Silica cristobalite

If fluids initially in equilibrium with quartz ascend rapidly, some metastable minerals (amorphous silica, cristobalite, wairakite) may precipitate because of supersaturation with respect to Si02 (e.g., Wolery, 1978 Bird and Norton, 1981). Important processes for the supersaturation and deviation from the equilibrium between fluids and rocks are adiabatic boiling, mixing of fluids and conductive cooling of fluids (Giggenbach, 1984). 

Figure 4 The structure of the two forms of silica cristobalite (crystalline, left) and silica glass (amorphous, right). (Reprinted with permission from Ref. 4.)...

Figure 1.50 X-ray diffraction patterns of vitreous silicon, crystalline silica (cristobalite), and sol-gel-derived silica. Reprinted, by permission from H. Scholze, Glass, p. 97. Copyright 1991 by Springer-Verlag.

All the forms of silica and silica-alumina studied are similar in the rates at which catalytic activity was induced in them by radiation (82). For instance, the yields of increased activity per unit dose were within a factor of about 100 of each other for silica gel, quartz, vitreous silica, cristobalite, and Cab-o-sil, although before irradiation the quartz and vitreous silica were lO to 10 times as active as any of the gels. In a series of experiments using reactor radiation in which silica gel was compared with silica-aluminas, the yield differed by less than a factor of 100. In other experiments the differences have been even less (83). These results are consistent with a single kind of defect being introduced at... 

The non-equilibrium condition of most groundwater systems with respect to many primary minerals, or similarly the metastability which exists with respect to many semi-crystalline or amorphous phases are common problems, especially for silicates. Some clear identification is needed for system reaction time, or the rate at which equilibrium is approached, and similarly identification is needed for metastable plateaus of pseudo-equilibrium, especially for compounds such as amorphous silica, cristobalite, quartz, clay minerals, etc. The likely magnitude of saturation indices which could apply to a given mineral could be specified for a variety of conditions. In this volume, Glynn, and elsewhere others, have recently shown that some error occurs in the calculated saturation values for trace elements when pure end member minerals are assumed to be present, when actually the phases are solid solutions. The consensus among modelers appears to be that error is present and significant the challenge is to develop procedures that quantify the error, so models become tools that provide realistic and interpretable results. 

Quartz Silica Cristobalite Silica Tridymite Silica Oil of vitriol... 

ACUTE HEALTH RISKS exposure to very high levels of Silica, Cristobalite can cause a serious lung disease called Silicosis Simple Silicosis may only cause changes in chest x-ray... 

One should remember that in some cases the expansion depends on the fineness of reactive forms of silica. According to Hobbs [123], the expansion is the highest at the particle size of reactive component, for example opal, in the range from 0.1 to 1 mm. At the fineness lower than 10 pm the expansion is eliminated and the reactive silica reveals the nature of pozzolanic material. The behaviour of chalcedony is not typical despite of the presence of active form of silica (cristobalite) the expansion is not always observed. 

XRD patterns before and after the oxidation for 100 h are shown in Figure 4.11. For Zl, see Figure 4.11a, there are no visible changes for oxidation at 1000 "C compared with the as-sintered product. However, after 1200 "C oxidation, a visible silica (cristobalite) peak was detected and a very few muUite peaks were also detected. When the temperature reached to 1400 "C, the intensity of mullite peaks became stronger and more peaks were visible, and the peak of cristobalite was still... 

Amorphous silica, cristobalite, and tridymite all give a single band at 790 to 800 cm although the absorption of the crystalline species is distinctly sharper than that of the amorphous. 

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