Vaterite

Abstract: Inorganic carbonate in soil occurs predominantly as the sparingly soluble alkaline-earth carbonates, calcite (CaCOs) and dolomite (CaMg(CO,),). Calcite is usually the dominant form in active pedogenic environments (Doner & Lynn, 1977; Nelson, 1982). There are only a few reported occurrences of aragonite (CaCOs) and vaterite (CaCOs) in soils. Sodium carbonate and magnesium carbonate and hydroxycarbonate are common in evaporates or in regions of high-salt deposition in soil. Concentration of dissolved carbonate is controlled by equilibrium relations of the solid-phase carbonates and gas-phase CO*. Concentration of dissolved carbonate is likely to be higher in systems with high partial pressures of C02, e.g., in flooded soils or in microenvironments of high microbial activity, or in sodic soils, because of the high solubility of Na&Os. Also, calcite and dolomite usually control the activities of Ca2+(aq) and Mg2+(aq) in soils containing these minerals. Calcium carbonate contents of carbonate-influenced soils range from traces to greater than 80%. Calcite exists in a variety of forms, from nodules of 1 cm or greater diameter to submicrometer particles to well-formed rhomboids. The presence of carbonates is usually associated with neutral to alkaline soils, but solid-

Abstract: The kinetics and mechanisms of nanoparticulate amorphous calcium carbonate (ACC) crystallization to calcite, viavaterite, were studied at a range of environmentally relevant temperatures (7.5–25 °C) using synchrotron-based in situ time-resolved Energy Dispersive X-ray Diffraction (ED-XRD) in conjunction with high-resolution electron microscopy, ex situX-ray diffraction and infrared spectroscopy. The crystallization process occurs in two stages; firstly, the particles of ACC rapidly dehydrate and crystallize to form individual particles of vaterite; secondly, the vaterite transforms to calcitevia a dissolution and reprecipitation mechanism with the reaction rate controlled by the surface area of calcite. The second stage of the reaction is approximately 10 times slower than the first. Activation energies of calcite nucleation and crystallization are 73 ± 10 and 66 ± 2 kJ mol−1, respectively. A model to calculate the degree of calcite crystallization from ACC at environmentally relevant temperatures (7.5–40 °C) is also presented.