Free energy and relative stability of the enstatite Mg2Si2O6 polymorphs

Detailed lattice dynamical studies of the equations of state and the phase diagram of the geophysically important mineral enstatite have been undertaken. There are several enstatite polymorphs: ortho, proto and clinoenstatite, whose structures are characterized by double MgO6 octahedral ribbons and single silicate chains. The computed equations of state are found to be in good agreement with available experimental data and ab initio results. The calculations reveal that the stable phase at ambient conditions is orthoenstatite, which transforms to the protoenstatite phase at high temperatures. The monoclinic C2/c clinoenstatite phase is found to be stable at high pressures. The computed phase diagram is in overall qualitative agreement with the experimental data. These studies have enabled a microscopic understanding of the factors contributing to the relative stability and indicate that while the orthoenstatite to protoenstatite transition is temperature driven, the orthoenstatite to clinoenstatite transition is pressure driven.