Otavite (CdCO3) recovered from experiments at P > 170 kbar and T ≈ 1000 °C contained material with the aragonite structure, indicating that a phase transition occurred. The lattice parameters of the aragonite-type CdCO3 are a = 4.989 ± 0.003, b = 7.822 ± 0.003, and c = 5.713 ± 0.004 Å at ambient conditions. The ambient molar volume is 33.57 ± 0.06 cm3, which is 2.1% smaller than that of otavite. The same kind of transition was not observed in rhodochrosite (MnCO3), which is stable at least up to 280 kbar at ~1000 °C. Lack of any observations of another phase transition type suggests that the aragonite-type structure is the only likely high-pressure modification of the smaller-cation carbonates such as MgCO3 at high pressures.

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