Pyroxenes from non-carbonaceous chondritic meteorites

SUMMARY. Chemical, optical, and X-ray data are presented for pyroxenes from a representative selection of chondrites. The Mg-Fe pyroxenes originally crystallized above iooo-q2oo ~ as the protoenstatite-protohypersthene polymorph, and inverted on cooling to the polysynthetically twinned clinoenstatite-clinohypersthene polymorph. In most chondrites the latter became converted to orthopyroxene during recrystallization at temperatures ranging up to 9oo to 950 ~ The contents of certain minor elements changed uring the clino-ortho inversion. Ancestral protoenstatite-protohypersthene may have been slightly non-stoichiometric. Accessory Ca-rich pyroxenes are discussed briefly. RECENT studies, directed towards large collections rather than single individuals, have led to greatly improved petrographic and genetic understandings of chondritic meteorites. By providing a more satisfactory basis for the selection of representative material than existed in the past, these advances have stimulated a new investigation of major constituents in a number of chondrites from the extensive collections of the British Museum (Natural History), London. Of the several mineral groups examined, it is appropriate to discuss the pyroxenes separately, for, as indicated by the followin