The High Temperature Grystal Ghemistry of Glinohypersthenel

Single crystal X-ray intensity data have been obtained from a crystal of clinohypersthene' MgonFeou"Caoo,uSiO " at 2A",2W', 40A",600', 70O", 760 ' and 825'C. Refinements of the crystal structure using three dimensional data and anisotropic thermal models have been made at each temperature. Clinohypersthene undergoes a first-order transition from space grottp PL/c to C2/t at 725 ' + 10'C. Cell dimensions indicate a discontinuous change in the length of the axis of greatest thermal expansion at the transition. The h { k odd, diffractions remain sharp and observable up to the temperature of transition and exhibit a discontinuous drop to zero intensity at the transition. The high clinopyroxene refinements appear to represent a real struc-furo rather than an average of the two primitive silicate chain configurations. There are slight tendencies for clinohypersthene to exhibit more ordering of Mg and Fe on Ml and M2 and for Fe-Mg exchange to begin at higher temperatures and to have a slightly higher AG of exchange than does orthopyroxene