Silica minerals

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. 

Polymorph Quartz Coesite Stishovite Cristobalite Tridymite 

Formula unit SiOj Si02 Si02 Si02 Si02 

System Trigonal Monoclinic Tetragonal Tetragonal Monoclinic 

The amount of H2O in amorphous silica (number n of H2O molecules per unit formula) varies between 0.14 and 0.83 (Frondel, 1962). Nevertheless, the thermodynamic properties of the phase are not particularly affected by the value of n (Walther and Helgeson, 1977). The molar volume of opal is 29 cm /mole. The same volume of a-quartz may be adopted for chalcedony see table 5.68 for the other polymorphs. 

Silica minerals are a primary mineral classified as tectosilicates, characterized by repeating SiO units in a framelike stracture. Quartz, one of the most abundant minerals on earth, often comprises up to 95% of aU sand and silt fractions. It therefore is representative of the structure and properties of sihca minerals. 

Wilding et al. (1977) showed that the sihca tetrahedron in quartz is almost sym-mehical and has a Si-O distance of 0.16nm. They noted that the structure of quartz can be visualized as a spiral network of silica tetrahedra around the z-axis. From 

The term silica refers to silicon dioxide (SiOj), in its various forms, and silica minerals are the second most abundant group, behind silicate minerals discussed in Chapter 8 (Monger and Kelly 2002). The silica minerals include quartz (in several crystalline forms) and opals. All of them are tectosilicates, formed from strongly bound Si04 tetrahedra, which were discussed in Section 8.1. 

Si02 is scarcely soluble, its solubility being dependent on aystallinity and impurities moreover, its dissolution is a slow process. In soil, the average solubility of silica, as undissociated silicic acid (H4Si04), is around 10 M (Lindsay and Elgawhary 1972) at pH below 8 at higher pH values, ionization causes a sharp increase in solubility. This increased solubility can lead to weathering of silica minerals, which is otherwise very slow. 

Owing to the almost complete absence of heterovalent isomorphic substitution and low amounts of silanol groups, silica minerals have a very low adsorption capacity, either by cation exchange or by specific adsorption. However, cation-exchange capacity increases somewhat at high pH values. In general, these minerals have a small contribution to soil-ion binding. 

FIGURE 9.1 Crystalline structures of a-quartz (a) and a-cristobalite (b) showing different forms of linking Si04 tetrahedra (two out of nine). The cristobalite structure is more open, thus less dense and can contain more impurities than quartz. (Reprinted from Monger, H. C. and Kelly, E. F., Soil Mineralogy with Environmental Applications, Soil Scienee Soeiety of America, Madison, WI, 611-636, 2002, with kind permission.) 

Various names, such as amethyst, citrine, and smolqr or black quartz, have been used for colored rock-crystal, whereas for the cryptocrystalline aggregate of quartz, names such as chalcedony and jasper are used. Agate and cornelian, for example, are types of chalcedony that have specific textures or colors. In this chapter, we analyze how a variety of morphologies of high-temperature and low-temperature quartz appear, and how textures of polycrystalline aggregate seen in agate and other crystals are formed. 

Although the principal morphology of rock-crystal is a hexagonal prismatic Habitus, natural crystals may deviate from this. In Goldschmidt s Atlas der Kristallformen (see ref. [1], Chapter 9), 855 crystal figures are compiled in 54 plates. A few examples are shown in Fig. 10.2, in which various forms are observed, such as malformed hexagonal prisms, tapered prisms, platy, and scepter forms. (See also Fig. 1.1.) 

In contrast to the hexagonal prismatic morphology of low-temperature quartz, it has been assumed that the characteristic morphology of high-temperature quartz is hexagonal bipyramidal where no 1010 faces appear on the crystal 

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Foo CWP, Huang J, Kaplan DL (2004) Lessons from seashells silica mineralization via protein templating. Trends Biotechnol 22 577-585... 

The alkali process uses sodium hydroxide and is well known as Bayer s process. It involves relatively simple inorganic and physical chemistry and the entire flowsheet can be divided into caustic digestion, clarification, precipitation and calcination. Although mineral assemblage in bauxites is extensive, processing conditions are primarily influenced by the relative proportions of alumina minerals (gibbsite and boehmite), the iron minerals (goethite and hematite), and the silica minerals (quartz and clays-usually as kaolinite). 

Many natural waters are supersaturated at low temperature, primarily because less stable minerals dissolve more quickly than more stable minerals precipitate. Relatively unstable silica phases such as chalcedony or amorphous silica, for example, may control a fluid s SiC>2 concentration because quartz, the most stable silica mineral, precipitates slowly. 

