TEM Texture and x-ray Single-Crystal Study of Serra de Mage Orthopyroxene

Details TEM Texture and x-ray Single-Crystal Study of Serra de Mage Orthopyroxene TEM Texture and x-ray Single-Crystal Study of Serra de Mage Orthopyroxene

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29..460d&link_type=abstract

Meteoritics, vol. 29, no. 4, page 460

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Eucrite, Geothermometry, Pyroxene, Serra De Mage, Structure

Scientific paper

A combined 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 Angstroms irregularly spaced within the orthopyroxene matrix, with orientation parallel to (100). The cell orientation for the lamellae was a*(sub)opx=a*(sub)aug, [101](sub)opx=c*(sub)aug, b*(sub)opx*b*(sub)aug. Ledges at the augite-orthopyroxene interface were observed with a periodicity of about 500 Angstroms. EDAX analysis on a lamella-free zone showed about 0.03 Ca atoms per formula unit. Streaking along a* was observed in SAD patterns, although no Guiner-Preston zones were detected according to [1]. 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 0kl "forbidden" reflections (with k = 2n + 1, l = 4n + 2) that may be explained either by P21ca symmetry of the orthopyroxene or by overlap of reflections from the exolved C2/c clinopyroxene [2], structural refinement in space group P2(sub)1ca 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 P2(sub)1ca space group. Calibration of K(sub)D (K(sub)D = (Fe^2++Mn)(sub)M1Mg(sub)M2/(Fe^2++Mn)(sub)M2Mg(sub)M1) vs. 1/T [3] gave a closure temperature T(sub)c for the intracrystalline Fe^2+-Mg exchange reaction of about 470 degrees C, consistent with a relatively slow cooling rate. References: [1] Harlow G .E. et al. (1979) EPSL, 43, 173-181. [2] Sasaki S. and Prewitt C. (1984) Am. Min., 69, 1082-1089. [3] Molin G. et al. (1991) EPSL, 105, 260-265.

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