High pressure phase transformations in aragonite-type carbonates

 High-temperature and high-pressure experiments conducted in a diamond-anvil cell revealed phase transformations in the aragonite-type carbonates of strontianite (SrCO3), cerussite (PbCO3), and witherite (BaCO3) at pressures below 4 GPa and ∼1000 °C. The powder X-ray diffraction patterns of these high-pressure phases can be reasonably indexed with the same type of orthorhombic cell having a space group of P2122 (17). By assuming 16 MCO3 (M=Sr, Ba or Pb) molecules in a unit cell, the transition from the aragonite form to a new phase was concomitant with a volume contraction of 4.23, 2.38, and 2.34% for SrCO3, PbCO3, and BaCO3, respectively. If the same phase transition were to occur in CaCO3, it has been estimated that the transition would accompany a 7% volume contraction.