Calcite, CaCO 3 , has been reported to exist in as many as seven different structural forms. The structure at room temperature and pressure (space group R $$\overline{3 }$$ 3 ¯ c , ‘Phase I’) was established by Bragg many years ago. A phase transition to a higher temperature phase (space group R $$\overline{3 }$$ 3 ¯ m, ‘Phase V’) was noted to occur at around 1240 K—this may proceed via an intermediate phase (space group again...