X-RAY STUDY OF THE PHASE TRANSFORMATIONS OF ENSTATITE

The mechanisms of the phase transformation among the three polymorphs of enstatite were investigated by heating experiments. Below 1, 000°C, clino-enstatite shows a tendency to realize in part the ortho-structure through partial disordering of the clino-structure. Well-ordered ortho-enstatite inverts itself into the proto-phase at 1, 150°C. When proto-enstatite is cooled from 1, 100°C at the rate of 100°C/hour, the phase persists down to 650°C, where it suddenly changes to disordered clino-enstatite with a remarkable shortening of the c axis. Possible structural mechanisms for the above transformations have been proposed. During the investigations mentioned above, it was concluded that β-MgSiO3 with a=36.4 Å is, in all probability, identical with twinned clino-enstatite. The refinement of the crystal structure of proto-enstatite, which is stable above 985°C, was carried out with the intensity data obtained at 1, 100±6.5°C by high temperature precession technique, the residual R being 0.128 for 49 observed reflections.