Experimental evidence for the stability of synthetic Ca-bearing orthoenstatite near 1400 °C at atmospheric pressure in the system Mg2Si2O6-CaMgSi2O6 is difficult to reconcile with other experimental results, given present thermodynamic models for pyroxenes on that join. The hypothesis that the phase stable under these conditions is not ordinary orthoenstatite, but is instead a structurally and mineralogically distinct phase, has been tested by optical, sem, and tem studies of crystals synthesized at 1370 °C and 1 atm, combined with a single-crystal X-ray structure refinement at 25 °C and with X-ray powder diffractometry at 25 °C and 1425-1440 °C. The results indicate that (1) the structure assumed by the quenched phase at room temperature is that of ordinary orthoenstatite and (2) the structure above 1400 °C is at least very similar to, and probably identical to, that of ordinary orthoenstatite and does not match the derivative structures seen at that temperature for analogous compositions of Mg2Si2O6-LiScSi2O6 pyroxenes.

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