Silica polymorph

V. V. Murashov, I. M. Svishchev. Quartz family of silica polymorphs comparative simulation study of quartz, moganite, and orthorhombic silica, and their phase transformations. Phys Rev B 57 5639, 1998. 

Polymorph Analogous silica polymorph d/(gcm Ordered (O) or ) disordered (D) positions... 

As a second example, we choose quartz (or any silica polymorph) as a component for a system containing an aqueous fluid and quartz. Now the mole number for the quartz component includes not only the silica in the quartz mineral, the real quartz, but the silica in solution in species such as SiC>2(aq) and IGSiO. Again, the mole numbers of component quartz and real quartz are not the same. A common mistake in geochemical modeling is confusing the components used to describe the composition of a system with the species and phases that are actually present. 

According to Mottl and McConachy (1990), amorphous silica (SiC>2) is the only silica polymorph present in the smoke at the site. To allow it to form in the calculation, we suppress each of the more stable silica polymorphs. We also suppress hematite (Fe203) in order to give the iron oxy-hydroxide goethite (FeOOH) a chance to form. 

From a plot of the saturation states of the silica polymorphs (Fig. 23.7), the fluid s equilibrium temperature with quartz is about 100 °C. Quartz, however, is commonly supersaturated in geothermal waters below about 150 °C and so can give erroneously high equilibrium temperatures when applied in geothermometry (Fournier, 1977). Chalcedony is in equilibrium with the fluid at about 76 °C, a temperature consistent with that suggested by the aluminosilicate minerals. 

They made several assumptions about which minerals could precipitate from the fluid. The alkaline lakes tend to be supersaturated with respect to each of the silica polymorphs (quartz, tridymite, and so on) except amorphous silica, so they suppressed each of the other silica minerals. They assumed that... 

To keep our discussion simple for the moment, we suppress the silica polymorphs tridymite and chalcedony. In the calculation results (Fig. 26.1), the silica concentration gradually decreases from its initial value and, as in the previous calculation, approaches equilibrium with quartz after about half a year. 

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.

By suppressing the other silica polymorphs, we prevent them from forming in the calculation. We will show, however, that except at large At these minerals tend to become supersaturated in the low-temperature end of the fracture. In reality, therefore, these minerals would be likely to form within the fracture under such conditions. 

Ostwald s step rule holds that a thermodynamically unstable mineral reacts over time to form a sequence of progressively more stable minerals (e.g., Morse and Casey, 1988 Steefel and Van Cappellen, 1990 Nordeng and Sibley, 1994). The step rule is observed to operate, especially at low temperature, in a number of min-eralogic systems, including the carbonates, silica polymorphs, iron and manganese oxides, iron sulfides, phosphates, clay minerals, and zeolites. 

Once the amorphous silica has nearly disappeared, the cristobalite that formed early in the calculation begins to redissolve to form quartz. The cristobalite dissolves, however, much more slowly than it formed, reflecting the slow rate of quartz precipitation. After about 300 000 years of reaction, nearly all of the cristobalite has been transformed into quartz, the most stable silica polymorph, and the reaction has virtually ceased. 

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.

ITQ-4, which has the formula Si32064.(C 4I I20NF)216, is another relatively new 12-MR ID channel-type zeolite that forms a pure silica polymorph (50-52). The structure of ITQ-4, which has been assigned the structure code IFR, is shown in Fig. 8. This view along the 001 direction reveals the distorted 12-MR pores that are... 

Many of the zeolites described in Section II. A form all silica polymorphs, so it is reasonable to expect the isoelectronic germanium to form related structures. However, until recently there were relatively few examples of open framework germinates. By using an organocation as a template, a series of Ge02 molecular... 

