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: Na0.06Ca0.02Mg0.71Fe0.20Al0.11 Si0.94O3. 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/cm3. 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, (Na0.08Ca0.03Mg0.95 Fe0.26Al0.15Si0.250.28)2Si1O4, with 0.28 formula units of vacancies (□), 0.11 of Na+ plus Ca2+, and 0.25 of Si4+, 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.

You do not have access to this content, please speak to your institutional administrator if you feel you should have access.