Molecular dynamics simulation of crystal dissolution from calcite steps
N. H. de Leeuw
*
and S. C. Parker
†
School of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom
J. H. Harding
‡
Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom
Received 22 March 1999
Molecular-dynamics simulations were used to model two stepped 101
¯
4 surfaces of the calcium carbonate
polymorph calcite. The acute monatomic steps were found to be more stable than the obtuse monatomic steps.
The initial stages of dissolution from the steps were considered in vacuo and in water. In vacuo CaCO
3
was
shown to dissolve preferentially from the obtuse step. In aqueous environment both stepped surfaces are
stabilized by the presence of the water molecules although the relative stabilities remain similar. Using poten-
tial parameters that reproduce experimental enthalpies of the dissolution of calcite crystal, the formation of the
double kinks on the obtuse step is shown to cost less energy than dissolution from the acute step, probably due
to the lower stability of the obtuse surface. The simulations suggest that formation of the kink sites on the
dissolving edge of the obtuse step of calcite is the rate determining step and this edge is predicted to dissolve
preferentially, which is in agreement with experimental findings of calcite dissolution under aqueous condi-
tions. S0163-18299912039-3
INTRODUCTION
Calcite is one of the most abundant minerals in the envi-
ronment and of fundamental importance in many fields, both
inorganic and biological. It is a building block of shells and
skeletons
1
and is used as a carbon isotope counter in marine
carbonates, with a view to assessing the relationship between
the CO
2
-induced greenhouse effect and climate.
2
Further-
more, calcium carbonate is important in ion exchange, due to
its strong surface interactions with heavy metals in the
environment,
3,4
in energy storage where the products of its
endothermic decomposition into CaO and CO
2
can be stored
and subsequently reacted exothermically to re-release the
energy
5
and in industrial water treatment.
6
Hence, calcite has
been the subject of extensive and varied research. One area
of research, which has attracted much attention is crystal
growth and dissolution, e.g., Refs 7–10. As the concentration
of calcium carbonate in many natural waters exceeds the
saturation level, the precipitation of calcite in industrial boil-
ers, transportation pipes and desalination plants is of
concern
11
and it is therefore important to learn how crystal
growth and dissolution are affected and modified. Often
studies have concentrated on the incorporation in the crystal
of foreign ions such as copper and manganese,
12
iron
13
and
other divalent cations,
14–16
phosphate species,
6,17
or organic
matter.
18–20
Alternatively, side reactions like the oxidation of
pyrite and ammonia affect the rate of CaCO
3
dissolution.
21
Earlier computational studies
22,23
have confirmed experimen-
tal findings
24
that lithium and HPO
4
2-
impurities radically
change the morphology of calcite, and predicted that magne-
sium ions would do likewise, which was later confirmed by
Compton and Brown
14
who found that magnesium ions in-
hibit calcite growth.
The 101
¯
4 surface is by far the most stable plane of
calcite and dominates the observed morphology.
25–27
Hence,
it has been the subject of many investigations, both in ultra-
high vacuum such as the scanning electron microscopy
SEM study by Goni, Sobrado, and Hernandez,
28
in air
29
and under aqueous conditions such as the atomic force mi-
croscopy AFM investigations by Ohnesorge and Binnig
30
and Liang et al.
31
However, no experimental surface is truly
planar and there are always defects present like steps and
kinks. Indeed, calcite growth is found to occur through
steps
32
and spiral dislocations,
33
often in monolayers from
the step as observed by Liang et al.
31
in their AFM study of
the calcite 101
¯
4 plane under aqueous conditions and by
Stipp, Gutmannsbauer, and Lehmann
29
who used scanning
force microscopy SFM to study the same surface in air over
some days and found the steps to spread one layer at a time.
Foreign ions can be incorporated at the growing steps, e.g.,
boron oxyanions.
34
Recent models of step dissolution have
included a terrace-ledge-kink model, successfully describing
the initial stages of pit growth on the 101
¯
4 surface,
35,36
and
a kinetic Monte Carlo model which reproduces experimental
pit-growth behavior.
37
In addition, atomistic simulation
methods have been used to model growth inhibition by in-
corporation of diphosphates into the steps
38
although they
did not explicitly include solvent effects.
The aim of the work described in this paper is to use
molecular-dynamics simulations to investigate the energetics
of key stages in calcite dissolution, which is achieved by
modelling the dissolution of CaCO
3
units from two different
monatomic steps on the 101
¯
4 surface. In addition to study-
ing calcium carbonate removal from the steps in vacuum, we
have extended our study to include the effect of water on the
stepped surfaces to begin to understand the influence of
aqueous conditions on the growth and dissolution process.
THEORETICAL METHODS
The surface and adsorption energies of the calcite surfaces
were modeled using classical molecular dynamics simula-
PHYSICAL REVIEW B 15 NOVEMBER 1999-I VOLUME 60, NUMBER 19
PRB 60 0163-1829/99/6019/137928/$15.00 13 792 ©1999 The American Physical Society