is

s

0 a n d 2. A s

buildups edges

Numerous

c

between

sites

bulk

militates

naturally

makes

i f D were

rate. )

o f the

concentration i n

dissolution

Wilson

to diffusion

(2J_,22) o n n a t u r a l l y has a

these

(e.g.,

dessication

coefficient

(Diffusion

usually

The " p r o t e c t i v e - s u r f a c e - l a y e r " ,

equivalent

affinity)

constant,

concentration

are patchy,

form

diffusion-inhibiting diffusion

undersaturation

n

the development

and Holdren

on w e a t h e r e d that

u p , and

control.

microscopy Berner

ions

up" i n the

goes

chemical

the rate

hypothesis.

(21,22,27,32-40),

of a diffusion

because the

(dislocations,

attack.

on both

"pile

the dissolved

around

features

of etch

pits

mineral

Electron

(which

energy

preventing

protective-surface-layer etch

k is

c is

local

rates

site-selective

observed

c )

of the surrounding

and any surface

the presence

-

s

transport-controlled

uniformly,

indicate that

crystal

that

of

rate,

causes

dissolution is

o f the degree

and η i s r a t i o n a l ,

dissolution

result

is

by

general

concentration,

diffusion-inhibition

pits

As d i s s o l v e d

they

This

= k(c

the d i s s o l u t i o n

the

etch

the notion

concentration

down.

determined

dissolved

surfaces.

t h e thermodynamic

R where

contradicts

the local

techniques, was

(that i s ,

A mineral

rounded

of attack.

by a t t a c k i n g t h e

dissolution,

slows

pits

the resultant

attack

however,

by p a r e n t - m i n e r a l

diffusion-inhibiting

microscopic scientists

counting

(28-30).

are sites

and etch

and m a t e r i a l s

and then

crystal

to selective

of dislocations

Site-selective

control),

surface

electron

of

Dislocations,

in a crystal,

dislocations

and d e n s i t i e s

a microscope.

protective


between

arrays

crystal.

susceptible

of sophisticated

surface-reaction

are

imperfections

Metallurgists

distributions

(linear

etc.) in a

and a r e r e a d i l y

the advent

crystal

bonds,

reflect

energy,

correspondence

under

of dislocations

mismatched

and therefore

l i k e mud

10"^ i s not



30.

VELBEL

Feldspar

Weathering

Rates

623

c o n s i s t e n t w i t h the p r o t e c t i v e - s u r f a c e - l a y e r h y p o t h e s i s o f f e l d s p a r weathering. P e t r o v i c ( 4 Ί ) had reached a s i m i l a r c o n c l u s i o n on t h e o r e t i c a l grounds and Fung and S a n i p e l l i (j\2) c o n f i r m e d the earlier findings. The r a p i d l y growing body o f m i c r o m o r p h o l o g i c e v i d e n c e m i l i t a t e d a g a i n s t the p r o t e c t i v e - s u r f a c e - l a y e r h y p o t h e s i s . H e l g e s o n and o t h e r s (J_8) s u g g e s t e d t h a t e t c h - p i t t i n g o f m i n e r a l s i n c r e a s e s the s u r f a c e roughness o f the g r a i n s such t h a t t o t a l m i n e r a l s u r f a c e a r e a i n the system can i n c r e a s e t o a maximum v a l u e more than an o r d e r o f magnitude h i g h e r t h a n i n i t i a l v a l u e s as the r e a c t i o n p r o g r e s s e s . ( T h i s phenomenon was demonstrated e x p e r i m e n t a l l y f o r o l i v i n e by G r a n d s t a f f , 4 3 ) . They note t h a t these t h e o r e t i c a l f i n d i n g s are i n at l e a s t q u a l i t a t i v e accord with o b s e r v a t i o n s o f s u r f a c e morphology u s i n g the SEM. Depending on the p r e c i s e f u n c t i o n a l i t y o f t h e roughness f a c t o r , s u r f a c e a r e a may i n c r e a s e a p p r e c i a b l y a f t e r as l i t t l e as 0.1% o f the s t a r t i n g m a t e r i a l has been d i s s o l v e d . G e n e r a l l y , however, the s u r f a c e a r e a i n c r e a s e s by one o r d e r o f magnitude o r more (the r e a s o n f o r c h o o s i n g one o r d e r o f magnitude as the c r i t i c a l v a l u e f o r d i s c u s s i o n w i l l become a p p a r e n t i n a l a t e r s e c t i o n ) o n l y a f t e r some 10% o f t h e s t a r t i n g m a t e r i a l has been d i s s o l v e d , and s u r f a c e a r e a r e a c h e s i t s maximum when about 55% o f the s t a r t i n g m a t e r i a l has been consumed. As a c o n s e q u e n c e , s u r f a c e a r e a i n c r e a s e s do n o t a f f e c t room-temperature l a b o r a t o r y s t u d i e s , which a r e g e n e r a l l y o f such s h o r t d u r a t i o n t h a t o n l y a s m a l l f r a c t i o n o f the s t a r t i n g m a t e r i a l has r e a c t e d . Surface