In Chapter 16, we wrote rate laws for simple dissolution and precipitation reactions, such as those for the silica minerals forming from SiC>2(aq). Rewriting Equation 16.22 in terms of volumetric concentration C , assuming the activity coefficient Yi does not vary over the reaction, gives the rate law,... 

Several chemical geothermometers are in widespread use. The silica geothermometer (Fournier and Rowe, 1966) works because the solubilities of the various silica minerals (e.g., quartz and chalcedony, Si02) increase monotonically with temperature. The concentration of dissolved silica, therefore, defines a unique equilibrium temperature for each silica mineral. The Na-K (White, 1970) and Na-K-Ca (Fournier and Truesdell, 1973) geothermometers take advantage of the fact that the equilibrium points of cation exchange reactions among various minerals (principally, the feldspars) vary with temperature. 

Fig. 23.7. Calculated saturation indices (log Q/K) of silica minerals for Gjogur hot spring water. Chalcedony is approximately in equilibrium at 80 °C, but quartz is supersaturated at this temperature.

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... 

Each silica mineral dissolves and precipitates in our calculations according to... 

Table 26.1. Rate constants k+ (mol cm 2 s l )for the reaction of silica minerals with water at various temperatures, as determined by Rimstidt and Barnes (1980)...

Impressive, highly ordered centimetre-sized fibres are obtained whose synergistic growth mechanism based on the kinetic cross-coupling of a dynamical supramolecular self-assembly and a stabilizing silica mineralization may well be the basis of the synthetic paths used by Nature to obtain its materials with well-defined multiscale architectures in biological systems. 

Frondel C. F. (1962). The System of Mineralogy of James Dwight Dana and Edward Salysbury Dana. 7th ed., vol. 8, Silica Minerals, New York John Wiley. 

When all four oxygens of the tetrahedra are shared (Si 0 = 1 2), a fully polymerized ion results, and a three-dimensional framework is formed. Quartz, an example of this type of silicate array, is discusssed in the section on silica minerals. 

In earlier literature reports, x-ray data of a-based ceramics, the /3-like phase observed in certain silica minerals was explained by a structural model based on disordered Q -tridymite. However, others have suggested that the structure of the stabilized jS-cristobalite-like ceramics is closer to that of a-cristobalite than that of Q -tridymite, based on the 29Si nuclear magnetic resonance (NMR) chemical shifts (Perrota et al 1989). Therefore, in the absence of ED data it is impossible to determine the microstructure of the stabilized jS-cristobalite-like phase. ED and HRTEM have provided details of the ceramic microstructure and NMR has provided information about the environments of silicon atoms in the structure. Infrared spectroscopy views the structure on a molecular level. 

Another representation of the stability relations of the silica minerals is shown in Figure 4. This diagram, developed in the classical studies early in the twentieth century (51), illustrates the relationship of vapor pressure to temperature. It is assumed that vapor pressure increases with temperature and that the form having the lowest vapor pressure is the most stable. The actual values of the vapor pressures are largely unknown. Therefore, the ordinate must be considered only as an indication of relative stabilities. This diagram does not show all the various forms of tridymite that have been identified. 

MIZUTANI (S.), 1970. Silica minerals in the early stages of diagenesis. SeHimentology 15, 419-36. 

Fig. 1. Concentration of dissolved silica as a function of time for suspensions of silicate minerals in sea water. Curves are for I g (<62 u) mineral samples in silica-deficient (S1O2 in water was initially 0.03 ppm) and silica-enriched (Sit) was initially 25 ppm) sea water at room temperature. Notice that the minerals react rapidly and that the dissolved silica concentration for individual minerals becomes nearly constant at values within 01 close to the range of silica concentration in the oceans (from Mackenzie. F. T., Cartels. R. M., Bricker, O P, and Bickley, F., Silica in sea water Control by silica minerals Science, 155. 1404 (1967)).

Felsic term used to describe the amount of light-colored feldspar and silica minerals in an igneous rock. 

Frondell, C., "The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana", 7th Ed. Vol III, Silica Minerals. 1-334. John Wiley and Sons Inc., New York. 

Fig. 9.10. Antifoam effectiveness F of a dispersion of hydrophobed silica/mineral oil antifoam as a...

Marvin U. B., Petaev M. I., Croft W. J., and Kilgore M. (1997) Silica minerals in the Gibeon IVA iron meteorite. In Lunar Planet. Sci. XXVIII. The Lunar and Planetary Institute, Houston, pp. 879-880. 

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