The past nearly six decades have seen a chronological progression in molecular sieve materials from the aluminosilicate zeolites to microporous silica polymorphs, microporous aluminophosphate-based polymorphs, metallosilicate and metaHo-phosphate compositions, octahedral-tetrahedral frameworks, mesoporous molecular sieves and most recently hybrid metal organic frameworks (MOFs). A brief discussion of the historical progression is reviewed here. For a more detailed description prior to 2001 the reader is referred to [1]. The robustness of the field is evident from the fact that publications and patents are steadily increasing each year. 

Dixon JB (1989) Kaolin and serpentine group minerals. In Dixon JB, Weed SB (eds) Minerals in Soils. Soil Science Society of America, Madison, Wl, pp 468-527 Dress LR, Wilding LP, Smeck NE, Senkayi AL (1989) Silica in Soils Quartz and disordered silica polymorphs. In Dixon JB, Weed SB (eds.) Minerals in Soil Environment, 2edn. Soil Science Society of America, Madison, Wl. 

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

As must be obvious, we have only touched on the natural occurrences of crystalline fibrous Si02- For example, Si02, usually quartz, often replaces fibrous minerals of another composition as rock masses are altered. Si02 also occurs in fibrous form within some plants (phytoliths) and in the soil. To summarize, in addition to the usual and common occurrences of the mineral quartz and its varieties, several silica polymorphs occur as fibers in a variety of biologic and geologic environments. 

Note that dense silica polymorphs or silica-rich zeolites (ZSM-5, KZ-2, ZSM-22) are formed at the expense of metastable ZSM-48 for long reactions times in monoamine and diamine bearing systems (see above), indicating that excess of Al in the initial hydrogel does not play a particular role in the formation of these phases. In contrast, a higher Al content in the... 

Summarizing the natural occurrences of the silica polymorphs, it is possible to make several statements about the silicate-silica equilibria. 

In weathering situations, saturation of fluids with SiC relative to any species of pure silica is probably only rarely achieved. In continental and shallow sea deposits, silica is precipitated in some initially amorphous form, opaline or chert when lithified or extracted by living organisms. Authigenically formed silicates are probably not in equilibrium with quartz when they are formed. As compaction increases in sediments, silica concentrations in solution are again above those of quartz saturation (15 ppm) and again it must be assumed that the diagenetic minerals formed are not in equilibrium with a silica polymorph except where amorphous silica is present. It is possible that burial depths of one or two kilometers are necessary to effectively stabilize that quartz form. It must be anticipated that the minerals formed under conditions of silica saturation near the earth s surface will be a minority of the examples found in natural rock systems. 

Whereas most effort has been directed toward the zeolite ZSM-5 and its all-silica polymorph silicalite, other frameworks such as zeolite A, faujas-ite, and ferrierite have also been considered. This theoretical interest mirrors the commercial relevance of these zeolites. 

Xenon has been considered as the diffusing species in simulations of microporous frameworks other than faujasite (10-12, 21). Pickett et al. (10) considered the silicalite framework, the all-silica polymorph of ZSM-5. Once again, the framework was assumed to be rigid and a 6-12 Lennard-Jones potential was used to describe the interactions between Xe and zeolite oxygen atoms and interactions between Xe atoms. The potential parameters were slightly different from those used by Yashonath for migration of Xe in NaY zeolite (13). In total, 32 Xe atoms were distributed randomly over 8 unit cells of silicalite at the beginning of the simulations and calculations were made for a run time of 300 ps at temperatures from 77 to 450 K. At 298 K, the diffusion coefficient was calculated to be 1.86 X 10 9 m2/s. This... 

V. A REPRODUCTION OF THE STRUCTURES AND RELATED PROPERTIES OF THE KNOWN SILICA POLYMORPHS... 

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. 

The first discovery of a zeolite was recorded in 1756 (1). Since that time numerous natural and synthetic zeolites, silica polymorphs, and aluminophosphate-based molecular sieves have been reported. The largest ring in these materials consists of 12 tetrahedral (12 T) atoms. This boundary has been in existence for over 180 years since the first zeolite to contain 12 T-atom rings, gmelinite, was discovered in 1807 (1). Recently, we have... 

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