31 Dislocation Etch Pits in Quartz S. L. Brantley, S. R. Crane, D. A. Crerar, R. Hellmann, and R. Stallard
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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
/ /
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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
