TEM TEXTURE AND X-RAY SINGLE-CRYSTAL STUDY OF SERRA-DE-MAGE ORTHO-PYROXENE

A combined Transmission Electron Microscopy (TEM), X-ray single crystal diffraction (XRD) and microprobe study was undertaken on the orthopyroxene of Serra de Mage feldspar cumulate eucrite. Two single crystals, natural and equilibrated at several temperatures, were examined in order to identify the true symmetry of the orthopyroxene and to estimate the cooling history of the meteorite. TEM texture analysis showed augite lamellae larger than 500 A irregularly spaced within the orthopyroxene matrix, with orientation parallel to (100). XRD data were collected using a four-circle diffractometer. Structural refinement carried out in space group Pbca converged to a discrepancy factor of R = 2.81%. Because of the presence of some Okl 'forbidden' reflections that may be explained either by P21ca symmetry of the orthopyroxene or by overlap of reflections from the exsolved C2/c clinopyroxene, structural refinement in space group P21ca was also attempted. The resulting discrepancy factor was very close to that obtained in Pbca; however, the anomalous values of many thermal parameters and cation-oxygen bond distances do not support the choice of the P21ca space group. Calibration or KD (KD = (Fe(2+) + Mn)M1(MgM2/(Fe(2+) + Mn)MZMgM1) vs. 1/T gave a closure temperature Tc for the intracrystalline Fe(2+)-Mg exchange reaction of about 470 C, consistent with a relatively slow cooling rate.