Dislocation Etch Pits in Quartz

mineral-water interactions. Dissolution of a crystal surface is initiated at sites of high surface energy: edges, corners, cracks, scratches, and hole...
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31 Dislocation Etch Pits in Quartz S. L. Brantley, S. R. Crane, D. A. Crerar, R. Hellmann, and R. Stallard

Downloaded by GEORGETOWN UNIV on June 2, 2018 | https://pubs.acs.org Publication Date: November 13, 1987 | doi: 10.1021/bk-1987-0323.ch031

Department of Geological and Geophysical Sciences, Princeton University, Princeton, NJ 08544 Quartz samples were etched hydrothermally at 300°C in etchants of controlled Si concentration to measure the concentration above which dislocation etch pits would not nucleate. The C for 300°C was predicted to be 0.6C and the measured C was 0.75C ±.15 (C = equilibrium concentration). Our observations suggest that for C > C , dissolution occurs at edges and kinks on the surface; while for C < C , dislocation etch pits form rapidly, contributing to the overall dissolution rate. Analysis of quartz particles from a soil profile revealed a transition from angularlypitted grain surfaces at the top to rounded surfaces at the bottom, suggesting that downward permeating fluids pass through the critical Si concentration. The theory of etch pit formation may be useful in interpreting the chemical conditions of low temperature mineral-water interactions. crit

crit

o

o

crit

crit

Dissolution surface

of

favorable cause

for

point at

theory and

twin

strain,

by Sears

(6)

showed

dissolution

that

these

alteration, We cation

apply

surfaces

etch

of experiments

to geochemical

weathering,

describe

here

and other i s a useful

which tool

such

to of (1),

first

strain.

consistent

out

as

implica-

and

are suggested

processes

dissolution

an experiment

p i t theory

appeared

mineral

types

by

(5_), a n d I v e s

pointed

histories.

the

lattice

recently

fluid

which

theories

pits

Frank

(7)

of d i s s o l v e d

are

trapped

theories

(3) d e v e l o p e d

on d i s l o c a t i o n

simple

high

interest

deformation.

Johnston and Sears

for interpretation

theories

of etch

and d e v e l o p i n g

and Levine

Lasaga

of

and holes

particular

and m a t e r i a l

tions

Several

of

i n testing

based

sites

and d i s l o c a t i o n s c a n

The formati-on

(4), Gilman,

of L i F .

at

scratches,

At the m i c r o - s c a l e ,

has been

interested

lattice

initiated

boundaries,

dissolution.

of etch p i t formation

Hirth

with

defects,

e t a l . (2_), a n d C a b r e r a

Experiments

i s

cracks,

dissolution.

dislocations

experimentalists dissolution,

surface

corners,

fast

enhanced

dissolution

Cabrera,

crystal edges,

sites

impurities, also

a

energy:

by

paleothese

hydrothermal

reactions.

indicates

that

in interpreting

the

dislo-

formation

0097-6156/ 86/ 0323-0635S06.00/ 0 © 1986 A m e r i c a n C h e m i c a l Society

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

of

636

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

etch

pits

the

in

theory

quartz

quartz

by

by

grains

sampled

approaches

suggested

literature

which

the

rates

Theory Pit

and

of

of

of

we

r,

experiments

of

etch

profile.

useful

natural

consider

one

this

energy,

soil

of

have

pits

F i n a l l y , we and

by

also

on

tested

surfaces

discuss

experiments

geochemical

dissolution

of

other tn

information

the about

processes.

Formation

If

radius

a

our

d i s s o l u t i o n . We

incidence

provide

intersecting

formation surface

by

mechanisms

formation.

hole

the

from

would

Etch Pit

dislocation

hydrothermal

documenting

the

atom

dissolution

layer

nucleus

and

a

surface

deep

w i l l

elastic

which

be

nucleus

consists

(a),

then

composed

strain

energy

of

the

of

free

a

term,

at

a

screw

a cylindrical energy

volume

of

energy,

respectively,

as

Downloaded by GEORGETOWN UNIV on June 2, 2018 | https://pubs.acs.org Publication Date: November 13, 1987 | doi: 10.1021/bk-1987-0323.ch031

follows: A G = i r where

is

τ

the

dislocation energy

of

opening which

an

a

pit

and

additional

on a

the

a

term

and

free

affinity

energy

per

unit

surface

energy

of

and

the

whether

and

where

C •

concentration

solubility constant the

free

The

core

of

and

g,

term well to

the

set

using 7.3

r a

-

G

A (9),

a

cm

values

of

the

readily (0.64C ), Q

chosen

and

to

all

adequately

the value

of

following

r

is

Q

other

of

0.48

energy

of

a

360

saturation of

deep

index, as

a

in quartz in

water.

