Stishovi te, a form of SiO. with Si in sixfold coordination, inverts tmder certain conditions to silica glass with Si in fourfold coordination. The inversion rate determined at ten temperatures between 3000 and 800°C for Meteor Crater stishovite can be extrapolated to the time for total inversion of stishovite to silica glass at each temperature. The resulting time-temperature curve constitutes a limit for thermal distribution models of Meteor Crater immediately after passage of the shock wave generated by meteoritic impact and initial cool ing by adiabatic e1."Pansion. The data indicate the virtual impossibility that stishovite can be formed and preserved at the surface of the earth by any mechanism other than meteoritic impact. The transformation proceeds by nucleation of glass centers along grain boundaries and dislocation surfaces in the stishovite grains, then by growth from these centers into the host materiaL The growth rate of the already formed glass centers is the principal mechanism cont.rolling the observed transformation rate in the temperature range of investigation. The transformation appears to proceed by the formation of a dense sixfold-coordinated glass from the stishovite, then by a breakdown of the dense glass to the observed fourfold-coordinated silica glass. The final concentrate consists of clumps and aggregates of submicron-sized grains of stisho vite with a mean index of refraction of 1.806. Even though the Meteor Crater stishovite i.s intimately intergrown with quartz, coesite, and p articularly with amorphous silica, and is diffi cult to concentrate entirely free of other phases, no silica phase other than stishovite could be detected in our samples. Stishovite and zircon are insoluble in HF in the concentration method used [Fahey, 1962J. The concentrate contains 1.3 per cent of zircon by weight. The zircon 5595
Twenty-eight chemically analyzed specimens for 19 of which the analyses have not been published previously and many unanalyzed specimens of serpentine-groups minerals were studied by various techniques, including chemical, optical, static dehydration, and differential-thermal-analysis (DTA), andThe serpentine group of minerals has associated with it an .abundance of mineral names. In making this study of the minerals of the serpentine group it was necessary to investigate some of these materials and to consult the work of others for additional data. Table 1 summarizes these data and gives our observations and interpretations of them.
Pecoraite is a new phase in the natural system H(2)O-NiO-MgOSiO(2), the nickel analog of clinochrysotile. It occurs in cracks in the Wolf Creek meteorite in Australia where it was formed under hydrothermal conditions. Particles of pecoraite are very small curved plates which have begun to coil; some have achieved spiral form.
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