Composition

The morphology o f weathered f e l d s p a r s u r f a c e s , and the n a t u r e o f the c l a y p r o d u c t s , c o n t r a d i c t s the p r o t e c t i v e - s u r f a c e - l a y e r hypothesis. The presence o f e t c h p i t s i m p l i e s a s u r f a c e - c o n t r o l l e d r e a c t i o n , r a t h e r than a d i f f u s i o n ( t r a n s p o r t ) c o n t r o l l e d r e a c t i o n . F u r t h e r m o r e , the c l a y c o a t i n g c o u l d not be " p r o t e c t i v e " i n the sense o f l i m i t i n g d i f f u s i o n . F i n a l l y , H o l d r e n and B e r n e r (11) demonstrated t h a t s o - c a l l e d " p a r a b o l i c k i n e t i c s " o f f e l d s p a r d i s s o l u t i o n were l a r g e l y due t o enhanced d i s s o l u t i o n o f f i n e particles. None o f t h e s e f i n d i n g s , however, a d d r e s s e d the q u e s t i o n o f the a p p a r e n t n o n - s t o i c h i o m e t r i c r e l e a s e o f a l k a l i s , a l k a l i n e e a r t h s , s i l i c a , and aluminum. T h i s q u e s t i o n has been approached both d i r e c t l y ( e . g . , XPS) and i n d i r e c t l y ( e . g . , m a t e r i a l b a l a n c e from s o l u t i o n d a t a ) . N i c k e l ( Q ) c a l c u l a t e d the t h i c k n e s s o f the proposed " r e s i d u a l l a y e r " on a l b i t e from the mass o f d i s s o l v e d a l k a l i s and a l k a l i n e e a r t h s r e l e a s e d d u r i n g l a b o r a t o r y w e a t h e r i n g and the measured s u r f a c e a r e a , and d e t e r m i n e d t h a t the t h i c k n e s s r a n g e s from 0 . 8 t o 8.0 nm i n the pH range o f n a t u r a l s u r f a c e w a t e r s . A l t h o u g h he i n t e r p r e t e d h i s r e s u l t s d i f f e r e n t l y , they a n t i c i p a t e l a t e r f i n d i n g s on the pH dependence o f r e s i d u a l l a y e r c o m p o s i t i o n s (see b e l o w ) . P e t r o v i c and o t h e r s (44) used X - r a y p h o t o e l e c t r o n s p e c t r o s c o p y (XPS) t o a n a l y z e the K, A l , and S i c o n t e n t s o f e x p e r i m e n t a l l y a l t e r e d K - f e l d s p a r g r a i n s , and found t h a t a l k a l i d e p l e t i o n , i f i t e x i s t e d a t a l l , c o u l d not e x t e n d t o g r e a t e r than 1.7 nm d e p t h ; i . e . , the " l e a c h e d l a y e r " , i f i t e x i s t s a t a l l , i s l e s s than 1.7 nm thick. H o l d r e n and B e r n e r (JM) u s i n g XPS o b s e r v e d a s l i g h t d e c r e a s e i n t h e Na/Si r a t i o o f e x p e r i m e n t a l l y weathered a l k a l i


GEOCHEMICAL PROCESSES AT MINERAL SURFACES

6 2 4

feldspar H

(or

+

surfaces,

K

proposed

from

+

by G a r r e l s

Otherwise, 2.0-4.0

they

nm o f

interior.

thickness


personal

detectable occurs

even

not

within

incongruent exist.

some

sort

hydrogen)

albite

below

the layer (23)

surface

They argue

of

challenged layers

that

of

altered

layer

between

alkalis,

the

calculations

order

of

only

suggest

tens

to the contrary,

chemistry

observations

Berner

and others studies

illusory".

Noting

with

t h e XPS d a t a

others

(47)

into

that

"tubes"

(to which

inaccessible) requirements

with of

control.

dissolution

only

until

quasi-steady-state the inner

portion

of

and o t h e r s

dissolution-void

-

reaction. (47)

interiors

of

be

accomodated

which

"tubes"

penetrate would

be

allowing

above,

agree,

noting

diffusion

layer

until

may w e l l

be

i t s

encroachment the

outer

a

(48) role

i n reconciling results

that

limits

attains

no f u r t h e r

the possible

and

balance

(48)

can occur

Berner

r e c o n c i l e XPS

s t i l l

that,

between

"may be

pits

Chou and W o l l a s t

regarding

surface

incongruent

could

surface

which

material be o n

their the

challenge,

etch

while

outlined

removed,

observed

refute.

could

surfaces

"microfront"

is

to

do

require

Their

with

and m a t e r i a l

after

the

studies

grains)

the r e s i d u a l

weathering

the layer

surface-controlled Berner

model

thickness

for

despite

(23)

instance,

studies,

indicate

Angstroms

the c o n t r a d i c t i o n

Chou and W o l l a s t

the quasi-steady-state

which

inhomogeneous

the i n t e r i o r

l i t t l e

t h i c k n e s s must

seek

that

the incongruency

laboratory

surface-reaction

of

(for

i s no

calculations

incongruency

of mineral

greater

very

several

which,

chemistry

spatially

(46)

from

and aluminum.

XPS) they

limited

that

there

to account

the layer

suggest

of

experiments

and that

than

Chou a n d W o l l a s t

the i n t e r i o r s

measurements

in

(47)

that

not inconsistent

(e.g.,

sets

along

is

and surface

suggest

dissolution deep

that

portion

sampled

losses

recent

balance

silica,

the

Mea a n d o t h e r s

XPS s t u d i e s

o f nanometers,

arguments

dissolution

i n order

of Κ

depletion.

thicker

material

outer

mineral

at distances

more

depth,

sodium

loss

estimate

indicate same

i n the

i n the outer

sodium

surface;

for

+

that

to the fresh

Delia

that

Na

place.

change

some

are negligible

this

incongruency balance

Finally,

communication)

Chou and W o l l a s t that

observed

the mineral

hydrogen

relative

they

of

like

be t a k i n g

(45)

observed

used)

no o t h e r

with

nm.

exchange

they

might

HF/H2SO45

6.0

leached

30-40 nm f r o m

(Petit,

with

techniques

experimentally than

and others

t o be a b o u t

i o n beam

(J_3)

surfaces

due t o exchange treated

some

t o document

the weathered

K-feldspar

using

able

that

solution

a n d Howard

were

Perry

(presumably a

and suggested

t h e KOH b u f f e r

agree

with

of of

previous

studies. Other sites" their

aspects

earlier

because sites

mean

area,

there.

