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Experimental Study on the Polymorphism of Enstatite



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American Minerulogist Volume 59 pages 345 352 1974 Experimental Studyon the Polymorphism of Enstatite JosBpHR Srrayrn Lunqr ScienceInstitute 3303 NasaRoad 1 Houston Texas77058 Abstract The polymorphism of enstatite has been studied undel various heating and cooling rates using high temperature single crystal X ray techniques Rapid quenches of protoenstatite produce twinned low clinoenstatite while slower cooling rates produce mixtures of blinoenstatite and up to fifty percentorthoenstatite Large strain energiesappear to account for the martensitic nature of coherent transformations between enstatite polymorphs and the observation that different quench rates produce differing products A modification of the transformation model of Sadanaga Okamura and Takeda 1969 and Coe 1970 better accountsfor the observed nature of the transformations No evidence of a stability field for high clinoenstatite above that of protoenstatite was observed when a crystal of protoenstatite underwent partial incongruent melting to forsterite plus liquid Introduction Since the first attempted synthesis of enstatite by Ebelman in 1851 the polymorphism of enstatite has been the subject of much study and the source of much confusion and controversy An understanding of the relative stabilities of the various polymorphs of enstatite is of primary importance to the study of meteorites and of the few terrestrial rocks in which they occur Although the polymorphs observed at room temperature orthoenstatite and clinoenstatite are essentiallyisomorphous with their iron rich equivalents hypersthene and pigeonite respectively there are some important differences between iron rich and iron free low calcium pyroxenes The most important difference is the occurrence for the enstatite composition of a third major structure typ protoenstatite which has not been observed for other pyroxene compositions The existence of this polymorph greatly complicates the stability relations in the enstatite corner of the



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