Original and alteration mineral phases in the NWA 10628 Martian shergottite determined by micro‐Raman spectroscopy assisted with micro‐energy dispersive X‐ray fluorescence imaging

Abstract

AbstractNorthWest Africa 10628 (NWA 10628) is a shergottite found in Tindouf (Algeria) in 2014. Little is known on this meteorite apart from the Meteoritical Bulletin report. In this work, original and altered mineral phases of NWA 10628 were characterised by micro‐Raman spectroscopy assisted with micro‐energy dispersive X‐ray fluorescence imaging. The matrix of the meteorite is dominated by high‐Ca pyroxenes, augite ((Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6) and diopside (CaMgSi2O6). Olivine and feldspars appeared as small crystals all along the matrix. Merrillite (Ca9NaMg(PO4)7) was found as big crystals in inclusions. As whitlockite (Ca9NaMg(PO4)6(HPO4)) can be transformed into merrillite at high temperature and pressure, we suggest that merrillite in the meteorite was originally whitlockite, before its ejection from Mars. Gypsum (CaSO4·2H2O) was found as small crystals and in fractures; this last one could come from terrestrial weathering because it was associated with calcite (CaCO3). Small anhydrite (CaSO4) crystals are shown in the bulk together with gypsum, probably from the partial dehydration of original Martian gypsum by the high‐pressure/temperature of ejection. Pyrite and marcasite (FeS2 polymorphs) minerals were detected, as well as their oxidation products, goethite (α‐FeO (OH)) and rozenite (FeSO4·4H2O), due to their terrestrial weathering. The identification of hematite (Fe2O3), magnetite (Fe3O4), ilmenite (FeTiO3) and anatase (TiO2) suggested that ulvospinel (Fe2TiO4) was the precursor on Mars, because ulvospinel is transformed into ilmenite by pressure and later ilmenite can be altered to magnetite/hematite and anatase by oxidative weathering on Earth. This non‐destructive analytical methodology provides deep mineralogical characterisation adequate for scarce samples like those returning from the Mars Sample Return mission.