Detailed lattice dynamical studies of the equations of state and the phase
diagram of the geophysically important mineral enstatite have been undertak
en. 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 orthoens
tatite, which transforms to the protoenstatite phase at high temperatures.
The monoclinic C2/c clinoenstatite phase is found to be stable at high pres
sures. The computed phase diagram is in overall qualitative agreement with
the experimental data. These studies have enabled a microscopic understandi
ng 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. (C) 2
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