Sapphirine I |
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| Authors: | John B Higgins Paul H Ribbe Richard K Herd |
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| Institution: | (1) Department of Geological Sciences, Virginia Polytechnic Institute and State University, 24061 Blacksburg, Virginia, USA;(2) Geological Survey of Canada, 601 Booth Street, K1A 0E8 Ottowa, Canada;(3) Present address: Department of Geological Sciences, University of Tennessee, 37916 Knoxville, TN, USA |
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| Abstract: | Microprobe analyses of 26 natural sapphirines from 17 localities indicate that the predominant chemical substitutions in this mineral occur along the solid solution joinVI(Mg,Fe)2++IVSi4+=VI(Al, Fe)3++IVAl3+. Chromium and manganese are minor substituents. Evidence for the substitution Si Al+1/2Mg+1/2 vacancy is absent within the limits of analytical error.A partitioning scheme based on electrostatic charge balance considerations has been devised permitting calculation of Fe2+ and Fe3+ from total iron content. Results are in good agreement with previous Mössbauer studies which indicate Fe3+ is sometimes in octahedral and/or tetrahedral coordination.Distribution coefficients for Fe2+-Mg exchange equilibria between sapphirine-spinel and sapphirine-orthopyroxene are similar for most mineral pairs and suggest that most of the assemblages equilibrated at about the same temperature or that the exchange reactions are insensitive to temperature.Compositions of synthetic sapphirines as a function of temperature and pressure are qualitatively predictable from crystal chemical considerations. Changes in sapphirine composition along the MgSi= AlAl solid solution join toward more aluminous compositions stabilize the sapphirine structure at high temperatures and low pressures. The limited extent of MgSi=AlAl solid solution observed in natural sapphirines appears to be related to the requirements of geometrical fit among octahedra and tetrahedra in the almost idealized cubic closest-packed anion framework. |
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