Molecular dynamics simulations of phase transition between high-temperature and high-pressure clinoenstatite

Phase transformation between high-temperature clinoenstatite (HT-CEn) and high-pressure clinoenstatite (HP-CEn) was studied using molecular dynamics (MD) simulations. Starting from HT-CEn, the MD simulations of isothermal compression were carried out at different temperatures (at 2800, 2500, and 2000 K). At 2800 and 2500 K, the transformation into HP-CEn occurred directly at high pressure and the discontinuous change of the MD-simulated cell volume between HT-CEn and HP-CEn showing an isosymmetric or first-order transition was observed. At 2000 K, on the other hand, low-temperature clinoenstatite (LT-CEn) appears between HT-CEn and HP-CEn. The crystal structures of three clinoenstatites (LT-CEn, HT-CEn, and HP-CEn) were compared one another, and it was also confirmed that the switching among the bonds between Mg atoms in the M2 sites and the coordinated O3 atoms occurs during the transitions among the three CEn’s. A phase relation among these three CEn’s is also proposed, although the stability fields of protoenstatite and orthoenstatite are appeared at low pressure.