"If

the 88).

the inhomogeneous

than

by W o l l a s t

estimates

restricting

would

smaller less

of

are discussed

more

a residual

However,

of

layer

thickness

(25),

layer

layer

exists,

surface

would

discussion

then

takes

the r e s i d u a l i t would

be i n c r e a s e d is

place

and a c c o r d i n g l y

that

are minima,

to the v i c i n i t y of

"active

who s u g g e s t

thickness

the reaction

the thickness

the t o t a l

their

residual

the r e s i d u a l

that

increasing

10% o f

calculated

of

distribution of

and Chou

of

active

over

a

layer

probably

cover

the value

proportionally"

inconsistent with

for (p.

the model


of

30.

VELBEL

weathered during form

surfaces

o f the walls

proposed o f "deep

preferential

attack

long

deep

"tubes"

surface

area

depth,

slightly

Chou

(25,

enough

p.

that

thinner,

deeper

o f the bulk

increases

surface

requires

that

suggest

area

that

the residual

(only

near

than

the holes area

would

If

layer

mass

sites),

surface

be t h i n n e r

loss

consistent

direct

agreement

with

thin

observations

during with

balance

be d i v i d e d n o t i n an

as suggested

o f mass

an u n d e t e c t a b l y

greatly

resulting

thickness

a greater

actually

loss

the calculations into

or

than

(e.g.,

so, material

measured

active

surfaces

(8,^,20,^9,23,25).

23,24)

area),

greater

weathering

workers

showing

a r e deep

surface

surface

are

as Wollast and

by p r e v i o u s

(e.g.,

the bulk

( i f the holes

depth),

feldspar £3).

local

but over

layer

3)·

t h e XPS sample

(23,24)

the bulk

of

the total

see Figure

be t h i c k e r

etch-pit

(see also

(25),

If

surface.

o f weathered

area

and Chou

calculated

47)

3)

an i n t e r n a l

i n the calculated

Wollast

bring

have

of feldspar,

(47;

(if

i n the

i n penetration

are than

thick

equals

the experimentally

a smaller

increase

equally area

Etching

e t c . " (47).

results

t h e XPS sample

surface mineral

Figure

that

than

observations

£1,22,37;

could

holes,

grains

the holes

layer

suggest,

88)

sites

(47).

o f new s u r f a c e

tubes,

by Chou a n d W o l l a s t

i f the holes

SEM

cracks,

and others

areas

significantly

(local)

their

large

the weathering

may i n c r e a s e

only

over

by B e r n e r

o n how much d e e p e r

calculated

625

Rates

o f the active

into

the actual

average

that

Weathering

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

Depending


Feldspar

area, than

that

T h i s would

dissolution layer,

o f the surface

then

experiments

XPS m e a s u r e m e n t s

residual

by

meaning

a n d make

morphology

of

(e.g., both weathered

feldspar. Much

remains

weathered feldspar surface the us

surfaces, analysis

mineral)

been

results

the correct

order

diffusion

Taken

creates which

data

that

exist,

a synthesis

Weathering

(8)

from

chemistry has

his results

are

hydrogen-exchange

i t i s a t most far too thin

exchange

a thin

o f hydrolyzed

layer

i s pH d e p e n d e n t . layer"

(about

Gardner

several t o be

i n their

thick.

workers

layer This

A Summary

initial

as

follows:

and a l k a l i n e

earths

the composition

(J_9) h a s c a l c u l a t e d , that

experiments Similar than

was a b o u t

calculations

several

exchange

using the

the thickness

(8,22,43,45,^6)

i s n o more

observations

proceeds

aluminosilicate,

( 9,JK)),

1 . 5 - 2 . 5 nm) t h i c k . by o t h e r

residual

nanometers

for alkalis

and Clemency

produced

-

o f a l l t h e above of feldspar

Hydrogen-ion

observations altered

of Feldspar

the weathering

o f Busenberg

"leached cells

Studies

as a whole,

1.

does

that

i . e . , i f a

and i s probably

tell

Nickel's

inhibiting.

Laboratory

suggests

layer

thick,

from

(which

thicknesses

work o n f e l d s p a r suggesting

o f magnitude,

surface

o f nanometers

and l a t e r

his results,

layer"

of

removed

experiments

to the solution).

"residual

work o n

the results

o f elements

dissolution

calculated

with

cation-depleted

from

composition o f

analytical

i n comparing

t h e amounts

was r e l e a s e d

concentration,

the surface

to actual

i s required

(determining

with

consistent

tens

of

work

for instance,

dissolved

regarding

In addition

how much m a t e r i a l

work,

of

t o be done

feldspars.

