Molecular dynamics simulations of the phase transition between low-temperature and high-temperature clinoenstatites

Phase transition between low-temperature clinoenstatite (LT-CEn) and high-t emperature clinoenstatite (HT-CEn) was studied by using molecular dynamics (MD) simulations, based on empirical potential parameters. Starting from LT -CEn, the MID calculations were carried out at atmospheric pressure and at elevated pressures (1-6 GPa). At elevated temperatures the transformation f rom the starting LT-CEn to HT-CEn occurred at any pressure. It was confirme d that the HT-CEn has the same space group C2/c as diopside but the M2 site is six-coordinated, unlike diopside. A significant difference in the MD-si mulated cell volumes between LT-CEn and HT-CEn was also observed, showing a first-order transition. In addition, there were some temperature ranges wh ere LT-CEN and HT-CEn would be coexistent and very small thermal hystereses between increasing and decreasing temperatures during the transition. Thes e behaviors are consistent with the characteristic of a thermoelastic-marte nsitic transformation. The phase boundary between LT-CEn and HT-CEn was det ermined for the first time. Its positive dT/dP slope strongly shows that th e high-pressure clinoenstatite is a significantly distinct phase from HT-CE n although the both phases have the same space group, C2/c.