gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1983.047.343.04 Chemical Distinction Between Igneous and Metamorphic Orthopyroxenes Especially Those Coexisting with Ca-rich Clinopyroxenes: A Re-Evaluation Rietmeijer Frans J. M. Department of Petrology, Instituut voor Aardwetenschappen, Budapestlaan 4, 3584 CD Utrecht, The Netherlands 6 1983 47 343 143 151 15 5 1981 1 10 1982 Copyright © 1983, The Mineralogical Society 1983 Abstract

A previously suggested simple relationship in a diagram of(Al2O3 vs. MgO + FeO + Fe2O3)opx for the distinction between igneous and metamorphic orthopyroxenes is shown to be inadequate. A revised diagram is presented and its limitations are discussed. A diagram showing Fe2+/(Fe2+ + Mg) vs. 100Ca/(Fe2+ + Mg + Ca) for orthopyroxenes coexisting with Ca-rich clinopyroxenes proves to be useful for the discrimination of igneous and metamorphic orthopyroxenes and for evaluation of the extent and physical conditions of regional orthopyroxene re-equilibration.