Calcium
carbonate is one of the most abundant minerals on Earth
and a component of natural biogenic materials and man-made cements.
The interactions between hydrated calcium carbonate surfaces at nanometer
confinement are relevant in dissolution and crystallization processes
as well as in adsorption of organic fluids in natural reservoirs.
In this work we quantify using atomistic molecular dynamics simulations
the water mediated interactions between calcium carbonate surfaces
at nanometer separations. We investigate two calcium carbonate polymorphs,
calcite and aragonite. We show that the adsorption behavior of water
on the (101̅4) surface of calcite and the (001) surface of aragonite
is very different. These differences are reflected in intersurface
forces between the two mineral surfaces. The interactions between
surfaces feature an oscillatory behavior whose origin is connected
to the structuring of water at an intersurface separation <1 nm.
We observe adhesion between the surfaces and demonstrate that it can
be reduced or eliminated in the case of aragonite, when the calcium
carbonate surfaces are shifted out of registry. Our work highlights
the sensitivity of mineral–mineral interactions to the topography
of the mineral surface and the distinctive structure of liquid water
in nanoconfinement, hence providing microscopic insight that might
be relevant in biomineralization and gas sequestration processes.
Deformation twinning provides a mechanism for energy dissipation in crystalline structures, with important implications on the mechanical response of carbonate biogenic materials. Carbonate crystals can incorporate magnesium, e.g. in the sea, modifying their elastic response significantly. We present a full atom computational investigation of the dependence of the twinning response of calcite with magnesium content, covering compositions compatible with three main structures, calcite, dolomite and magnesite. We find, in agreement with experiments that the incorporation of magnesium disfavors twinning as a dissipation mechanism in ordered structures (dolomite, magnesite), however the response is strongly dependent on the arrangement of the magnesium ions in the crystal structure. We show that structures with a high content of magnesium (>33%) in a disordered arrangement, lead to plastic response before twinning or fracturing. We demonstrate that the position of the magnesium ions plays a key role in the determination of the crystal deformation mode. This observation is correlated with the formation of percolation clusters of magnesium in magnesian calcites.
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