Crystal Structure of Basic Calcium Carbonate and Its Decomposition Process in Water

Abstract

The crystal system of BCC (basic calcium carbonate) was determined by analyzing XRD patterns and SAED spot patterns. It was proved that the BCC was assigned to an orthorhombic system, having the lattice constants of a=7.094Å, b=8.627Å, and c=6.558Å. The decomposition process of BCC in water at 40-80°C was investigated by means of XRD, SEM, TEM, and SAED. XRD measurement showed that BCC decomposed to Ca(OH)₂, calcite, and aragonite. Elevation of the slurry temperature resulted in an exponential increase of the decomposition rate and a decrease of the yield of aragonite. SEM, TEM observations and SAED measurement indicated that CaCO₃ (calcite and aragonite) deposited on the surface of BCC particles resulting from the dissolution of Ca²⁺ ion from BCC in the early stage of decomposition. With progress of decomposition, the aragonite grew in the [001] direction and the number of calcite particles increased. Ca²⁺ ion dissolved into the liquid phase precipitated as Ca(OH)₂ and grew to large single crystals with progress of the decomposition.