CRYSTALLOGRAPHIC ANALYSIS OF THREE MODIFICATIONS OF CaCO3: CALCITE, ARAGONITE, VATERITE

The crystallographic analysis of three modifications of CaCO3 - rhombohedral calcite (R (3) over barc), orthorhombic aragonite (Pmcn), and monoclinic vaterite (C2/c) - reveals different mechanisms of atomic ordering in the structures. An important role in the formation of a specific structure is played by the point symmetry of energetically stable atomic groups (templates) in the precrystallization phase, which depends on the crystallization conditions, impurities, and local heterogeneities. Then the template symmetry forms the structure symmetry by placing heavy atoms and mass centers of stable CO3 atomic groups mainly in special positions with fixed coordinates. The details of this crystal dynamics (the ordering of atomic positions by translational and point symmetries) are shown.