High temperature single crystal X-ray diffraction studies of the ortho-proto phase transition in enstatite,Mg2Si2O6 at 1360 K

A high temperature single-crystal X-ray diffraction study of enstatite, Mg₂Si₂O₆ was undertaken at 296, 900, 1200, 1360 and 1400 K. During the X-ray data collection at 1360 K (T₀), orthoenstatite (Pbca) transformed to protoenstatite (Pbcn). The unit cell parameters measured at T₀ are a=18.456(4), b=8.960(2) and c=5.270(1) Å for ortho and a=9.306(1), b=8.886(1) and c=5.360(1) Å for proto. The discontinuous increase in c and decrease in b due to the ortho to proto transformation are associated with the drastic unkinking of the silicate chains, whereas the abrupt increase in a results from the large expansion of the M2 — O distances along a coupled with the increase in the out-of-plane tilting of the silicate tetrahedra. Stacking faults form in ortho prior to the phase transition, as well as in proto between 1360 and 1400 K. With increasing temperature, the silicate B chain in ortho straightens faster than the A chain as the configurations of the SiA and SiB tetrahedra tend to become similar. At T₀, the A and B chains with the O3-O3-O3 angles (O3 being the bridging oxygen atom) of 163.0° and 149.5° in ortho, respectively, attain an identical angle of 168.4° in proto. The configuration of the silicate chain in proto resembles that of the A chain in ortho. Rigid-body thermal vibration analysis suggests that between 1200 and 1400 K the largest, the second largest and the smallest thermal librational motions of the SiO₄] tetrahedra in both ortho and proto are approximately around a, c and b, respectively. Below 1200 K, the largest thermal librational amplitudes of the SiA and SiB tetrahedra in ortho are quite different, but become nearly equivalent at T₀. In contrast to the results reported for all iron-bearing orthopyroxenes at high temperature, switching of the O3B atoms coordinated with the M2 cation occurs during the ortho to proto transformation, but not in ortho below T₀. The ortho-proto transition does not affect the configuration of the M1 octahedron significantly, but results in a decrease of the mean M2 — O bond distance by 0.043 Å and a highly distorted M2 octahedron in proto.