gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1966.035.274.05 Unit Cell Volumes of Synthetic Orthoenstatite and Low Clinoenstatite Stephenson D. A. 1 Sclar C. B. 2 Smith J. V. 1 Department of Geophysical Sciences, University of Chicago, Chicago, Illinois Physics of Solids Division, Battelle Memorial Institute, Columbus, Ohio 6 1966 35 274 838 846 4 11 1965 Copyright © 1966, The Mineralogical Society 1966 Summary

Cell dimensions and densities derived from least-squares refinement of X-ray powder diffractometer patterns are: orthoenstatite, a 18·2249±0·0013, b 8·8149±0·0008, c 5·1746±0·0008 Å, V 831·30±0·14 Å3, D 3·208 g/cc ; low clinoenstatite, a 9·6065±0·0011, b 8·8146±0·0007, c 5·1688±0·0006 Å, β = 108·335± 0·009, V 415·46±0·09 Å3, D 3·210 gm/cc. Because orthoenstatite transforms reversibly to clinoenstatite at high pressure and low temperature, and because a statistical test shows that orthoenstatite is less dense than clinoenstatite at the 90 % confidence level (in accord with the direction of slope of the experimental transition curve), it is no longer possible to regard low clinoenstatite as a metastable form of MgSiO3.