Quartz cristobalite

Table 3. Thermodynamic Properties of Quartz, Cristobalite, and Liquid Si02...

Although natural quartz, cristobalite and opal are used as fillers, only synthetic products (fumed and precipitated silicas) find use as fillers in rubber base adhesives. 

As a result of its unique chemical and physical properties, silica gel is probably the most important single substance involved in liquid chromatography today. Without silica gel, it is doubtful whether HPLC could have evolved at all. Silica gel is an amorphous, highly porous, partially hydrated form of silica which is a substance made from the two most abundant elements in the earth s crust, silicon and oxygen. Silica, from which silica gel is manufactured, occurs naturally, either in conjunction with metal oxides in the form of silicates, such as clay or shale, or as free silica in the form of quartz, cristobalite or tridymite crystals. Quartz is sometimes found clear and colorless, but more often in an opaque form, frequently colored... 

Figure 1.142 shows the dependence of solubility of Si02 minerals (quartz, cristobalite) on temperature. As described already, cristobalite occurs in peripheral and shallower part of hydrothermal alteration zone. Quartz is present in zones occurring in deeper and closer to the gold-quartz veins. Such zoning from quartz to cristobalite is also common in main active geothermal systems (Hayashi, 1973 Takeno et al., 2000). 

Fig. 26.3. Silica concentration (bold lines) in a fluid packet that cools from 300 °C as it flows along a quartz-lined fracture of 10 cm aperture, calculated assuming differing traversal times At. Fine lines show solubilities of the silica polymorphs quartz, cristobalite, and amorphous silica.

Fig. 26.7. Variation in silica concentration (top) and saturation indices (log Q/K) of the silica polymorphs (bottom) over the course of the reaction path shown in Figure 26.6. The dashed lines in the top diagram show Si02(aq) concentrations in equilibrium with quartz, cristobalite, and amorphous silica.

Silica is of particular importance because of its use as a stable catalyst support with low acidity and its relationship to zeolite catalysts, which will be discussed in chapter 4. Silicon is an abundant material in the earth s crust and occurs in various forms including silica. Silica is also polymorphous with the main forms being quartz, cristobalite and trydimite. The stable room temperature form is quartz (Si02). Recently, a new family of stable silica-based ceramics from chemically stabilized cristobalites has been described using electron microscopy (Gai et al 1993). We describe the synthesis and microstructures of these ceramic supports in chapters 3 and 5. 

Six different silica modifications were used vitreous silica, quartz, cristobalite, tridymite, coesite, and stishovite. Two of these—cristobalite and tridymite—were prepared from fine amorphous silica powder by tempering samples at 950°C. with 1% of a mineralizer (K2CO3 and KH2P04, respectively). Vitreous silica was obtained from fused rock crystals. All other samples were natural minerals pure specimens of Brazilian rock crystal were used as quartz coesite and stishovite were obtained as fine powders by isolation from Coconino sandstone of the Barringer Meteor Crater in Arizona (4). 

Preliminary tests with samples of vitreous silica, quartz, cristobalite, and tridymite not treated with NaOH showed an effect which for quartz was previously described (7, 13) repeated tests on the same sample did not reproduce the dissolution pattern but gave reduced values. The example in Figure 8 shows three of four repeated runs on the same cristobalite sample. Although the surface area remained constant, the concentration increase with time was always slower than in the preceding... 

The listed chemical formulae are ideal and most of these minerals contain trace and minor elements which undoubtedly affect the CL. Several of these minerals have polymorphic or compositional varieties which also may, or do, show CL (e.g. the silica polymorphs quartz, cristobalite, tridymite phosphate compositional varieties apatite, whitlockite, farringtonite, buchwaldite carbonate compositional varieties calcite, dolomite, magnesite). Glass and maskelynite (shock modified feldspar), although not strictly minerals, are relatively common. Below are described the CL observations for the most common phases including enstatite, feldspar and forsterite and they are related to their use for interpreting the mineralogy of meteorites. The observations for the other minerals are sporadic and many details have yet to be studied. 

Silica is one of the most abundant chemical substances on earth. It can be both crystalline or amorphous. The crystalline forms of silica are quartz, cristobalite, and tridymite [51,52]. The amorphous forms, which are normally porous [149] are precipitated silica, silica gel, colloidal silica sols, and pyrogenic silica [150-156], According to the definition of the International Union of Pure and Applied Chemistry (IUPAC), porous materials can be classified as follows microporous materials are those with pore diameters from 3 to 20 A mesoporous materials are those that have pore diameters between 20 and 500 A and macroporous materials are those with pores bigger than 500 A [149],... 

Silica, or Si02, occurs in nature as quartz, cristobalite, or tidymite. 

Silica dust can be present in industrial atmospheres in three forms,ok quartz, cristobalite and tridymite, depending upon the process temperatures. When cristobalite or tridymite are present, one-half the value of the threshold limit formula is used. The validity of the results depends on the cyclone and pump system... 

