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We report a new mineral that occurs in shock-induced melt veins of the Tenham L6 chondrite. The new mineral, identified by transmission electron microscopy (TEM), occurs as acicular nanocrystals in a glassy matrix at the edge of shock-induced melt veins that crystallized during rapid quench at high pressure. The elongate crystals have aspect ratios up to 25. Widths range from -5 to -40 nm and lengths are up to 500 nm. Energy-dispersive X-ray spectroscopy (EDS) analyses provide the relative cation abundances that are consistent with a pyroxene-like stoichiometry: Na 0.06 Ca 0.02 Mg 0.71 Fe 0.20 Al 0.11 Si 0.94 O 3 . Selected area electron diffraction (SAED) patterns from single-crystal and polycrystalline aggregates indicate an olivine structure with refined cell parameters: a = 4.78, b = 10.11, and c = 5.94 Å and a calculated density of 3.32 g/cm 3 . Synchrotron X-ray microdiffraction data are consistent with an olivine structure and provide similar cell parameters: a = 4.778, b = 10.267, c = 5.937 Å. The pyroxene composition represents a large deviation from olivine stoichiometry, (Na 0.08 Ca 0.03 Mg 0.95  Fe 0.26 Al 0.15 Si 0.25□0.28 ) 2 Si 1 O 4 , with 0.28 formula units of vacancies (□), 0.11 of Na +  plus Ca 2+ , and 0.25 of Si 4+ , in octahedral sites. Our observations indicate that a metastable and nonstoichiometric olivine structure can crystallize from a silicate melt during rapid quench. Trace amounts of such defects may be present in stable olivines in the deep upper mantle

A new mineral with an olivine structure and pyroxene composition in the shock-induced melt veins of Tenham L6 chondrite