of the

two u n i t

and e m p i r i c a l

a l l suggest

that the

t o a few t e n s

accounts

for the

of

initial



626

G E O C H E M I C A L PROCESSES AT M I N E R A L SURFACES

n o n - s t o i c h i o m e t r i c r e l e a s e o f a l k a l i s and a l k a l i n e e a r t h s r e l a t i v e t o s i l i c a and aluminum (as p e r J_3). F u r t h e r n o n - s t o i c h i o m e t r y ( e . g . , between Na and K) may be a p p a r e n t o n l y . Gardner (19) p o s t u l a t e d t h a t such n o n - s t o i c h i o m e t r y may be due t o d i f f e r e n t i a l d i s s o l u t i o n o f two d i s t i n c t phases ( r e c a l l t h a t a l k a l i f e l d s p a r s usually consist of microperthitic exsolution features, consisting of d i s c r e t e s o d i c p l a g i o c l a s e lamellae i n d i s c r e t e potassium feldspar). Congruent d i s s o l u t i o n o f two d i s t i n c t but i n t i m a t e l y i n t e r g r o w n phases a t d i f f e r e n t r a t e s may appear t o be i n c o n g r u e n t d i s s o l u t i o n o f the b u l k p h a s e . W i l s o n and McHardy (4^) have documented d i f f e r e n t i a l a t t a c k on such p e r t h i t i c i n t e r g r o w t h s ; r e s u l t s o f H o l d r e n and Speyer (27) may be r e l a t e d t o t h i s phenomenon. However, i n t h e i n i t i a l s t a g e o f d i s s o l u t i o n , n o n - s t o i c h i o m e t r i c r e l e a s e ( e . g . , S i and A l ) i s r e a l , r e f l e c t i n g the pH-dependent c o m p o s i t i o n o f t h e r e s i d u a l l a y e r ( 2 3 - 2 5 ) . 2. C o n t i n u e d d i s s o l u t i o n removes whatever h y p e r f i n e p a r t i c l e s may have been c r e a t e d d u r i n g sample p r e p a r a t i o n f o r l a b o r a t o r y dissolution. Where f i n e s were removed, o r were n e v e r p r e s e n t (as i n most n a t u r a l m a t e r i a l s ) , f u r t h e r d i s s o l u t i o n breaks down t h e o u t e r s u r f a c e o f the r e s i d u a l l a y e r a t the same r a t e t h a t a l k a l i s a r e r e p l a c e d by hydrogen a t t h e i n t e r f a c e between t h e f r e s h f e l d s p a r and t h e r e s i d u a l l a y e r , r e l e a s i n g a l l c o n s t i t u e n t s t o solution. Element r e l e a s e d u r i n g t h i s s t a g e o f t h e r e a c t i o n i s s t o i c h i o m e t r i c (based on o b s e r v a t i o n s o f s u r f a c e morphology and s u r f a c e c h e m i s t r y ( e . g . , _H,22) and s o l u t i o n c h e m i s t r y ( e . g . , 2 5 ) ) , but f u r t h e r work on t h e s u r f a c e c h e m i s t r y o f n a t u r a l l y weathered f e l d s p a r i s r e q u i r e d to determine t h i s with c e r t a i n t y . This stage o f t h e r e a c t i o n i s a l s o l i n e a r w i t h time ( J J , 2 5 ) ; t h e r e s i d u a l s u r f a c e l a y e r i s n o t t h i c k enough t o i n h i b i t t r a n s p o r t ( 3 0 ) , i t a t t a i n s a quasi-steady s t a t e t h i c k n e s s (provided s o l u t i o n c o m p o s i t i o n remains c o n s t a n t ; 2 3 - 2 5 ) , and no c o n t i n u o u s , tenaceous l a y e r o f s e c o n d a r y p r e c i p i t a t e s forms ( 2 M , 2 2 , 4 Ί ) · The r a t e - l i m i t i n g s t e p i s t h e detachment o f s p e c i e s from t h e mineral-solution interface, i . e . , a surface-controlled reaction (21,22). Rates o f F e l d s p a r W e a t h e r i n g

i n the Laboratory

Numerous s u r f a c e c h a r a c t e r i s t i c s have been p o s t u l a t e d t o p l a y a r o l e d e t e r m i n i n g mechanisms, r a t e - l i m i t i n g s t e p s , and r a t e s o f f e l d s p a r d i s s o l u t i o n d u r i n g w e a t h e r i n g (as d i s c u s s e d a b o v e ) . These include: 1. Adhered f i n e p a r t i c l e s ( e . g . , 1 1 ) . 2. D e f e c t s and d i s l o c a t i o n o u t c r o p s ( e . g . , 2 8 , 1 1 ) . 3. A r t i f i c i a l l y s t r a i n e d s u r f a c e s ( e . g . , 7 , 8 ) . 4. R e s i d u a l s u r f a c e l a y e r s o f a l t e r e d c o m p o s i t i o n ( e . g . , 13)» 5. S u r f a c e l a y e r s c o n s i s t i n g o f secondary p r e c i p i t a t e s ( e . g . , 13,^4). 6. S u r f a c e l a y e r s whose c o m p o s i t i o n has been a l t e r e d c h e m i c a l l y d u r i n g sample p r e p a r a t i o n ( e . g . , 4 5 ) .


30.

Feldspar

VELBEL

Table

Weathering

Silica

I.

627

Rates

Release

Rates

Laboratory

from

System

5.6


4.5-5.1

Feldspars

Studies Rate

pH

Sodic

Treatment

χ

10

1 2

(moles/m^/sec)

Reference

13

(8) (9,10) (JL1)

None

15

(23)

Ultrasonic

22

(26)

Flowthrough

None

3.1

Batch

None

4.4

Ultrasonic

6

Batch

5.1

F l o w t h r o u g hι

5

Batch

Field

& HF

Studies

0.024 0.89

Numerous

laboratory

effects

of

Table

shows

I

specific

many

of

the

the

dissolution

results

experimental

with

natural

comparison

taken

directly

from

linear

portions

of

surface Table

using

area

1 compare Table

acidic

solutions and

basis

a

of

in

ambiguity silicate

(J8)

studies,

less come

to

arises and

from

determined

data.

areas,

Rates

or

order

of

Lasaga

in

slightly

magnitude,

despite

c o n c l u s i o n on

of

the

published

not

conclude

of in

by

identical numerous

from

were

Results

rates

the

Rates

estimates

data.

compilation

(but

of

waters.

pretreatments.

of

They

pH's to

sample

i n c l u d i n g many

the

area

an

analysis

interpretation

hydrolysis about

and

The

chosen

were

dissolution

t h a n one

the

surface

size

similar

for

or

graphed

feldspar

compilation.

attending

surface

a

were

(fresh)

surface

tested

characteristics.

studies. I

results,

conditions

rigorous

this

uncertainty which

by

such

particle

with

that

vary

others

more

experimental included

shows

have

surface

Table

or

reported

reported

experimental

Helgeson

reported

favorably

(50).

different

I

on

in

tabulated

either

based

several

used

facilitate

normalized

of

runs

were the

studies

aforementioned

(56) (57)

all) that

of

experimental

several

factors,

effective

surface

area,

surface

roughness

greatest

rates

which

and

change

those

the

the

with

of

include extent

to

reaction

progress. Rates

of

Feldspar

Geochemical

Mass

Geochemical

mass

budgets) flux the

of

invoke any

flux

of

that

calculated, place

Garrels

and

and

simple

of

a

into the

watershed.

profiles

and/or

Mackenzie

Weathering

(also

the

known

as

from

must

be

input-output

principle.

