Zhidong Xie, Thomas G. Sharp, Kurt Leinenweber, Paul S. DeCarli, Przemek Dera; A new mineral with an olivine structure and pyroxene composition in the shock-induced melt veins of Tenham L6 chondrite. American Mineralogist 2011;; 96 (2-3): 430–436. doi: https://doi.org/10.2138/am.2011.3437
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 Fe0.26Al0.15Si0.25□0.28)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.
Keywords: chain silicates, chemical composition, chondrites, crystal chemistry, crystal structure, formula, glass materials, L chondrites, lattice parameters, melts, metamorphism, meteorites, nesosilicates, new minerals, olivine, olivine group, ordinary chondrites, orthosilicates, pyroxene group, shock metamorphism, silicates, spectra, stoichiometry, stony meteorites, synchrotron radiation, TEM data, veins, X-ray diffraction data, X-ray spectra, Tenham Meteorite