Chenmingite, FeCr2O4 in the CaFe2O4-type structure, a shock-induced, high-pressure mineral in the Tissint martian meteorite

1Chi Ma,2Oliver Tschauner,1John R. Beckett,3Yang Liu,4Eran Greenberg,4Vitali B. Prakapenka
American Mineralogist 104,1521-1525 Link to Article [https://doi.org/10.2138/am-2019-6999]

1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
2Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154, U.S.A.
3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, U.S.A.
4GSECARS, University of Chicago, Argonne National Laboratory, Chicago, Illinois 60637, U.S.A.
Copyright: The Mineralogical Society of America

Chenmingite (FeCr2O4; IMA 2017-036) is a high-pressure mineral, occurring as micrometer- to submicrometer-sized lamellae within precursor chromite grains along with xieite and Fe,Cr-rich ulvöspinel next to shock-induced melt pockets, from the Tissint martian meteorite. The composition of type chenmingite by electron probe analysis shows an empirical formula of (Fe2+0.75Mg0.23Mn0.02) (Cr1.60Al0.29Fe3+0.06Fe2+0.04Ti0.02)Σ2.01O4. The general and end-member formulas are (Fe,Mg)(Cr,Al)2O4 and FeCr2O4. Synchrotron X-ray diffraction reveals that chenmingite has an orthorhombic Pnma CaFe2O4-type (CF) structure with unit-cell dimensions: a = 9.715(6) Å, b = 2.87(1) Å, c = 9.49(7) Å, V = 264.6(4) Å3, and Z = 4. Both chenmingite and xieite formed by solid-state transformation of precursor chromite under high pressure and high temperature during the Tissint impact event on Mars. The xieite regions are always in contact with melt pockets, whereas chenmingite lamellae only occur within chromite, a few micrometers away from the melt pockets. This arrangement suggests that chenmingite formed under similar pressures as xieite but at lower temperatures, in agreement with experimental studies.

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