A NEW HIGH-PRESSURE CALCIUM ALUMINOSILICATE (CaAl2Si3.5O11) IN MARTIAN METEORITES: ANOTHER AFTER-LIFE FOR PLAGIOCLASE AND CONNECTIONS TO THE CAS

Introduction: Most martian meteorites are highly shocked, with plagioclase transformed to maskelynite or even melted and partially crystallized [1,2]. These responses to shock conditions on Mars provide clues to the nature of the ejection events for martian meteorites and to local shock conditions. Where new-to-science phases are discovered, meteoritic occurrences can also inform the study of earth systems at high pressures and temperatures. During a nanomineralogy investigation of shock melt pockets in the basaltic shergottites Zagami and NWA 856, we identified a new high-pressure aluminosilicate mineral (‘HAS’), CaAl2Si3.5O11, with a P63/mmc hexagonal structure (Figs. 1-3). We characterized its composition, structure and petrography using high-resolution SEM, electron back-scatter diffraction (EBSD), electron microprobe analysis (EPMA), and synchrotron X-ray diffraction. The mineral is currently under review by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association. Occurrence, chemistry, and crystallography: The Zagami and NWA 856 meteorites consist mainly of pyroxene (mostly zoned augite and pigeonite) and maskelynite (shock-generated plagioclase glass) with accessory ilmenite, titanomagnetite, baddeleyite, merrillite, apatite, and Fe sulfide, plus shock melt veins and pockets. In addition to HAS, which is described below, we observed the high pressure minerals liebermannite, lingunite, stishovite, and a new type of Fig. 1. Backscatter electron (BSE) image showing a HAS-bearing shock melt pocket in Zagami.