Orthopyroxene from the Serra de Mage Meteorite; structure refinement and estimation of C2/c pyroxene contributions to apparent Pbca diffraction violations

Abstract

Transmission electron microscopy (TEM) of microstructures in the orthopyroxene of the Serra de Mage cumulate eucrite shows augite lamellae thicker than 500 A, irregularly spaced within the orthopyroxene matrix and parallel to (100), with a~px ==a!ug, [lOl]~px ==c!Ug,and b~px ==b!ug. Streaking along a* owing to stacking disorder was observed in selected-area electron diffraction (SAD) patterns. Slabs of clinopyroxene were observed, but no Guinier-Preston zones were detected. Single-crystal X-ray diffraction (XRD) data were collected and structure refinements completed in space group Pbca, converging to Robs= 2.81 and 5.17% for the two samples. Because of the presence of some Ok! forbidden reflections with k = 2n + 1 and! = 4n + 2, structure refinement was attempted in space group P2lca, as suggested by previous studies, and in space groups P12/cl and Pll2/a, as suggested by the Laue symmetry of the reflections, which is 21m rather than mmm. However, the high residual indices R and the anomalous values of many displacement parameters and bond distances did not support the lower-symmetry space groups. The violations of the extinction conditions of the Pbca space group and the monoclinic symmetry of the reflections are due to overlap of reflections from the C21c augite lamellae. From the intensity of the reflections forbidden in Pbca space group, the calculated augite content ranges from approximately 4 to 12%.