(e.g.,

watershed

difference

via

(e.g.,

between due

to

If

the

streams),

and

via

the

the

atmospheric

two

can

sura o f

involving

that

element

which

mass

balance

studies

watersheds

and

Cleaves

include and

those

Bricker

be

all

Pioneering small

(.51,52)

Profiles

Watersheds

watershed

difference

transformations the

Natural

studies

known,

this

in

Small

conservation-of-mass

leaving

element

are

and

within

weathering

balance a

element

precipitation) reactions

Weathering Balance

took on

of

and


their


628

coworkers

(53,54).

a s t h emost

weathering

rates

Many mass

o f unit

of

rates

those

published

studies

calculated studies rates

rates

with

from

orders

h i sestimate, surface

despite

this

outside

t h e range

of

attributes feldspar

this

surfaces

experiments. are

fresh,

would

rough,

rounded" "partly

surfaces covered

inhibitors with is

between

widely

believed

dissolution

clay

of naturally

rounded,

but are instead

theoretical (and

both

Despite

area)

discrepancy

between

laboratory

rate

by whatever

minerals

from

forested

control

Ridge

watershed

o f North

formalized

by Plummer

andBack

estimates

o f mineral

abundance

estimates

of mineral

grain

thickness

o f t h eweathering

results

o f t h e mass

watersheds moles/m

( i n moles

o f mineral

2

experimental rates

balance

rate

slower

(58)

size

than

rate

are often

weathering rates

roughness progress.

explanation

results

themselves a r e

f o r seven

small

i n the southern with

Blue

equations volumetric data),

andt h e average transformed the

(57)

o f watershed/yr) i n which The

these into laboratory

results

are approximately

determined

them.

of individual

f o r oneo f

reported.

for the

constants and

of linear

theunits

3);

surface

balances

Velbel

Figure

reaction

to explain

andgeometry,

profile,

that the

3) a n d

on pétrographie

calculations

surface/sec,

data

that

combined

(based

o f reaction/ha

o f plagioclase

magnitude

a system

(e.g.,

Figure

with

i t

feldspar

a r e n o t smooth a n d

suggested

mass

be

for instance,

established

etched

watersheds

Using

i s at

notinhibit

o f weathering

geochemical

(unmanipulated)

Carolina.

variance

43 a n d

i s invoked

rates

w h i c h may a l s o f o r the

thequantitative

argument

calculated

(57)

anddeeply

(56)

which

"smooth,

rates

well

o f Paies»

feldspars

terraces

explanation

could

(JjB) i n d i c a t e

constants,

old,

inferences;

increase

i n laboratory

and

feldspar

His

(e.g.,

(56)

w h i c h may a c t a s

natural

h i sfield-determined

unaffected

than

grains

may a c t u a l l y

the inadequacy

Velbel

rough

used

ledges

feldspar

observations

orders

Pac*es

dissolved

I t i s also

weathered

considerations

surface

1861). and

coatings

_41,jM,22).

surfaces

furthermore,

products

andt h e o r e t i c a l

that

(e.g.,

weathered

(p.

laboratory

rate.

falls

feldspar

onet o three

t o those

by k i n k s ,

values

natural

i n the character o f

experimentally rates

thearea o f However,

o f estimated (56),

drainage

are associated

fluids.

laboratory

a s opposed

weathering

oligoclase

constants;

t o Paces

dissolution

many o b s e r v a t i o n s

of

to thedifference

that

of dissolution"

discrepancy

these

laboratory

i n estimating

range

h i s favored

of naturally

with

such

of

of

of variability

rate

characterized

i n higher

Discussion

percolating

according

i n nature

Only two

to renormalize

constant

thetotal

He s u g g e s t s

result

with

discrepancy

and cannot,

paper.

o f sodium i n European

o f magnitude

than

as a

comparison

rates.

o f laboratory are,

rates,

area.

due t o u n c e r t a i n t i e s

slower

rates

do n o t permit

f o r comparisons

balance

i n contact

rates

magnitude

surface

therate

uncertainty,

are widely mineral

weathering

of this

f a r attempted

weathering

t h e mass

mineral

report surface

dissolution

the basis

estimated

Several

weathering

thus

forms

studies

o f estimating

to theobjectives

t o mineral

natural (j>6)

dissolution basins.

laboratory

have

therefore

Paies

with

with

which

o f landscape

contribute

balance

means

(55).

studies

area

mass

reliable

i n nature

balance

function

therefore,


Geochemical

recognized

(57) f o r

oneorder o f

i n laboratory

experiments


30.

