EXSOLUTION LAMELLAE OF KIRSCHSTEINITE IN MAGNESIUM-IRON OLIVINE FROM AN ANGRITE METEORITE

Exsolution phenomena of kirschsteinite (CaFeSiO4) in olivine have been studied by single-crystal X-ray diffraction techniques and scanning e lectron microscopy (SEM) of oriented polished thin sections (PTS) of t hree single crystals separated from the Antarctic angrite LEW86010, su pplemented by micro-area X-ray diffraction with the Laue method (MXL) by synchrotron radiation (SR) for PTS of a rock chip of LEW86010. The cell dimensions of the host olivine and exsolved kirschsteinite are a = 4.79(3), b = 10.39(5), and c = 6.06(3) Angstrom, and a = 4.87(5), b = 11.14(10), and c = 6.36(5) Angstrom, respectively, from the precessi on photos. The PTS of olivine single crystals oriented parallel to (10 0) show exsolution lamellae of kirschsteinite up to 10 mu m in width. The two sets of lamellae are symmetrically related and parallel to (03 1) and (0(3) over bar1$). Electron microprobe analysis gave SiO2 33.1, TiO2 0.07, Al2O3 0.03, FeO 49.4, MnO 0.61, MgO 13.4, CaO 2.2, Cr2O3 0 .02, V2O3 0.01, NiO 0.05 (sum 99.4 wt%) for the host olivine and SiO2 33.3, TiO2 0.03, FeO 31.5, MnO 0.39, MgO 5.4, CaO 28.5, Cr2O3 0.02, Ni O 0.05 (sum 99.2 wt%) for the exsolved kirschsteinite. The results fro m MXL for the olivine crystals on the rock PTS are compatible with the observation on the single crystals that the lamellae are parallel to (031) and (0(3) over bar1$). The (031) and (0(3) over bar1$) planes ha ve been known to be twin planes for olivine, and the twinning is by re ticular pseudomerohedry based on a quadruple lattice. Although other r eported exsolved precipitates in meteoritic olivines exist as inclusio ns, kirschsteinite in LEW86010 olivine takes the form of lamellae. Our explanation is that LEW86010 olivine is Fe-rich and that lamellar pre cipitates are more easily formed than inclusions because exsolution la mellae along {031} in Fe-rich olivine maintain lattice coherency.