From in-situ structure and site occupancy refinements of a synthetic orthopyroxene, (Mg0.75Fe0.25)2Si2O6, by single-crystal X-ray diffraction at 296, 1000, 1100, 1200, and 1300 K (with reversals), two significant phenomena were observed: (1) with increasing temperature, the silicate B chain straightens much faster, especially above 1200 K, and becomes straighter than the A chain at 1300 K, with the 03-03-03 angles of the A and B chains being 170.8 and 173.1°, respectively; and (2) the In KD values (where KD = FeM1MgM2/FeM2MgM1) vary linearly with 1/T (K) between 1000 and 1200 K, but nonlinearly between 1200 and 1300 K. The drastic straightening of the silicate chains at 1300 K is accompanied by the attainment of very similar configurations of the SiO4 tetrahedra, SiA and SiB, in shapes, sizes, and out-of-plane tiltings. The rigid-body thermal vibration analysis suggests that the librational motion of the SiA tetrahedron (8.1°2) is slightly larger than that of SiB (5.2°2) at 296 K but becomes considerably smaller (48.0°2) than that of SiB (77.3°2) at 1300 K. The M1 octahedron remains nearly regular from 296 to 1300 K, whereas the M2 coordination changes from sixfold (296 K) to sevenfold (1200 K) and to sixfold (1300 K) because of the bridging 03B moving out and O3B′ moving into the coordination sphere (r ≤ 3.0 Å) at elevated temperatures. At 1300 K, the highly distorted M2 octahedron shares two edges with SiA and SiB, rather than one with SiA at 296 K, and the structure has all features of the high-temperature orthopyroxene phase predicted by Pannhorst (1979), which, in fact, is a transitional structural state between orthopyroxene and the so-called protoenstatite. On the basis of the configurations of the silicate chains in Mg- and Fe-rich orthopyroxenes at high temperature, an explanation is given as to why Mg-rich orthopyroxenes tend to transform to the protoenstatite structure with increasing temperature, whereas Fe-rich ones tend to transform to a clinopyroxene with space group C2/c. The anomalous behavior of the Fe-Mg order-disorder above 1200 K is attributed to the existence of the transitional state, particularly the changing charge distribution around the M2 site.

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