There are 16 SiO units per unit cell the space group is C2/c. Coesite persists as a basically stranded phase at atmospheric pressure and is a stable phase at  [c.476]

Other forms of SiOj can be made at high pressure (Fig. 9.2). Coesite was first made  [c.343]

By definition, the stable phases show no changes in properties with time at constant temperature and pressure, even when in contact with solutions or melts. Metastable or unstable phases may persist essentially indefinitely at temperatures and pressures outside thek ranges of stabiUty. The distinction is that metastable phases can be stimulated to change into a different form when placed in contact with that form unstable phases change without the need for such contact. Thus, tridymite and cristobahte may be investigated at temperatures within the stabiUty range of quartz and may undergo transitions of the high—low type at these temperatures. Such thermodynamically unstable but kineticahy stable phases are said to be thermally stranded. An analogous situation occurs when pressure rather than temperature is the controlling variable in phase equiUbria. Such phases are said to be baricahy stranded. For example, coesite is stable only at high pressure, but nevertheless persists when brought to atmospheric pressure and room temperature. Under particular conditions, a metastable or unstable phase may convert either to the phase that is stable under those conditions, or into some other phase that is lower in free energy than the fkst but also unstable or metastable. When heated to 867—1470°C, pure quartz, for example, usually converts to a disordered cristobahte rather than to tridymite.  [c.473]

In addition to the three principal polymorphs of siUca, three high pressure phases have been prepared keatite [17679-64-0] coesite, and stishovite. The pressure—temperature diagram in Figure 5 shows the approximate stabiUty relationships of coesite, quart2, tridymite, and cristobaUte. A number of other phases, eg, siUca O, siUca X, sihcaUte, and a cubic form derived from the mineral melanophlogite, have been identified (9), along with a stmcturaHy unique fibrous form, siUca W.  [c.474]

Coesite. Coesite, the second most dense (3.01 g/cm ) phase of silica, was first prepared ia the laboratory by heating a mixture of sodium metasibcate and diammonium hydrogen phosphate or another mineraliser at 500—800°C and 1.5—3.5 GPa (14,800—34,540 atm). Coesite has also been prepared by oxidation of silicon with silver carbonate under pressure (67). The stmcture is monoclinic = 717 pm, Cg = 1.238 pm, and 7 = 120°.  [c.476]

Pressure—temperature diagrams for the coesite—quart2 equilibrium have been summari2ed (23). Coesite has been found ia nature ia the meteor crater ia Ari2ona.  [c.476]

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  [c.476]

Increased pressures can lower the temperature at which crystallisation occurs. Experiments performed using Spectrosil (Thermal Syndicate Ltd.) and G.E. Type 204 (General Electric Company) fused siUcas (see Eig. 2) show that at pressures above 2.5 GPa (<25, 000 atm), devitrification occurs at temperatures as low as 500°C and that at 4 GPa (<40, 000 atm), it occurs at as low as 450°C (107). Although the temperatures and pressures were in the coesite-phase field, both coesite and quarts were observed. Both the devitrification rate and the formation of the stable phase (coesite) were enhanced by the presence of water. In the 1000—1700°C region at 500—4000 MPa (<5, 000-40,000 atm), a- and p-quarts were the primary phases. Crystal growth rates  [c.502]

See pages that mention the term Coesite : [c.358]    [c.237]    [c.471]    [c.476]    [c.98]    [c.342]    [c.343]    [c.613]   
Chemistry of the elements (1998) -- [ c.342 , c.343 ]