The other clay mineral, illite, also has some strontium sorption, but the distribution coefficient is about one order of magnitude lower than that of montmorillonite and rectorite. Since, in illite, the layer charge is compensated by non-exchangeable cations (Chapter 1, Table 1.2), cation sorption can takes place only on the deprotonated edge sites. This is the case for tectosilicates (quartz, cristobalite). 

Silica can exist in many crystalline forms such as quartz, cristobalite, and tridymite. Fumed silica on the contrary tends to be amorphous, which could be attributed to the fabrication process of the abrasive. The amorphous nature is probably caused by the rapid cooling employed in the process [83]. Colloidal silica, which is usually synthesized via wet chemical methods, is highly amorphous as well. In addition, colloidal silica particles are usually spherical and highly hydrated in nature, which makes them far less likely to cause scratches on metal substrate surface. 

Meagher, E. P., J. A. Tossell, and G. V. Gibbs (1979). A CNDO/2 molecular orbital study of the silica polymorphs quartz, cristobalite and coesite. Phys. Chem. Mineral. 4, 11-21. 

High-temperature electrolysis of tridymite brings about migration of impurities towards the cathode, while tridymite in the vicinity of the anode is converted to quartz below 1050 °C and to cristobalite above 1050 °C this temperature is approximately equal to that of quartz-cristobalite inversion, according to data of other authors. 

Silica refractories contain more than 93% Si02 and are made from natural quartzites comprising in excess of 96% Si02 which is converted to cristobalite and tridymite on firing while decreasing the density of the material. Besides the residual quartz, cristobalite and tridymite are the main crystalline phases of the product, which, in contrast with the materials described so far, undergoes volume expansion and even increase in porosity on firing. 

Coating of SiOi Sand (Quartz Cristobalite) with Iron Oxides... 

Dissolution of silicon oxides is geographically important since silica is one of the most abundant minerals in the Earth s crust. At least nine different Si02 phases are found in natural and engineered earth systems in the form of quartz, cristobalite,... 

Representative Chemicals Quartz Cristobalite Stishovite Tridymite Coesite... 

In general, the variation of the intensity ratio IJI p with is not linear, as shown by the curves of Fig. 14-2. The experimental points were obtained by measurements on synthetic binary mixtures of powdered quartz, cristobalite, beryllium oxide, and potassium chloride the curves were calculated by Eq. (14-9). The agreement is excellent. The line obtained for the quartz-cristobalite mixture is straight because these substances are two allotropic forms of silica and hence have identical mass absorption coefficients. When the mass absorption coefficients of the two phases are equal, Eq. (14-9) becomes simply... 

In filler applications, the silicates group of greatest interest is in the subclass of tektosilicates. Four minerals (quartz, tridymite, cristobalite, and opal) belong to the silica group and three of them (quartz, cristobalite, and opal) are used as fillers or materials for their production. 

Unlike quartz, cristobalite has an open structure, allowing some fraction of silicon (2-3%) to be replaced by other elements, such as, Al, Na, or Ca. Still, 95% of the mineral is formed by Si02. The natural cristobalite does not exist in concentrations that make mining feasible therefore it is produced by synthesis (see separate section on cristobalite). Both minerals are found in volcanic rocks, but quartz, which constitutes 12.5% of the Earth s crust, is found eveiywhere, since it does not change or erode. Sandstone is one of the sources of quartz. 

The potential of Eq. (1) with parameters determined in Refs. [10, 11] was thoroughly tested in computer simulations of silica polymorphs. In Ref. [10], the structural parameters and bulk modulus of cc-quartz, a-cristobalite, coesite, and stishovite obtained from molecular dynamics computer simulations were found to be in good agreement with the experimental data. The a to / structural phase transition of quartz at 850 K ha.s also been successfully reproduced [12]. The vibrational properties computed with the same potential for these four polymorphs of crystalline silica only approximately reproduce the experimental data [9]. Even better results were reported in Ref. [5] where parameters of the two-body potential Eq. (1) were taken from Ref. [11]. It was found that the calculated static structures of silica polymorphs are in excellent agreement with experiments. In particular, with the pressure - volume equation of state for a -quartz, cristobalite, and stishovite, the pressure-induced amorphization transformation in a -quartz and the thermally induced a — j3 transformation in cristobalite are well reproduced by the model. However, the calculated vibrational spectra were only in fair agreement with experiments. 

Silica [7631-86-9 (colloidal), 60676-86-0 (Quartz, Cristobalite, sand), 112945-52-5 (fumed)]. Purification of silica for high technology applications uses isopiestic vapour distillation from concentrated volatile acids and is absorbed in high purity water. The impurities remain behind. Preliminary cleaning to remove surface contaminants uses dip etching in HF or a mixture of HCl, HoO and deionised water [Phelan Powell Analyst 109 1299 1984]. 

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