(0.73C ), Q

rnJm"

radius

correspond calculations

300o

to

a

Q

used

G

the

(8).

to

the

a

radius

down

a

r e

m

(>

quartz a

of

molar r

for

1 we

r )

elec­ strain

perature

energy. are

at

300°C, of

volume

of

different plotted

C for

where

correspond

Β (0.36C ), Q

a

experimental

run

which

to

C (0.51

conditions. is

strictly

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

pit

quartz the

C ),

Concentrations

Q

not

chosen

vector

have

and

Q

r. the

and

have

Burger's

curves

1,

of

elastic

we

gas

expresses

volume

and

and

(0.89C ).

Equation

is

dislocation

For

(9),

t

a

chemical

d i s l o c a t i o n core

of

Lettered

selected

have

c >

at

radius,

equilibrium R

Q

the

In F i g u r e

A (0.04C ), Q

r

breaks

a function Q

of

chosen

form

the

»

0

central

(10),

2

C/C .

F (0.82C ),

C

Q

Pa

function at

g as

r < r ,

the

1

w i l l

Equation 1

the

workers 1

is

corrections:

undetermined,

χ 1 0

strain

(creation

volume,

predict of

c a n c a l c u l a t e Δ G as

concentrations: Ε

of For

energetics

crystal

of Δ G with

species,

molar

from

important.

that terms

(2)

continuum approximation

dissolves

following

pit

radius

the free

/ V

the

values

molecule

the

is the

Q

of

geometry pit

temperature.

the

the

become

modulus we

is

energy

etch

is

surface

,

calculated

that

b,

shear

22.688

one

enlarging

nature

understood,

V

volume

hole

r ,

Equation 1 cannot

Because

)

Q

r is

dislocation

activity

dissolving

absolute

Q

where

energies

of

the

in

radius,

of

a

of

defining

neglecting

species,

Τ is

energy

dislocation tronic

the

of

variation

g * RT In ( C / C

g

E q u a t i o n 1 shows

free

an

the

dissolution,

and

a c o m p e t i t i o n between

cylindrical

determine

volume

(2 , 3 ) .

(1)

vector,

energy,

release

increases The

to

Burger's

is

/4 π

Q

(dissolution

medium

predict

want

the

( In ( r / r ))

2

surface

unit volume

which

To

we

ax b

is

the

crystal

area).

simplicity.

b

Ύ is

free

surface

dislocation, r,

modulus,

radius,

undersaturated

energy)

2irraY-

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

of

decrease

into

for

shear

core

a g +

2

r

Q

Ό

were Note valid

31.

B R A N T L E Y ET AL.

Dislocation

Etch Pits in Quartz

/ /

Downloaded by GEORGETOWN UNIV on June 2, 2018 | https://pubs.acs.org Publication Date: November 13, 1987 | doi: 10.1021/bk-1987-0323.ch031

20-

Ό

637

6

At α

dislocation

io-




2

at

a macroscopic

this

occurs

C =

C

c

^ ,

r

Figure etch

is

1).

an

crit

pit,

any

pit

solutions

and

nucleated

pits

C

crit

^

o

r

l

c

u

Above C to

the

neously

If

pic

t

G

^

m

t

pit.

that

including barrier

guartz

metastable bution,

rate

above

is

tions,

and

is

is

included

i n the

even

pit

for

solutions

in

with

Dissolution

kinetics

to

2).

that

one)

is

pit

grows If

Equation

the

3

a

and

small

open

up

to

which

:

(4)

maximization

AG function barrier

C




rates

pits)

young"

c

t

analyzed

cm

"reactively

C ^

r

deep

formation

With

c

(20)

sand

that,

decreases.

etch

low

transition

topmost

be

when

Venezuela

pit

C

fluid.

cm

50

C