Thermal behaviour of calcite-structure carbonates: a powder X-ray diffraction study between 83 and 618 K

The thermal behaviour of five calcite-structure carbonates (space group R-3c), magnesite (MgCO₃), smithsonite (ZnCO₃), rhodochrosite (MnCO₃), otavite (CdCO₃) and calcite (CaCO₃), was explored by in situ powder X-ray diffraction over the temperature range 83–618 K. The unit-cell and crystal-structure parameters relative to temperature in theses carbonates were determined. All the carbonates in this study display obvious anisotropic thermal expansion: the values of the thermal expansion coefficients along the c-axis (α₀(c)) are much larger than those along the a-axis (α₀(a)). The a cell parameter of otavite increases slightly from 123 to 223 K and subsequently contracts at higher temperature. Negative thermal expansion along the a-axis is also observed in calcite. In contrast to those of magnesite and smithsonite, the O₁–O₂ bond distances of otavite and calcite do not expand with increasing temperature, which may explain their negative thermal expansion along the a-axis. Moreover, the values of α₀(V) and α₀(a) for the calcite-structure carbonates (MgCO₃, ZnCO₃, MnCO₃, CdCO₃, and CaCO₃) are shown to exhibit an inverse correlation with the cation radius. However, no obvious relationship is noted between the α₀(c) values and the size of the cations. Structural mechanisms for the systematic thermal behaviour of the five carbonates are also discussed.