Silica quartz transformation

Dissolution and redeposition phenomena are currently used to reach this goal. The dissolution of convex asperities is balanced by deposition on concave regions. The net result is an increase in the neck size between the elementary particles and the smoothing of asperities. The smallest pores disappear first. Several parameters may be used to increase the dissolution phenomenon. At room temperature amorphous silica is insoluble (5 ppm) in alcohol, but its solubility is around 70 ppm in water [13]. Under basic conditions the solubility of silica increases. The temperature is a parameter that can be used to increase silica solubility, and these experiments are advantageously performed under pressure in an autoclave. However, the temperature must be low enough (<180°C) to avoid silica-quartz transformation. 

In Figure 5.15, the endotherm peaked at 576°C (onset at 572°C) resulted from the a-/3 quartz transformation in silica. The broad., low intensity endotherm which peaked at 65l°C correlates to the broad endotherm in the soda ash-dolomite system (Figure 5.12) and coincides with continuous reductions in dolomite content indicated in the XRD traces at 600, 660, and 680° C. 

Chapter IX, page 99.—An interesting case of a condensed system, namely, the affinity of the transformation of amorphous silica (quartz glass) into crystallized quartz, has been worked out by R. Wietzel (" Zeitsch. anorg. Chem., 116, p. 71, 1921) in my old laboratory. Since, however, the specific heat of quartz glass falls off so slowly that it was not possible to get anywhere near the region of the T3-Law, the experimental investigation of this case is not yet complete. This is one of the cases where it is very desirable that accurate measurement of specific heat should be continued down to helium temperatures. This is the more important in that no direct experimental test of the application of the Heat Theorem to amorphous substances has yet been made with satisfactory reliability. The transformation from quartz into cristobalite could, however, be followed with adequate accuracy from the standpoint of the Heat Theorem. 

Williams AO, Knapton AD, Ifon ET, Safflotti U. Transforming growth factor beta expression and transformation of rat lung epithelial cells by crystalline silica (quartz). Int J Cancer 1996 65 639-649. 

V. V. Murashov, I. M. Svishchev. Quartz family of silica polymorphs comparative simulation study of quartz, moganite, and orthorhombic silica, and their phase transformations. Phys Rev B 57 5639, 1998. 

Once the amorphous silica has nearly disappeared, the cristobalite that formed early in the calculation begins to redissolve to form quartz. The cristobalite dissolves, however, much more slowly than it formed, reflecting the slow rate of quartz precipitation. After about 300 000 years of reaction, nearly all of the cristobalite has been transformed into quartz, the most stable silica polymorph, and the reaction has virtually ceased. 

Crystalline Silica Three principal polymorphic forms exist at atmospheric pressure. These are quartz, tridymite, and cristobalite. Quartz is stable below 870°C. It transforms to tridymite form at about 870°C. Tridymite is stable up to 1,470°C and transforms to cristobahte at 1,470°C. High cristobalite melts around 1,723°C. Other than these three polymorphs, there are also three high pressure phases of crystalline sihca keatite, coesite, and stishovite. 

For minerals featuring modifying transformations with temperature increase, the decline in hardness is not continuous in character—a temporary increase in hardness takes place in the vicinity of the transformation temperature. This phenomenon is presented in Fig. 6.4.8 (curve 4) for a quartz sample. It is also true for silica glass processed by quartz melting. The curves for silica glass (Fig. 6.4.8, curves 1, 2, 3) show two breakdowns... 

For reversible transformations such as melting/solidification or the Q to (3 quartz inversion in silica, heat flux DSC and power compensated DSC can each be equivalently precise in determining the latent heat of transformation. Transformations of... 

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