Hydrated divalent metal carbonates exhibit a diverse range of structure and composition in geochemical environments.  Whereas a simple model based on dication size and electrostatic interactions is consistent with structure trends in anhydrous carbonates (calcite and aragonite series), hydrated carbonates are much more complex.  Mg+2 and Ca+2 form numerous distinct hydrate structures with complementary stoichiometry; Fe+2, which is nearly the same size as Mg+2, does not form any known hydrated carbonates. Density functional theory is used to determine the role of covalency, steric and repulsive electrostatic interactions, directionality of active d-orbitals, cation size, and hydrogen bonding networks in determining structure.   Ab initio thermodynamics is employed to calculate the phase diagrams for known and hypothetical hydrated carbonates as a function of temperature, pressure, and chemical potential of species to help elucidate the role of thermodynamics versus kinetics to explain the existence of some hydrates and the absence of others.

Ab initio thermodynamics of hydrated magnesium and calcium carbonates and their iron analogs

245th National Meeting (2013)

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