VELBEL

Feldspar

Weathering

Rates

629


under s i m i l a r h y d r o g e o c h e m i c a l c o n d i t i o n s . V e l b e l (57) suggested t h a t two major s o u r c e s o f the r e m a i n i n g r a t e d i s c r e p a n c y a r e (a) the c h a r a c t e r o f a r t i f i c i a l l y t r e a t e d m i n e r a l s u r f a c e s i n l a b o r a t o r y e x p e r i m e n t s , which r e n d e r s them more r e a c t i v e t h a n t h e i r n a t u r a l c o u n t e r p a r t s ( e . g . , V O and/or (b) d i f f i c u l t i e s i n e s t i m a t i n g the r e a c t i v e m i n e r a l s u r f a c e a r e a i n n a t u r a l s y s t e m s . These r e s u l t s a r e d i s c u s s e d a t l e n g t h i n V e l b e l (57). Rates e s t i m a t e d i n the above s t u d i e s a r e shown i n T a b l e I . W a t e r s h e d - s c a l e g e o c h e m i c a l mass b a l a n c e s t u d i e s y i e l d c a l c u l a t e d f e l d s p a r w e a t h e r i n g r a t e s one t o t h r e e o r d e r s o f magnitude s l o w e r than r a t e s determined i n l a b o r a t o r y experiments. Summary and E v a l u a t i o n S t u d i e s o f w e a t h e r i n g r a t e s i n n a t u r e have i d e n t i f i e d two major sources of u n c e r t a i n t y a s s o c i a t e d with c a l c u l a t i o n o f n a t u r a l w e a t h e r i n g r a t e s : 1) d i f f e r e n c e s i n s u r f a c e c h a r a c t e r i s t i c s between n a t u r a l and a r t i f i c i a l f e l d s p a r s u r f a c e s , and 2) u n c e r t a i n t i e s i n the e f f e c t i v e s u r f a c e a r e a i n n a t u r a l s y s t e m s . As shown i n t h i s r e v i e w , however, t h e s u r f a c e c h a r a c t e r i s t i c s which p l a y major r o l e s i n i n f l u e n c i n g f e l d s p a r d i s s o l u t i o n r a t e s have e i t h e r not been i d e n t i f i e d i n n a t u r a l m a t e r i a l s (see d i s c u s s i o n o f 56 a b o v e ) , o r do not r e s u l t i n s u f f i c i e n t v a r i a b i l i t y i n ( l a b o r a t o r y determined) r a t e s t o a c c o u n t f o r the d i f f e r e n c e between l a b o r a t o r y - and f i e l d - d e t e r m i n e d r a t e s (as was o p t i m i s t i c a l l y s u g g e s t e d by V e l b e l , 57). In o t h e r w o r d s , the range o f v a r i a b i l i t y i n f e l d s p a r w e a t h e r i n g r a t e s due t o f i n e p a r t i c l e s , s t r a i n e d s u r f a c e s , d e f e c t s and d i s l o c a t i o n o u t c r o p s , and s u r f a c e l a y e r s i s too s m a l l t o a c c o u n t f o r the d i f f e r e n c e between l a b o r a t o r y and f i e l d - e s t i m a t e d rates of feldspar weathering. T h e r e f o r e , e i t h e r the s u r f a c e c h a r a c t e r i s t i c s o f n a t u r a l m a t e r i a l s f a l l w e l l o u t s i d e t h e range which i n c l u d e s a l l l a b o r a t o r y e x p e r i m e n t s , o r f a c t o r s o t h e r than those h i t h e r t o i n v e s t i g a t e d must a c c o u n t f o r the d i s p a r i t y between l a b o r a t o r y - and f i e l d - d e t e r m i n e d r a t e s o f m i n e r a l w e a t h e r i n g . F u r t h e r m o r e , the sense and magnitude o f the d i s p a r i t y s u g g e s t s t h a t e i t h e r 1) the volume (modal abundance) and c o r r e s p o n d i n g s u r f a c e a r e a have been v a s t l y o v e r e s t i m a t e d i n n a t u r a l s y s t e m s , o r , 2) t h e f r a c t i o n o f t h e n a t u r a l m i n e r a l s u r f a c e a r e a which a c t u a l l y r e a c t s w i t h s o l u t i o n s i s much s m a l l e r t h a n t h a t i n l a b o r a t o r y experiments. The l a t t e r c o u l d s t i l l be due t o some p r o p e r t y o f t h e mineral surfaces themselves. Alternatively, hydrological factors c o n t r o l l i n g the d i s t r i b u t i o n o f water c i r c u l a t i o n i n w e a t h e r i n g p r o f i l e s c o u l d e x e r t a s i g n i f i c a n t i n f l u e n c e on how much o f the available mineral surface w i l l a c t u a l l y p a r t i c i p a t e i n mineral-solution interactions. In o t h e r w o r d s , the r o l e o f s u r f a c e a r e a and s u r f a c e c h a r a c t e r i s t i c s i s not w e l l u n d e r s t o o d , because 1) e s t i m a t e s o f t o t a l s u r f a c e a r e a i n n a t u r a l systems may be f u n d a m e n t a l l y f l a w e d ; 2) t h e r a t i o o f e f f e c t i v e s u r f a c e a r e a t o t o t a l s u r f a c e i n n a t u r a l systems may be s i g n i f i c a n t l y l o w e r than i n l a b o r a t o r y e x p e r i m e n t s ; o r 3) t h e r e may be some a d d i t i o n a l h y d r o l o g i c a l f a c t o r which must be i n t r o d u c e d i n o r d e r b r i n g n a t u r a l w e a t h e r i n g r a t e s i n t o q u a n t i t a t i v e agreement w i t h l a b o r a t o r y r a t e data.



