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.

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