Anomalous Orthopyroxene Cell Volumes from Unshocked Equilibrated H Chondrites

Details Anomalous Orthopyroxene Cell Volumes from Unshocked Equilibrated H Chondrites Anomalous Orthopyroxene Cell Volumes from Unshocked Equilibrated H Chondrites

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.506f&link_type=abstract

Meteoritics, vol. 30, no. 5, page 506

Other

Chondrites, H, Crystal Chemistry, Meteorites, Acfer 314, Estacado, Frontier Mountain 90049, Frontier Mountain 90076, Frontier Mountain 90156, La Villa, Miami, Ragli, Orthopyroxene

Scientific paper

Thirteen orthopyroxenes were extracted from eight unshocked equilibrated H-chondrites representatives of the petrographic types 4 to 6 [1] for crystal-chemical analyses. Chemical compositions were determined through a WDS JEOL JX 8600 electron microprobe. Three to six spot analyses were run on each crystal and no significant chemical variation was detected. High quality single crystal X-ray diffraction data were obtained by a SIEMENS P4 diffractometer using MoK alpha radiation, and site occupancies by least squares structure refinements. Figure 1a shows a significant cell volume (Vc) increase with petrographic type, and a _1.5 Angstrom^3 spread within each petrographic type. In solid solutions, Vc is expected to mainly vary with the chemical composition: the higher the proportion of the large ions present, the larger the Vc. In particular, as shown by [2], Vc variations in orthopyroxenes are essentially linear with Fe/(Fe+Mg), and our data fall within this general trend. However, no such a correlation exists at the scale of our values (Fig.1b), rather, each petrographic type plots along a different roughly negative trend. Furthermore, as experimentally obtained by [3], the decrease of the Fe-Mg ordering between the M1 and M2 sites in orthopyroxenes (a temperature-time-dependent process), causes significant Vc increase due to the displacement of the large Fe2+ ions from the larger M2 to the smaller M1 sites. Again, in the Vc versus kD (i.e., the intracrystalline Fe-Mg distribution coefficient) diagram (Fig.1c), we observe no such a correlation. Contrary to the most immediate expectations, our data suggest that the net increase in Vc from H4 to H6 does not significantly depend upon chemical composition and degree of ordering, and demands that another as yet unidentified parameter accounts for the observed trends. Acknowledgments: We thank EUROMET for the Frontier Mt. samples, and PNRA for supporting this study. References: [1] Folco L. et al., this volume. [2] Sykes-Nord J. A. and Molin G. M. (1993) Am. Mineral., 78, 921-931. [3] Domeneghetti M. C. et al. (1985). Am. Mineral., 70, 987-995.

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