630


One a d d i t i o n a l f a c t o r which p l a y s a s i g n i f i c a n t r o l e (at l e a s t i n p r i n c i p l e ) i n d e t e r m i n i n g r a t e s o f m i n e r a l weathering i s the c h e m i c a l a f f i n i t y o f the w e a t h e r i n g r e a c t i o n - i . e . , how f a r the system i s from thermodynamic e q u i l i b r i u m . As shown by Aagaard and Helgeson (59) and Helgeson and o t h e r s Q8), the e f f e c t o f a f f i n i t y on r a t e s o f f e l d s p a r d i s s o l u t i o n i n most l a b o r a t o r y e x p e r i m e n t s i s negligibly small. T h i s i s because the e x p e r i m e n t s a r e u s u a l l y o f such s h o r t d u r a t i o n t h a t c o n c e n t r a t i o n s o f d i s s o l u t i o n p r o d u c t s have not b u i l t up t o l e v e l s s u f f i c i e n t t o s i g n i f i c a n t l y reduce the affinity. M i n e r a l - s o l u t i o n c o n t a c t t i m e s i n n a t u r a l systems a r e g e n e r a l l y much l o n g e r than i n l a b o r a t o r y s y s t e m s ; s a t u r a t i o n w i t h respect to secondary m i n e r a l products i s o b v i o u s l y widely a t t a i n e d , f o r such s e c o n d a r y m i n e r a l s ( e . g . , g i b b s i t e , k a o l i n i t e , e t c . ) abound i n n a t u r a l w e a t h e r i n g p r o f i l e s , and t h e i r p r e s e n c e can o f t e n be r e l a t e d d i r e c t l y t o the c o m p o s i t i o n o f c o e x i s t i n g s o l u t i o n s ( e . g . , 51,52,60-73)« Under t h e s e c o n d i t i o n s , m i n e r a l - s o l u t i o n i n t e r a c t i o n r e s u l t s i n s o l u t i o n s h i g h l y e v o l v e d toward equilibrium. The c h e m i c a l a f f i n i t y a n d , c o n s e q u e n t l y , t h e r a t e s o f the w e a t h e r i n g r e a c t i o n s , w i l l d e c r e a s e (Y7). Natural weathering r a t e s a r e , t h e r e f o r e , e x p e c t e d t o be s l o w e r than l a b o r a t o r y r a t e s ; t h e magnitude o f t h e d i f f e r e n c e i n r a t e s depends on t h e d i f f e r e n c e between the c h e m i c a l a f f i n i t i e s o f n a t u r a l v s . l a b o r a t o r y s y s t e m s . Some e f f e c t s o f s o l u t i o n c o m p o s i t i o n on r e a c t i o n r a t e s i n l a b o r a t o r y s e t t i n g s have r e c e n t l y been i n v e s t i g a t e d by Chou and Wollast (24). U n f o r t u n a t e l y , aqueous g e o c h e m i c a l d a t a f o r n a t u r a l w e a t h e r i n g systems a r e g e n e r a l l y i n a d e q u a t e t o p e r m i t d e f i n i t v e quantitative evaluation of t h i s hypothesis. One s u i t a b l e c o m p i l a t i o n o f d a t a f o r p e r f o r m i n g a p r e l i m i n a r y e v a l u a t i o n o f t h i s h y p o t h e s i s does e x i s t , t h a t o f P a i e s (64). Paces d e t e r m i n e d a d i s e q u i l i b r i u m i n d e x , I

= log (Q/K) 1 0

where Q i s the r e a c t i o n q u o t i e n t f o r the r e a c t i o n and Κ i s the equilibrium constant. A c c o r d i n g t o Aagaard and H e l g e s o n (59) t h e a f f i n i t y term, A = RTln(K/Q) has a n e g l i g i b l e e f f e c t on r e a c t i o n r a t e ( t h a t i s , the a c t u a l r a t e i s 95Î o r more o f i t s maximum p o s s i b l e v a l u e ) i f A/aRT > 3· If σ i s assumed t o be 1 (59, p . 2 5 9 ) , then P a i e s I i s r e l a t e d t o the a f f i n i t y term i n r a t e e x p r e s s i o n s based on t r a n s i t i o n s t a t e t h e o r y such t h a t (-2.31 = A / R T ) . R e a r r a n g i n g and r e l a t i n g the r e s u l t t o t h e i n e q u a l i t y from Aagaard and Helgeson (59) r e v e a l s t h a t s o l u t i o n c o m p o s i t i o n has a n e g l i g i b l e e f f e c t on r e a c t i o n r a t e i f (-1 >^ 1.3; o r , I < 1.3). The c o m p i l a t i o n o f P a i e s ( £ 4 ) s u g g e s t s t h a t many n a t u r a l s o l u t i o n s a r e s u f f i c i e n t l y c l o s e t o thermodynamic e q u i l i b r i u m w i t h p o t a s s i u m f e l d s p a r and c l a y s t o r e q u i r e the c o n s i d e r a t i o n o f the a f f i n i t y term i n r a t e e x p r e s s i o n s . However, many n a t u r a l s o l u t i o n s a r e s u f f i c i e n t l y f a r from e q u i l i b r i u m w i t h r e s p e c t t o a l b i t e and c l a y s t o r e n d e r the a f f i n i t y term (and any consequent r a t e v a r i a t i o n ) n e g l i g i b l e , and v i r t u a l l y a l l o f the solutions in Paies (^4) c o m p i l a t i o n a r e f a r enough from e q u i l i b r i u m w i t h r e s p e c t t o a n o r t h i t e and c l a y s so t h a t a f f i n i t y p l a y s no r o l e i n a f f e c t i n g n a t u r a l w e a t h e r i n g r a t e s o f a n o r t h i t e . 1

1


VELBEL

Feldspar

Furthermore,

many

30.

are

geothermal

far

from

Apparently, these

to

possible

of


using

Carolina the

Confirmation

are

close

solutions

of

to

less

equilibrium

than

25°C

watershed

pH)

any

individual

regarding

than

chemical in

the

effects

for

did

not

are

a

by

range

of Preliminary (57)

from

examination

equilibrium of

existing

available.

Finally,

not of

promising rate

determined

permit

rates.

data

of

their

for

generalizations

weathering

avenue

on

of

mineral

be

existing

to

the

negligible. constraints

do

to

in

much b e t t e r

system must

of

calculate

solutions

enough

on n a t u r a l

weathering

the full

Velbel

that

aluminosilicate

examination

rates

dissolution

used

far

from affinity

minerals.

expressions

data

affinity

offer

laboratory

were

presently

existing of

the

clay

detailed

natural

preliminary

affinity

relating

rate

more

are

cover

(64)

suggest

probably in

affinity

case;

as

rates,

especially,

specific

effect

Nevertheless,

both and,

reaction

stoichiometric data

are

term

not

of

far

chemical

Nevertheless,

PaSes

formation

same

the

slow

do

(6k)

example, for

sufficiently

that

systems.

Pa£es

by

the

are

significantly

For

requires

the

systems

weathering

aluminum

hydrolysis

that

not

affinity

(especially

dissolved

in

which

all

feldspars

laboratory

indices

natural

render

course,

in

disequilibrium

calculations,

each

will

unaccompanied

calculate

data

natural

situations.

feldspars

North

solutions

virtually

plagioclase

those

disequilibrium

to

the

many

with

systems

relative

indices

of

waters;

631

Rates

equilibrium.

equilibrium in

Weathering

suggests

future

research

natural

counterparts. Many

of

laboratory on

the

area

in

area

and

the

same

factors

studies

application

systems.

