Alpha cristobalite

D iogenically precipitated silica is a metastable silica polymorph which must eventually alter to quartz under the earth s surface conditions. Present observations of deep-sea sediments suggest that this transformation may occur directly or through an intermediate, alpha cristobalite. Several models have been proposed to ascertain the rate at which these processes occur. This series of papers tests these models and offers simple but powerful methods for detecting changes in crystal form as a function of geologic age. 

The equilibrium solubihty for low or alpha cristobalite in distilled water at25°C (by extrapolation from higher temperatures) is ca. 250 fiM (18), In a later paper (19) these authors showed that this material was saturated sevenfold in distilled water at room temperatures and that the above extrapolated value was not attained during the experiment (4.5 years). Although we beheve that the extrapolated value is valid, it... 

Yeganeh-Haeri, Weidner, D. J. and Parise, J. B. Elasticity of alpha-cristobalite A silicon dioxide with a negative Poisson s ratio. Science 257,1992, 650-652. 

Alpha-beta (a-jS) transitions of the condensed forms of Si02 quartz, try-dimite, and cristobalite may all be regarded as lambda transformations. Their kinetics are higher than those of quartz-trydimite, quartz-cristobahte, and quartz-coesite, which are first-order transformations. Figure 2.7 plots in detail the evolution of enthalpy, entropy, heat capacity, and volume at the transition zone... 

Fig. 4. Computer-generated crystal structure models nop row. left to right) Cuprite, zinc-blende, rutile, perovskite. iridymite (second row) Cristobalite. potassium dihydrogen phosphate, diamond, pyrites, arsenic (third rowt Cesium chloride, sodium chloride, wurtzite. copper, niccolite (fourth row) Spinel, graphite, beryllium, carbon dioxide, alpha i uanz. [AT T Bel Laboratories ...

SYNONYMS 43-63C, alpha-crystobalite, calcined diatomaceous earth, calcined diato-mite, cristobalite, crysvarl, metacristobalite, W006, WGL 300. 

Cristobalite (alpha) [14464-46-1] [Named after Cerro San Cristdbal near Pachuca, Mexico] (ICSD 47219 and PDF 39-1425) SiO, M = 60.0843 46.74 wt.% Si 53.26 wt.%0 (Tectosilicates, framework) Coordinence Si (4) Tetragonal as 497.1 pm cs 691.8pm (Zs4) P.G. 422 S.G. P4,2,2 Uniaxial (-) e= 1.482 0)= 1.489 S= 0.007 6-7 2330 Habit coarse aggregate. Color colorless. Streak white. Diaphaneity transparent to translucent. Luster vitreous( Le., glassy). Fracture conchoidaL Twinning (III). Oiemical resistant to Strong mineral acids, attacked by HF and molten alkali-metal hydroxides. Mdting point 1713 C... 

While tridymite and cristobalite may exist for indefinite periods of time at room temperature, tire low-temperature alpha-quartz is believed to be the form of silica truly stable at these temperatures. A considerable expansion accompanies the conversion of a-cristobalite to /J-cristobalite at 220-275°C, and a sudden expansion of -2.2% occurs during the inversion of a-quartz at 573°C. The conversion of quartz to tridymite at 870°C is accompanied by an expansion of -15%. 

Bentonite. Bentonite is of volcanic origin and is composed of a mixture of montmorillonite, feldspar, and biotite. Alpha-quartz and cristobalite are important contaminants. Bentonite is nsed in the oil industry as a catalyst and also in drilling muds. In addition, bentonite is nsed as an additive for paper, plastics, and paints. 

Absher M, Sjostrand M, Baldor LC, Hemenway DR, Kelley J. Patterns of secretion of transforming growth factor-alpha (TGF-alpha) in experimental silicosis. Acute and subacute effects of cristobalite exposure in the rat. Reg Immunol 1993 5 225-231. 

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