Silica chalcedony

Fukuda, J. Nakashima, S. (2008). Water at high temperatures in a microcrystalline silica (chalcedony) by in-situ infrared spectroscopy physicochemical states and dehydration behavior. Journal of Mineralogical and Petrological Sciences, Vol. 103, pp. 112-115... 

Acid refractory materials include fireclays, flint clays, china clays (kaolins), silica, flint, chalcedony, ganister and titanium dioxide. 

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. 

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. 

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. 

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.

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. 

Silica, or silicon dioxide, occurs in various forms including chalcedony, which is a decorative material chert, which is used in abrasives flint, which is used in abrasives and ceramics jasper, which is used for decorative purposes quartz, which is a constituent of sand tripoli, which is found in scouring powders, polishers, and fillers cristobalite, which is used in high temperature casting and specialty ceramics diatomaceous earth, which is used in filtration processes and as a filler and finally, silica gel, which is used in dehydrating and drying. Note, however, that the material of concern is silica, and not silicates, which are relatively harmless derivatives of silica, nor silicones, synthetic materials used especially as lubricants. Neither silicates nor silicones cause proliferative conditions. 

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. 

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 occurs widely in Nature as quartz, often in large transparent crystals of characteristic shape but also in the translucent agglomerations of microscopic crystals known as chalcedony, which includes cherts and flint. Other natural crystalline varieties of Si02 include tridymite and cristobalite (opal is a semiprecious stone that consists of microcrystalline, hydrous cristobalite). All forms of silica involve three-dimensional networks of corner-linked Si04 tetrahedra. 

The forms of SiC found in sediments and sedimentary rocks are quite varied but those which could be suspected of near surface origin are generally as follows quartz, chalcedony, opal, amorphous gels and ionic forms in solution. Natural occurrences indicate that the solid forms of silica precipitate which has crystallized after the time of initial deposition (Siever, 1962). 

High rates of water influx remove SiO at low solution concentrations. Normal ground water and streams carry about 17 ppm SiO and less in high rainfall areas (Davis, 1964). In some weathering profiles silicification or deposition of silica has been observed. Most often the form of the phase deposited is crypto-crystalline, either opal or chalcedony. In these cases quartz grains do not show overgrowths (Elouard and Millot,... 

Corroded quartz grains have been reported in association with chert in modern carbonates (Bartholome, 1966), and they are known to occur in sand and salt deposits (Dapples, 1959, 1962 Braitsch, 1971). Chalcedony is found associated with quartz in marine sediments formed from devitri-fied glass (Muller, 1961). Amorphous silica is common in recent ocean sediments (Biscaye, 1965 High and Picard, 1965 Calvert, 1971) and has also been identified in deltaic terriginous sediments (Millot, 1964). 

Most commonly, zeolites are found in series of sedimentary rocks which contain pyroclastic material and are formed during the devitrification of this material. If the rocks are silica-rich, the zeolite species formed seems dependent upon the bulk composition and burial depth or temperature of formation (Hay, 1966). They are most frequently accompanied by silica in an amorphous or cryptocrystalline form (opal, chalcedony). Analcite and all other compositional intermediates up to the silica-rich clinoptilolite are found in this association. The most comifton clay mineral in such tuffs is montmorillonite. Zeolites are sometimes found with glauconite (Brown, et al . 1969) or celadonite (Hay, 1966 Iijima, 1970 Read and Eisenbacher, 1974) in pelitic layers or acidic eruptive rocks... 

Next to materials of the glass-ceramics type, many varieties of chalcedony, such as agate, carneol, onyx, sardonyx, heliotrope and jasper, exhibit similar changes in hardness resulting from different consolidation of the cryptocrystalline structure of silica among mineral individuals. 

Chert is another organic marine sediment, less common than carbonate rocks, but found in huge deposits in some parts of the world. It initially consists of the skeletons of billions of tiny, single-celled animals called radiolaria. These skeletons are composed of microcrystalline quartz or chalcedony (Si02). Dense layers of this material accumulate on the ocean floor, where they are buried and compressed over time. The term chert is sometimes also applied to any compact, very fine-grained siliceous sediment that has resulted from precipitation or consolidation of silica gel. There may be chert lenses or very thin layers within other types of sediments, such as limestone. 

SYNS AGATE AMETHYST CHALCEDONY CHERTS CRISTOBALITE FLINT ONYX PURE QUARTZ ROSE QUARTZ SAND SILICA FLOUR SILICON DIOXIDE TRIDYIvlITE TRIPOLI... 

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