Orthopyroxene from the Serra de Magé Meteorite: Structure refinement and estimation of C2/c pyroxene contributions to apparent Pbca diffraction violations

Transmission electron microscopy (TEM) of microstructures in the orthopyroxene of the Serra de Magé cumulate eucrite shows augite lamellae thicker than 500 Å, irregularly spaced within the orthopyroxene matrix and parallel to (100), with $aOpx*≡aAug*,[101]Opx*≡cAug*$ and $bOpx*≡bAug*$ 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 R_obs = 2.81 and 5.17% for the two samples. Because of the presence of some 0kl forbidden reflections with k = 2n + 1 and l = 4n + 2, structure refinement was attempted in space group P2₁ca, as suggested by previous studies, and in space groups P12₁/c1 and P112₁/a, as suggested by the Laue symmetry of the reflections, which is 2/m 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 C2/c augite lamellae. From the intensity of the reflections forbidden in Pbca space group, the calculated augite content ranges from approximately 4 to 12%.