Chalcedony

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

Some treatments are practiced so widely that untreated material is essentially unknown ia the jewelry trade. The heating of pale Fe-containing chalcedony to produce red-brown carnelian is one of these. Another example iavolves turquoise where the treated material is far superior ia color stabiUty. Such treatments have traditionally not been disclosed. Almost all blue sapphire on the market has been heat treated, but it is not possible to distinguish whether it was near-colorless comndum containing Fe and Ti before treatment, or whether it had already been blue and was only treated ia an attempt at marginal improvement. The irradiation of colorless topa2 to produce a blue color more iatense than any occurring naturally is, however, self-evident, and treatments used on diamond are always disclosed. 

A microcrystalline form of native quartz, more opaque and granular than chalcedony. Used as an abrasive and in ceramics. 

Deposition in the 300 to 500 °F (149-260 °C) range is alpha-quartz (a-Si02), chalcedony (Si02), and crystabalite (a- and 0-Si02)... 

Fig. 2.11. The temperature dependence of cation/proton activity ratios of geothermal well discharges in Japan. The lines in the figure are recalculated temperature dependences of cation/proton ratios in Icelandic geothermal waters. The dashed curve in B represents the reaction 1.5 K-feldspar + H+ = 0.5 K-mica + 3 quartz (or chalcedony) + K+ (Chiba, 1991). Open circle Takigami, open triangle Kakkonda, open square Okuaizu, solid circle Kirishima, solid triangle Sumikawa, solid square Nigoiikawa.

Fig. 2.13. (A) Temperature dependence of pH in Japanese thermal waters. Lines indicate the temperature dependence of pH when pH is buffered by the K-feldspar-K-mica-quartz (or chalcedony at less than 200°C) assemblage at a Na + K concentration of 0.1 and 0.01 mol/kg H2O. Symbols are as in Fig. 2.11. (B) Temperature dependence of pH of Icelandic thermal waters. Large circles indicate well discharges. Small dots represent hot spring waters (Chiba, 1991).

Chalcedony Waxy Transparent or opaque Various Splintery fracture... 

Chalcedony A heat-resistant, chemically inert form of microcrystalline quartz. A decorative material. Rare in industry. 

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. 

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