Quartz polymorphs

Determination of electron density maps for the u-quartz polymorph establishes that the charge transfer between silicon and oxygen is not complete and that a residual charge of +1.0 ( 0.1) electron units (e.u.) remains localized on silicon, whereas a charge of —0.5 ( 0.1) e.u. is localized on each oxygen atom. The interpretation of this fact in terms of the bond ionicity is not as univocal as it may appear at first glance. 

For analysis of experimental results, the static model density must be used to eliminate noise, truncation effects, and thermal smearing. Some caution is called for, because the reciprocal space representation of the Laplacian is a function of F(H) H2, and thus has poor convergence properties.2 This difficulty is only partly circumvented by use of the model density, as high-resolution detail may be quite dependent on the nature of the model functions, as is evident in the experimental study of the quartz polymorph coesite discussed in chapter 11. 

Further information on the electronic structure of SiOy and other silicates may be obtained from various spectroscopic studies. An excellent review of the various spectra and their interpretation can be found in Gris-com (1977). We will consider, in turn, the photoelectron, x-ray emission, x-ray absorption, uv absorption, Auger, NMR, and NQR spectra of SiOj, primarily in the quartz polymorph. A discussion of the spectra of other polymorphs of SiOj and their differences compared to quartz will be presented in Chapter 7. 

Wright AF, Fitch AN, Wright AC (1988) The preparation and structure of the a- and P-quartz polymorphs of beryllium fluoride. J Solid State Chem 73 298-304... 

Silicon dioxide [7631-86-9] Si02, exists in both crystalline and glassy forms. In the former, the most common polymorph is a-quartz (low quartz). All commercial appHcations of crystalline quartz use a-quartz, which is stable only below ca 573°C at atmospheric pressure. Some of the properties of a-quartz are Hsted in Table 1. 

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. 

Different modifications of a compound are frequently designated by lower case Greek letters a, j3,..., e.g. a-sulfur, j3-sulfur, or by roman numerals, e.g. tin-I, tin-II etc. Polymorphic forms of minerals have in many cases been given trivial names, like a-quartz, P-quartz, tridymite, cristobalite, coesite, keatite, and stishovite for Si02 forms. 

Any two samples of a particular mineral, whatever their source or place of origin, have the same basic composition and characteristic crystal structure moreover, no two different minerals have identical chemical composition and crystal structure (see Textboxes 8 and 21). Quartz, for example, is a common and abundant mineral composed of silicon dioxide, a compound that occurs naturally not only as quartz but also in other crystal structures, known as polymorphs (polymorphs are minerals that have the same chemical composition but different crystal structure), some of which, listed in Table 23, have been used for a variety of purposes. The crystal structure, which is essential for the characterization of solid materials, is just one of a wide range of physical properties, that is, properties not involving chemical differences, which provide convenient criteria for characterizing and identifying solids. 

Polymorphism in Si02 leads to approximately 20 different forms of the compound. Among others, it occurs in quartz, tridymite, and cristobalite, each of which exists in a and (3 forms. The structure for C02 ... 

Preferred orientation of the particles must be minimized. One of the most effective ways to achieve this is to reduce the particle size by grinding the sample [1], As already discussed in Section III.A, however, grinding can disorder the crystal lattice. Grinding can also induce other undesirable transitions, such as polymorphic transformations [59]. In order to obtain reproducible intensities, there is an optimum crystallite size. The crystallites have to be sufficiently small so that the diffracted intensities are reproducible. Careful studies have been carried out to determine the desired crystallite size of quartz, but no such studies have been reported for pharmaceutical solids [60]. Care should be taken to ensure that the crystallites are not very small, since decreased particle size can cause a broadening of x-ray lines. This effect, discussed earlier (Eq. 9), usually becomes apparent when the particle size is below 0.1 jum. 

Fig. 7. Molar volumes of quartz and cristobalite polymorphs of PON as a function...

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. 

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.

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.

Pure silica zeolites or molecular sieves are metastable with regards to the thermodynamic stable polymorph at ambient conditions, a-quartz. However, they are... 

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). 

For many years, it was thought, purely on an empirical basis, that if the enthalpy change for a given reaction were negative, that is, if heat were evolved at constant pressure, the transformation could occur spontaneously. This rule was verified for many reactions. Nevertheless, numerous exceptions, exist such as the polymorphic transformation of a quartz to (3 quartz at 848 K and atmospheric pressure ... 

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. 

Figure 5.47 Extrinsic stability curves for muscovite and paragonite, based on (A) equilibria 5.139 and 5.144 (quartz absent) and (B) equilibria 5.140 and 5.145 (quartz present). Incipient melting curves of granite for = 0.5 to 1, and stability curves of Al2Si05 polymorphs according to Richardson et al. (1969), are superimposed.

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. 

---