Exsolution lamellae of kirschsteinite in magnesium-iron olivine from an angrite meteorite

Abstract

Exsolution phenomena of kirschsteinite (CaFeSi04) in olivine have been studied by single-crystal X-ray diffraction techniques and scanning electron microscopy (SEM) of oriented polished thin sections (PTS) of three single crystals separated from the Antarctic angrite LEW86010, supplemented by micro-area X-ray diffraction with the Laue method (MXL) by synchrotron radiation (SR) for PTS of a rock chip of LEW8601O. The cell dimensions of the host olivine and exsolved kirschsteinite are a = 4.79(3), b = 10.39(5), and c = 6.06(3) A, and a = 4.87(5), b = 11.14(10), and c = 6.36(5) A, respectively, from the precession photos. The PTS of olivine single crystals oriented parallel to (100) show exsolution lamellae of kirschsteinite up to I 0 ~m in width. The two sets of lamellae are symmetrically related and parallel to (031) and (031). Electron microprobe analysis gave Si02 33.1, Ti02 0.07, Al203 0.03, FeO 49.4, MnO 0.61, MgO 13.4, CaO 2.2, Cr203 0.02, V203 0.01, NiO 0.05 (sum 99.4 wt%) for the host olivine and Si02 33.3, Ti02 0.03, FeO 31.5, MnO 0.39, MgO 5.4, CaO 28.5, Cr203 0.02, NiO 0.05 (sum 99.2 wt%) for the exsolved kirschsteinite. The results from 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 (031). The (031) and (031) planes have been known to be twin planes for olivine, and the twinning is by reticular pseudomerohedry based on a quadruple lattice. Although other reported exsolved precipitates in meteoritic olivines exist as inclusions, kirschsteinite in LEW86010 olivine takes the form of lamellae. Our explanation is that LEW86010 olivine is Fe-rich and that lamellar precipitates are more easily formed than inclusions because exsolution lamellae along {031} in Fe-rich olivine maintain lattice coherency.