Otavite (CdCO 3 ) recovered from experiments at P>170 kbar and T nearly equal 100 degrees C contained material with the aragonite structure, indicating that a phase transition occurred. The lattice parameters of the aragonite-type CdCO 3 are a = 4.989+ or -0.003, b = 7.822+ or -0.003, and c = 5.713+ or -0.004 Aa at ambient conditions. The ambient molar volume is 33.57+ or -0.06 cm 3 , which is 2.1% smaller than that of otavite. The same kind of transition was not observed in rhodochrosite (MnCO 3 ), which is stable at least up to 280 kbar at approximately 1000 degrees 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 MgCO 3 at high pressures.

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