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Stishovite, a high‐pressure high‐density polymorph of SiO2, isostructural with rutile, was synthesized at pressures between 75 and 125 kb and temperatures greater than 800°C. Significant conversion to the rutile form was obtained in a modified girdle apparatus at 125 kb; trace amounts were obtained in a belt‐type apparatus between 75 and 100 kb. Stishovite is colorless, acicular in habit, and uniaxial positive with ε = 1.845 ± .005, ω = 1.800 ± .005. The optic axis is commonly highly inclined to the morphological axis. The transformation of coesite (4‐fold coordination) to Stishovite (6‐fold coordination) exemplifies the decrease in molar refraction with increasing coordination predicted by Dachille and Roy.

A unique crystalline phase was the sole constituent in newly opened containers of commercial reagent-grade "ammonium carbonate." The chemical composition, optical constants, x-ray powder diffraction pattern, infrared absorption spectrum, and density indicate a double salt, ammonium bicarbonate-carbamate (NH(4)HCO(3).NH(4) CO(2)NH(2)), which is unstable in air at ambient temperature and decomposes rapidly to ammonium bicarbonate.

Clinoenstatite breaks down to forsterite plus stishovite at 115 kb and about 650°C. The calculated approximate equation of the equilibrium boundary is P (kb) = (97.5 ± 5) + 0.027T (°C). This reaction line is located on the high‐pressure side of the coesitestishovite equilibrium boundary. These results suggest a petrological model of the upper mantle in which successive solid‐solid phase transformations and reactions with increasing pressure provide an explanation for the broad 200‐ to 900‐km seismic discontinuity zone in the mantle.

Infrared spectra of the four-coordinated quartz and coesite polymorphs of SiO(2), the rutile six-coordinated (stishovite) polymorph of SiO(2), and the quartz and rutile polymorphs of GeO(2) show that a change from tetrahedral to octahedral cation coordination results in (i) a 23-per-cent increase in the wavelength of the main absorption band for both the SiO(2) and GeO(2) polymorphs and (ii) a significant increase in the force constant of the same magnitude for the SiO(2) and GeO(2) polymorphs. The quartz and the rutile isostructural pairs for SiO(2) and GeO(2) show that the effect of increasing mass is to increase proportionally the wavelength of the respective main absorption bands. The infrared data for the rutile form of SiO(2) fit the empirical equation of Dachille and Roy relating cation coordination, mass, atomic number, valence, and main absorption wavelength.