Calcite (calcium carbonate) is the main component of limestone. Interaction of aquated ions with calcite surfaces leads to growth inhibition and concentric, sector, and intrasector zoning. Reactivity of carbonate growth surfaces is influenced by the geometry of the surface and by solution species bound to the surface-hydroxide, water, hydrogen carbonate, carbonate, and metal ions in various stages of hydration. This work is theoretically characterizing, at the atomic level, the energetically favored species at the calcite surface that may be responsible for growth features.
Many carbonate crystals display concentric zoning, the nonhomogeneous distribution of minor and trace elements in bands parallel to growth surfaces. At least in some cases, zoning arises from surface-controlled kinetics rather than from changing growth conditions. Minor and trace elements, including Mg, Mn, and Sr, also display concentration zoning in different growth sectors. Surface control related to the different atomic configurations on the different faces is likely. Sharp Mg, Mn, and Sr zones have also been identified within sectors. It has been argued that such intrasector zoning is systematically related to the structure and orientation of steps and kink sites. Several ions slow calcite growth. It is inhibition by magnesium that has been most thoroughly studied. The mechanism may be distortion of the near-surface calcite lattice or slow dehydration of adsorbed magnesium.
Rachel Green, Graduate Student Researcher
John Podobinski, Undergraduate Student Researcher
Preliminary embedded-cluster calculations are complete for attachment of ions to CaCO3. The Hartree-Fock method in GAMESS and the Ahlrichs+* basis set was used for these calculations. Preliminary results indicate that a few hundred point charges will be adequate. Correction for electron correlation and basis set superposition errors is necessary.
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