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A new high‐pressure silicate, (Mg,Fe,Si)2(Si,□)O4 with a tetragonal spinelloid structure, was discovered within shock melt veins in the Tenham and Suizhou meteorites, two highly shocked L6 ordinary chondrites. Relative to ringwoodite, this phase exhibits an inversion of Si coupled with intrinsic vacancies and a consequent reduction of symmetry. Most notably, the spinelloid makes up about 30–40 vol% of the matrix of shock veins with the remainder composed of a vitrified (Mg,Fe)SiO3 phase (in Tenham) or (Mg,Fe)SiO3‐rich clinopyroxene (in Suizhou); these phase assemblages constitute the bulk of the matrix in the shock veins. Previous assessments of the melt matrices concluded that majorite and akimotoite were the major phases. Our contrasting result requires revision of inferred conditions during shock melt cooling of the Tenham and Suizhou meteorites, revealing in particular a much higher quench rate (at least 5 × 103 K s−1) for veins of 100–500 μm diameter, thus overriding formation of the stable phase assemblage majoritic garnet plus periclase. [ABSTRACT FROM AUTHOR]

We report the first natural occurrence and single-crystal X-ray diffraction qstudy of the Fe-analog of wadsleyite [a = 5.7485(4), b = 11.5761(9), c = 8.3630(7) Å, V = 556.52(7) A3; space group Imma], spinelloid-structured Fe2SiO4, a missing phase among the predicted high-pressure polymorphs of ferroan olivine, with the composition (Fe2+ 1.10Mg0.80Cr3+ 0.04Mn2+ 0.02Ca0.02Al0.02Na0.01)Σ2.01(Si0.97Al0.03)Σ1.00O4. The new mineral was approved by the International Mineralogical Association (No. 2018-102) and named asimowite in honor of Paul D. Asimow, the Eleanor and John R. McMillan Professor of Geology and Geochemistry at the California Institute of Technology. It was discovered in rare shock-melted silicate droplets embedded in Fe,Ni-metal in both the Suizhou L6 chondrite and the Quebrada Chimborazo (QC) 001 CB3.0 chondrite. Asimowite is rare, but the shock-melted silicate droplets are very frequent in both meteorites, and most of them contain Fe-rich wadsleyite (Fa30 -- 45). Although the existence of such Fe-rich wadsleyite in shock veins may be due to the kinetic reasons, new theoretical and experimental studies of the stability of (Fe,Mg)2SiO4 at high temperature (>1800 K) and pressure are clearly needed. This may also have a significant impact on the temperature and chemical estimates of the mantle's transition zone in Earth. [ABSTRACT FROM AUTHOR]

Tuite is a high-pressure γ-form of Ca3( PO4)2. An occurrence of tuite partly transformed from merrillite and chlorapatite was observed in the chondritic area adjacent to the shock veins in the Suizhou meteorite. Tuite grains are found in contact with both merrillite and chlorapatite, indicating two different transformation pathways. Tuite isochemically transformed from merrillite contains much higher contents of Na2O and MgO than those transformed from chlorapatite. Tuite transformed from merrillite does not contain Cl, but tuite transformed from chlorapatite contains 1.90-3.91 wt% of Cl, hence indicating an incomplete phase transformation from chlorapatite to tuite. P-T conditions of above 12 GPa and 1100 °C are probably required for the transformation from merrillite and chlorapatite to tuite. A temperature gradient from the hot vein at 2000 °C to the surrounding chondritic area at 1000 °C corresponds to the partial phase transitions in the Suizhou phosphates. Fast cooling of the thin shock veins plays a key role in the preservation of phosphates that suffered partial high-pressure phase transformation. [ABSTRACT FROM AUTHOR]

Shock-produced akimotoite was identified in the Suizhou chondritic meteorite, which occurs in two kinds of occurrence. The first is the irregular layers of akimotoite up to 4 μm in thickness occurring in fractures and cracks of low-Ca pyroxene enclosed in the shock veins. The second is the zonal polycrystalline aggregates of akimotoite in shocked pyroxene grains close to the shock vein, where akimotoite occurs in a zonal area in between pyroxene and MgSiO-glass as irregular small clumps up to 5 μm in size. This investigation suggests a solid-state transformation mechanism of pyroxene to akimotoite, and that akimotoite should have nucleated and grew in the area with abundant defects caused by shock deformation because the defect significantly enhances the solid-state reactivity and the kinetics of nucleation of high-pressure phase. The spatial relationship among the composed grains of pyroxene, akimotoite and MgSiO-glass (possibly vitrified perovskite) demonstrates a temperature gradient from the vein wall to the unmelted chondritic meteorite. [ABSTRACT FROM AUTHOR]

Tuite, γ-Ca3(PO4)2, was first discovered as the high-pressure phase of whitlockite in shock veins of the Suizhou L6 meteorite. This study reports the finding of tuite in a shock vein of the same Suizhou chondrite as a product of decomposition of chlorapatite, where it coexists with coarse-grained ringwoodite, majorite, lingunite, fine-grained majorite-pyrope solid solution, and magnesiowüstite. Moreover, we also successfully synthesized tuite with a multianvil apparatus from chlorapatite at 15 GPa and 1573 K over 24 h. Both natural and synthetic tuite crystals were examined by means of optical microscopy, scanning electron microscope, electron microprobe analysis, X-ray diffraction, and Raman spectroscopy. Our results suggest that the Na2O, MgO, and Cl contents in natural tuite may serve as good indicators for distinguishing the precursor phosphate mineral, chlorapatite or whitlockite. [ABSTRACT FROM AUTHOR]