The crystal structure of calcite, CaCO3 at room temperature was examined in the early days of crystallography (Bragg, 1914). A phase transition at around 1240 K was also reported ( Boeke 1912). But experimental studies at higher temperature are problematic because calcite starts to decompose at around 1100 K.
We have used computation, specifically molecular dynamics (MD), to explore behaviour at high temperatures (Yang et al., 2025a). Above about 2000 K the CO32- entities that at room temperature lie in planes re-orient to becomes effectively spherical, and the structure reduces to the halite structure, cubic, in space group Fm-3m. There is a phase transition around 2000 K, below which the CO32- develop a tendency to lie in a particular plane but are disordered with this plane – the structure is described in space group R-3m. There is another transition computed to be at 1525 K, corresponding to the transition reported to occur at 1240 K, to the known room temperature structure. This has the CO32- lying in planes and ordered within these planes, the CO32- orientation reversing between one plane and the next. The space group is R-3c and on account of the alternation just mentioned the repeat is twice that in R-3m.
We have considered the order parameters for these two transitions. The coupling between them is linear-quadratic, and the interaction between them is reported. We have also made a detailed study of twinning in calcite (Yang et al., 2025b).