Quartz, formation experimental

The formation of crystalline quartz from a gel at low temperatures has not been established experimentally. Direct crystallization of quartz from sea water has been observed (Mackenzie and Gees, 1971). In the opinion of Harder and Fleming (1970), quartz is formed only by adsorption of Si02 by hydroxides of Fe, Al, Mg etc. from undersaturated solutions, while amorphous silica arises in supersaturated solutions. The mechanism of crystallization of gels to quartz was studied in detail in our laboratory by Mitsyuk (1974). It was established that in amorphous silica the process of quartz formation goes on at an appreciable rate in hydrothermal conditions —T— 150-250°C and F = 0.5-4.0 kbar—and is characterized by the following features ... 

Figure 20-1 presents the data from Table 20-1 in both cumulative and frequency format. In order to smooth out experimental errors it is best to generate the frequency curve from the slope of the cumulative curve, to use wide-size intervals or a data-smootning computer program. The advantage of this method of presenting frequency data is that the area under the frequency curve equals 100 percent, hence, it is easy to visually compare similar data. A typical title for such a presentation would be Relative and cumulative mess distributions of quartz powder by pipet sedimentation. 

The deformation lamellae can be used to provide an estimate of the stress during the formation of the deformed envelope of the fault. In metals and halides, deformation lamellae form only in the exponential creep regime, which occurs above a critical stress level (Tsenn and Carter 1987). Experimental data on quartz suggest that lamellae form only at high stress, but no particular association with exponential creep can be discerned in the available data. The normalized transition stress in different... 

Koziol A. M. and Newton R. C. (1995) Experimental determination of the reactions magnesite -F quartz = enstatite -F CO2 and magnesite = periclase -F CO2 and the enthalpies of formation of enstatite and magnesite. Am. Mineral. 80, 1252-1260. 

Prior to the main study the experimental conditions for performing kinetic measurements were established. CH4 conversions of below 10% (reactants diluted with H O) could be obtained at temperatures below 923 K. The carbon limits, which correspond to the maximum partial pressure and minimum temperature under which formation of carbon could be observed as a weight increase by the microbalance, were also determined. In addition, test runs at the most severe conditions, i.e. 1073 K and 4.3 bar CH4, revealed that the support as well as the quartz and the alumina surfaces are sufficiently inert, and that gas phase activation of CH4 is not important. 

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