Stishovite structures

These curves were generated by fitting a standard equation of state to computed energies as a function of volume. Consistent with experiment, the lowest energy phase is the a-quartz structure which is a distortion of the 0-quartz structure. The stishovite structure is nearly as stable as the a-quartz structure, but the equilibrium molecular volume is considerably smaller owing to the higher coordination in the stishovite structure. The CaCl2... 

Structure, which corresponds to a distorted stishovite structure[20, 21], is nearly degenerate in energy with stishovite in our calculations. 

Another area where structural energy calculations can be of great utility concerns the solid-state order-disorder transformation. Such transformations can be induced by pressure. At high pressures, a-quartz subsists as a metastable phase which gradually transforms to an amorphous form and, subsequently to a rutile-like crystalline structure[34]. Evidence for the onset of the amorphization has been reported at about 15 GPa from single crystal analysis[35].In powder measurements[36], the transition is observed to be complete by 35 GPa. Experiments performed on powered samples at pressures above 60 GPa indicate a crystalline structure which is thought to resemble the stishovite structure[37]. 

The non-spherical distortions are as large in the close-packed stishovite structure as in the open cristobalite structure therefore, stishovite is not significantly more ionic than cristobalite (or quartz). However, the distortions are smaller in the perovskite structure than in the cristobalite, stishovite or spinel structures, and so the bonding in perovskite can be considered more ionic than in these other structures. 

The calculated enthalpies for silica in the quartz and stishovite phases are shown in Figure 3 as a funetion ofpressure. The stishovite structure beeomes more stable than the quartz strueture at 3.5 GPa with the distorted ion model, and at 21 GPa with the spherical ion model. In comparison, the experimental zero temperature transition pressure for the quartz to stishovite phase transition is estimated to be 5.5 GPa from thermodynamic data [53], and the transition pressure for the similar cristobalite to stishovite phase transition is caleulated to be 6 GPa by periodie Hartree-Fock methods [54]. The non-spherical distortions improve the modeling of this phase transition by stabilizing stishovite with respeet to quartz the greater stabilization ofstishovite occurs because the distortions strengthen three bonds per anion in stishovite, and only two bonds per anion in quartz (the bonds are significantly covalent in both structures, as shown above in the plots of the electron density distributions). 

Figure 3. Enthalpy of silica in quartz and stishovite structures as a function of pressure Key squares quartz, circles stishovite open symbols distorted ion electron gas model closed symbols spherical ion electron gas model.

Figure 4.15. Shock pressure versus specific volume for calcia and fused quartz indicating three regimes fused quartz, low-pressure regime is fused quartz, mixed phase regime, and high-pressure regime representing stishovite. In the case of calcia, the low-pressure phase is the B1 structure, mixed phase is indicated, and the high-pressure phase regime is in the B2 structure.

Stishovite is a high-pressure modification of Si02 having the rutile structure. Should it have longer or shorter Si-O bond lengths than quartz ... 

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. 

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. 

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. 

Crystalline Silica. Silica exists in a variety of polymorphic crystalline forms (23,41—43), in amorphous modifications, and as a liquid. The literature on crystalline modifications is to some degree controversial. According to the conventional view of the polymorphism of silica, there are three main forms at atmospheric pressure quartz, stable below about 870°C tridymite, stable from about 870—1470°C and cristobalite, stable from about 1470°C to the melting point at about 1723°C. In all of these forms, the structures are based on Si04 tetrahedra linked in such a way that every oxygen atom is shared between two silicon atoms. The structures, however, are quite different in detail. In addition, there are other forms of silica that are not stable at atmospheric pressure, including that of stishovite, in which the coordination number of silicon is six rather than four. 

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. 

Stishovite. Stishovite was first prepared (68) in the laboratory in 1961 at 1200—1400°C and pressures >16 GPa (158,000 atm). It was subsequendy discovered, along with natural coesite, in the Arizona meteor crater. It has been suggested that these minerals are geological indicators of meteorite impact structures. Stishovite (p = 4.35 g/cm3) is the densest known phase of silica. The structure, space group P42/nmn, is similar to that of... 

It is of interest that atomic hydrogen centres, which are unstable even at 4 K in quartz, were observed in both coesite and stishovite at 77K.66 The electron centre associated with Ti impurity was observed in stishovite, but not clearly in coesite. Electronic structures of these paramagnetic centres were calculated with the discrete variational (DV)-Xa method to establish a model for centres having substitutional impurities of Al, Ge and Ti.67... 

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... 

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