Zhidong Xie

Natural occurrence of Fe2SiO4-spinel in the shocked Umbarger L6 chondrite

Abstract Here we report the first natural occurrence of Fe2SiO4-spinel in a shock-induced melt pocket of the Umbarger L6 chondrite. Optical microscopy, scanning electron microscopy, electron microprobe analysis, and analytical transmission electron microscopy were used to examine the sample. Fe2SiO4- spinel was identified by TEM using selected-area electron diffraction and energy-dispersive X-ray spectroscopy. The symmetry of the diffraction patterns, the ratios of d-spacings, and interplanar angles are consistent with the spinel structure. However, the cell parameter of Fe2SiO4-spinel (8.52 Å), calculated from d-spacing data, is 3.5% larger than that of synthetic Fe2SiO4-spinel (8.235 Å). Chemical analyses of the spinel show olivine stoichiometry with Fe/(Fe + Mg) ratios ranging from 0.62 to 0.99. Fe2SiO4-spinel and stishovite occur within FeO-SiO2-rich zones in the melt pocket, surrounded by SiO2-rich glass and Fe-rich phyllosilicates. Fe2SiO4-spinel plus stishovite also occur with other high-pressure minerals in the melt pocket: ringwoodite, akimotoite, augite, and hollanditestructured plagioclase. We infer that the Fe2SiO4-spinel crystallized from a zone of FeO-SiO2-rich melt within the shock-induced melt pocket. Two models for FeO-SiO2-rich melt are discussed: it was either a residual melt after crystallization of MgO-rich silicates in a chondritic melt pocket, or it was produced by shock melting of FeO-SiO2-rich material.

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

Abstract 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: Na0.06Ca0.02Mg0.71Fe0.20Al0.11Si0.94O3. 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