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

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_0.06Ca _0.02Mg_0.71Fe_0.20Al_0.11 Si _0.94O₃. 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 A. The pyroxene composition represents a large deviation from olivine stoichiometry, (Na _0.08Ca_0.03Mg_0.95 Fe_0.26Al _0.15Si_0.25□_0.28)₂Si ₁O₄, 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.