Synthesis and optical crystallography, of stishovite, a very high pressure polymorph of SiO₂

Abstract

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.

Laser probe argon‐40/argon‐39 dating of coesite‐ and stishovite‐bearing pseudotachylytes and the age of the Vredefort impact event

“…1b). In general, the impact theory has gained wider acceptance, particularly since the discovery of shatter cones (Dietz, 1961;Hargraves, 1961), which require near-surface stresses of 20-60 kbar (Roddy and Davis, 1977) and, within quartzites of the collar rocks (Martini, 1978), the high pressure Si~po1ymorphs coesite, formed at >30 kbar (Steffler, 1971) and stishovite, formed at >75 kbar (Sclar et al,' 1962). The presence of planar deformation features in quartz at Vredefort has also been considered indicative of shock metamorphism, despite overprinting by high temperature, postshock annealing (Grieve et al, 1990;Leroux et al, 1994).…”

Synthesis and properties of a high-pressure rutile-type AlAsO4

“…The ratio for the A1AS0 4 polymorphs is 1.51 and the ratio for the Si0 2 polymorphs is 1.49 (SCLAR et al, 1962). The mean of the three published values for the GeO a polymorphs, which range from 1.48 to 1.54, is 1.51 (LIPPIN- COTT et al, 1958;DACHILLE andROY, 1959, andSCLAR et al, 1962). The effect of coordination on the force constant, therefore, is apparently independent of the chemical composition.…”

“…DACHILLE and ROY (19596) suggested that the ratio of the squares of the wavelengths of the main absorption band for pairs of polymorphs may indicate the magnitude of the effect of coordination on the force constant as based on the expression for the simple-harmonic oscillation of two masses about a point (/ = \π )>ΊίΙμ where / is the frequency, k is the force constant, and μ is the reduced mass). The ratio for the A1AS0 4 polymorphs is 1.51 and the ratio for the Si0 2 polymorphs is 1.49 (SCLAR et al, 1962). The mean of the three published values for the GeO a polymorphs, which range from 1.48 to 1.54, is 1.51 (LIPPIN- COTT et al, 1958;DACHILLE andROY, 1959, andSCLAR et al, 1962).…”

“…The specific refractivity (Gladstone-Dale relationship) and the specific refractive capacity of ALLEN (1956) for each of the reconstructively related pairs of polymorphs of Ge0 2 , Si0 2 , and A1AS0 4 do not show systematic numerical differencs. The per-cent difference in specific refractivity relative to the quartz form is -0.6, -6.9, and -4.9 for the A1AS0 4 , Si0 2 , and Ge0 2 polymorphs, respectively; the per-cent difference in specific refractive capacity is +12.1, -0.1, and +7.4 for the A1AS0 4 , Si0 2 , and GeO¡¡ polymorphs, respectively (Table 2 and SCLAR et al, 1962). It is of interest to note that the Gladstone-Dale relationship is empirical and that the specific refractive capacity is based on the volume of the optical indicatrix whereas the molar Indices of refraction of the quartz form of AlAsO, are ε -1.610, ω = 1.595, both i!z 0.002, ε-ω = 0.015, measured on trigonal trapezohedral crystals up to 52 microns long and 26 microns wide produced by reaction of Al2Oa and As205 in air at 1200 C; ε was measured on crystals whose orientation was determined conoscopically.…”

“…This generalization was based on comparative molar refraction values of the 4-coordinated quartz form of Ge0 2 and the 6-coordinated rutile form of Ge0 2 and on molar refraction data for 4-and 6-coordinated aluminum and magnesium in glass. SCLAR et al (1962) Z. Kristallogr.…”