Silica Stishovite

Soluble silica does not cause silicosis. The crystalline form of silica, stishovite, dissolves in water to give a concentration of soluble silica much more concentrated than from quartz (309). If soluble silica is the active agent, then stishovite should be even more toxic than quartz. Instead, it is harmless. ... 

Crystalline Silica. Sihca exists in a variety of polymorphic crystalline forms (23,41—43), in amorphous modifications, and as a Hquid. The Hterature on crystalline modifications is to some degree controversial. According to the conventional view of the polymorphism of siHca, there are three main forms at atmospheric pressure quart2, stable below about 870°C tridymite, stable from about 870—1470°C and cristobaHte, stable from about 1470°C to the melting point at about 1723°C. In all of these forms, the stmctures are based on SiO tetrahedra linked in such a way that every oxygen atom is shared between two siHcon atoms. The stmctures, however, are quite different in detail. In addition, there are other forms of siHca that are not stable at atmospheric pressure, including that of stishovite, in which the coordination number of siHcon is six rather than four. 

Stishovite. Stishovite was first prepared (68) ia the laboratory ia 1961 at 1200—1400°C and pressures >16 GPa (158,000 atm). It was subsequentiy discovered, along with natural coesite, ia the Ari2ona meteor crater. It has been suggested that these minerals are geological iadicators of meteorite impact stmctures. Stishovite (p = 4.35 g/cm ) is the densest known phase of silica. The stmcture, space group P42/nmn is similar to that of... 

Demiralp E, Cagin T, Goddard WA(1999) Morse stretch potential charge equilibrium force field for ceramics Application to the quartz-stishovite phase transition and to silica glass. Phys Rev l tt 82 1708-1711 Dewar MJS (1977) Ground states of molecules. The MNDO method. Approximations and parameters. J Am Chem Soc 99 4899-4907... 

Only with silica was the nature of the surface groups studied as extensively as with carbon. Silica, like carbon, has several polymorphs. Apart from the amorphous state, it is known to exist in numerous crystalline modifications. The most important forms are quartz, tridymite, and cristobalite. Each of these can occur in a low-temperature form and in a high-temperature form of somewhat higher symmetry. Tridymite is only stable if small amounts of alkali ions are present in the lattice 159). Ar. Weiss and Al. Weiss 160) discovered an unstable fibrous modification with the SiSj structure. Recently, a few high-pressure modifications have been synthesized keatite 161), coesite 162), and stishovite 16S). The high-pressure forms have been found in nature in impact craters of meteorites, e.g., in the Arizona crater or in the Ries near Nbrdlingen (Bavaria). 

Stishovite is very interesting because it has the rutile structure with octahedral coordination of silicon. In all other forms of silica, each silicon atom is surrounded tetrahedrally by four oxygen atoms. 

The surface chemistry of coesite and stishovite was studied by Stiiber (296). The packing density of hydroxyl groups was estimated from the water vapor adsorption. More adsorption sites per unit surface area were found with silica of higher density. Stishovite is especially interesting since it is not attacked by hydrofluoric acid. Coesite is dissolved slowly. The resistance of stishovite is ascribed to the fact that silicon already has a coordination number of six. Dissolution of silica to HaSiFg by hydrogen fluoride is a nucleophilic attack. It is not possible when the coordination sphere of silicon is filled completely. In contrast, stishovite dissolves with an appreciable rate in water buffered to pH 8.2. The surface chemistry of. stishovite should be similar to that of its analog, rutile. 

Silica has 22 polymorphs, although only some of them are of geochemical interest—namely, the crystalline polymorphs quartz, tridymite, cristobahte, coesite, and stishovite (in their structural modifications of low and high T, usually designated, respectively, as a and jS forms) and the amorphous phases chalcedony and opal (hydrated amorphous silica). The crystalline polymorphs of silica are tectosilicates (dimensionality = 3). Table 5.68 reports their structural properties, after the synthesis of Smyth and Bish (1988). Note that the number of formula units per unit cell varies conspicuously from phase to phase. Also noteworthy is the high density of the stishovite polymorph. 

Table 5.70 Thermodynamic data for silica polymorphs. Stishovite and tridymite from Saxena et al. (1993) remaining polymorphs from Helgeson et al. (1978). ...

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. 

Stishovite is the most dense phase of silica. Its density is 4.35 g/cm. It has a rutde-type crystal structure in which the sdicon atom is octahedrally surrounded by six oxygen atoms. Four Si—O bonds are 1.76A and two 1.8lA. Stishovite has been prepared similarly to coesite but at temperatures between 1,200 to 1,400°C and a pressure above 150,000 atm. Both the coesite and stishovite are found in nature in certain meteorite craters resulting from meteorite impacts. 

In addition to the three principal polymorphs of silica, three high pressure phases have been prepared keatite [17679-64-0], coesite, and stishovite. The pressure—temperature diagram in Figure 5 shows the approximate stability relationships of coesite, quartz, tridymite, and cristobalite. A number of other phases, eg, silica O, silica X, silicalite, and a cubic form derived from the mineral melanophlogite, have been identified (9), along with a structurally unique fibrous form, silica W. 

When fine powders of vitreous silica, quartz, tridymite, cristobalite, coesite, and stishovite of known particle-size distribution and specific surface area are investigated for their solubility in aqueous suspensions, final concentrations at and below the level of the saturated concentration of molybdate-active silicic acid are established. Experimental evidence indicates that all final concentrations are influenced by surface adsorption of silicic acid. Thus, the true solubility, in the sense of a saturated concentration of silicic acid in dynamic equilibrium with the suspended silica modification, is obscured. Regarding this solubility, the experimental final concentration represents a more or less supersaturated state. Through adsorption, the normally slow dissolution rates of silica decrease further with increasing silicic acid concentrations. Great differences exist between the dissolution rates of the individual samples. 

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). 

In view of this, the dissolution patterns of all other silica modifications were interpreted disregarding the condensation reaction. This seemed particularly adequate for stishovite, a high pressure, high temperature material, first produced in an autoclave by Stishov and Popova (17) in 1961 and detected at Meteor Crater one year later (6). The lattice... 

The mineral stishovite, Si02, was found in the Canyon Diablo meteorite. It is the form of silica formed at very high pressure. The crystal structure... 

An important use of high mechanical pressiues is to force solids to assume crystal structures and coordination numbers in which they are normally unstable. Thus, four-coordinate Si in Si02, typified by quartz or try dimite, can be forced to assume the six-coordinate rutile stracture under very high pressures. Once formed, these structures are kinetically stable. The high-pressure polymorph of silica, called stishovite is found terrestrially as the result of meteor impact. 

Fig. 7.1. Equation of state (EOS) of silica calculated using the modified electron-gas method by Bukowinski and Wolf (1985) and showing pressure-density relations for stishovite and fluorite-type SiOj.

Qualitative molecular-orbital analysis of computational results. Rather than employing purely qualitative, symmetry-based theory, we can also perform calculations on solids or clusters and then analyze them using qualitative MO arguments. We will consider two such studies (1) an EHMO study of SiOj in (3-quartz, stishovite, and hypothetical silica-w structures (Burdett and Caneva, 1985), and (2) an MS-SCF-Aa study of electron-rich transition-metal compounds (Tossell and Vaughan, 1981). 

The best-known compound containing Si(VI) is the high-pressure silica polymorph stishovite, which has a chemical shift of 191.3 ppm (Stebbins and Kanzaki 1991). Other high-pressure silicate phases also known to contain Si( VI) are shown in Table 4.4, together with their Si chemical shifts. 

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