3)

the

kinetic

of

which

are

complicate

among

laboratory

the

dissolution

These

include

uncertainty

about

natural

systems

(.56,57);

the

surface

roughness

magnitude

natural

of

2)

change

solution

with

the

most

rate

1)

the

extent

data

to

natural

which on

surface

surface

progress

effects

of

limitations

effective

to

reaction

composition

interpretation

important

(JJ3) ;

rates

and

in

systems.

Conclusions 1.

Numerous

surface

mechanisms, during fine

weathering

particles;

Artificially altered

F.

Sample

designed

to

magnitude

magnitude 4. fine

or in

The

feldspar slower range

particles,

outcrops,

and

of

whose

role

in

determining

feldspar

These

dissolution

include:

outcrops;

Residual

layers

and

surface

consisting composition

varied

eliminate the

the

do

A.

Adhered

C. layers

of

of

secondary

has

experimental effects

not

result

experimental

geochemical rates

rates in

surfaces,

layers

is

too

of

been

altered

procedures

one

i n more

dissolution

mass

balance

one

determined

variability

strained

surface

D.

a

of

preparation.

weathering

than

play

rates

above).

layers

characteristics

change

may

and

dislocation

Surface

pretreatment

Watershed-scale

calculated

and

surfaces; E.

sample

control

aforementioned 3.

discussed

Surface

during

steps,

Defects

strained

precipitates;

of

(as

B.

composition;

chemically 2.

characteristics

rate-limiting

to

in

small

of

experiments. rates

dislocation

account

the order

yield

orders

weathering

to

of

one

rates.

laboratory and

more

than

studies

three

feldspar defects

or

for

the


due

to

632

G E O C H E M I C A L PROCESSES AT M I N E R A L SURFACES


d i f f e r e n c e between l a b o r a t o r y weathering.

and f i e l d - e s t i m a t e d

rates

of

feldspar

5. E i t h e r t h e i n t r i n s i c s u r f a c e c h a r a c t e r i s t i c s ( e . g . , r a t i o o f e f f e c t i v e - t o - t o t a l surface area) o f natural materials are s i g n i f i c a n t l y d i f f e r e n t from t h o s e used i n l a b o r a t o r y e x p e r i m e n t s , o r o t h e r f a c t o r s must a c c o u n t f o r t h e d i s p a r i t y between l a b o r a t o r y and f i e l d - d e t e r m i n e d r a t e s o f f e l d s p a r w e a t h e r i n g . Possible n o n - m i n e r a l o g i c a l f a c t o r s i n c l u d e inhomogenous f l u i d m i g r a t i o n through the weathering p r o f i l e . Differences i n solution c o m p o s i t i o n between l a b o r a t o r y and n a t u r a l systems may a l s o p l a y a s i g n i f i c a n t r o l e , which remains t o be e v a l u a t e d . F u t u r e improvements i n t h e a p p l i c a t i o n o f l a b o r a t o r y d i s s o l u t i o n d a t a t o n a t u r a l systems w i l l come n o t ( o n l y ) from a d d i t i o n a l work on l a b o r a t o r y k i n e t i c s , b u t w i l l a l s o depend h e a v i l y on much more comprehensive s t u d i e s o f s u r f a c e a r e a d i s t r i b u t i o n , e v o l u t i o n , and a c c e s s i b i l i t y t o a t t a c k by f l u i d s i n n a t u r a l s y s t e m s , and by improved u n d e r s t a n d i n g o f thermodynamic properties of natural f l u i d s . O n l y i n t h i s way w i l l l a b o r a t o r y k i n e t i c d a t a c o n t r i b u t e t o s o l v i n g e n v i r o n m e n t a l problems such a s n u c l e a r waste d i s p o s a l and e v a l u a t i n g t h e impact o f a c i d deposition. Acknowledgments I a p p r e c i a t e t h e a s s i s t a n c e o f t h e e d i t o r s , an anonymous r e v i e w e r , a n d , e s p e c i a l l y , D. B r a n d t V e l b e l and W i l l i a m M. Murphy, f o r t h e i r comments and c r i t i c i s m s . D r . A l a n S . Pooley o f t h e Y a l e Peabody Museum a s s i s t e d i n t a k i n g t h e s c a n n i n g e l e c t r o n p h o t o m i c r o g r a p h s o f F i g u r e 3· P r e p a r a t i o n o f t h i s r e v i e w was s u p p o r t e d by NSF g r a n t BSR-8514328.

Literature Cited 1. Hyndman, D.W. "Petrology of Igneous and Metamorphic Rocks"; McGraw-Hill: New York, 1972. 2. Deer, W.A.; Howie, R.A.; Zussraan, J. "Rock-Forming Minerals, Volume 4 - Framework Silicates"; Longmans: London, 1963· 3. Deer, W.A.; Howie, R.A.; Zussman, J. "An Introduction to the Rock-Forming Minerals"; Longmans: London, 1966. 4. Goldich, S.S. J. Geology 1938, 46, 17-38. 5. Wollast, R. Geochim. Cosmochim. Acta 1967, 31, 635-48. 6. Correns, C.W.; von Engelhardt, W. Chimie der Erde 1938, 12, 1-22. 7. Huang, W.H.; Kiang, W.C. Amer. Mineralogist 1972, 57, 1849-59. 8. Nickel, E. Contributions to Sedimentology 1973, 1, 1-68. 9. Busenberg, E.; Clemency, C.V. Proc. Int'l. Symp. Water-Rock Interactions, 1976, pp. 388-94. 10. Busenberg, E.; Clemency, C.V. Geochim. Cosmochim. Acta 1976, 40, 41-9. 11. Holdren, G.R., Jr.; Berner, R.A. Geochim. Cosmochim. Acta 1979, 43, 1161-71. 12. Garrels, R.M. In "Research in Geochemistry"; Abelson, P.H., Ed.; Wiley, 1959, PP. 25-37.



30.

VELBEL

Feldspar

Weathering

Rates

633

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