Quartz chalcedony

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). 

Quartz, Chalcedony, Agate, and Opal, by Differential Thermal Analysis, J. Inst. Fuel (1970) 43, (354), 240-242. 

An agate is banded microcrystalline quartz (chalcedony). The bands are curved, and can be any color, opaque or translucent. 

The minerals found in United States coals continue to be studied with the availability of improved instrumental procedures such as x-ray diffraction, infrared absorption, and scanning electron microscopy beyond the traditional optical and chemical mineralogical techniques as applied to thin sections, polished pellets, and isolated particles. The minerals may be grouped into the silicates (kaolinite, illite montmorillonite, and chlorite), the oxides (quartz, chalcedony, hematite) the sulfides (pyrite, marcasite, and sphalerite) the sulfates (jarosite, gypsum, barite, and numerous iron sulfate minerals) the carbonates (ankerite, calcite, dolomite, and siderite) and numerous accessory minerals (apatite, phosphorite, zircon, rutile, chlorides, nitrates, and trace minerals). 

Iron oxides (< 1%) occur in sandstones of the distal Caioba area associated with dissolved, ferroan dolomite/ankerite and oxidized, coarsely crystalline pyrite. These sandstones display evidence of extensive dissolution and kaolinization of feldspars. Iron oxides (2-22%) also fill pores as alternating bands with quartz/chalcedony (4-13%) in some coarsegrained sandstones and conglomerates that outcrop in the middle-domain Japoata-Penedo area. In... 

Fig. 6.114 Grain of the quartz-chalcedony rock with the microcline in the gravel...

Gibbsite, boehmite, diaspora Quartz, chalcedony, kaolinite Goethite, hematite, siderite Rutile, anatase Calcite, magnesite, dolomite... 

Silicon dioxide is an intriguing, highly complex compound that has been the subject of more scientific scrutiny than any substance except water. Dozens of forms are known a-quartz, chalcedony, chert, agate, onyx, jasper, and many others. 

Fig. 1.18 Temperature dependence of the solubility of a-quartz, chalcedony, a-cristobalite, moganite, p-cristobalite and amorphous silica at different pressures, as specified (data computed by SUPCRT92). Also shown is the temperature dependence of the solubility of an unspecified tridymite (data from Wagman et al. 1982) (Marini 2007)...

The siliceous clay raw material is a source of both aluminum and silica. It may contain one or more types of clay minerals. Other sources of silica are quartz, chalcedony, opal, feldspar, etc. Table 1 shows the types of clay minerals and their suitability as raw mixes for cement manufacture. ... 

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 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. 

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. 

Si02 is one of the most abundant compounds in nature. It forms a number of minerals and several varieties whose names are quite familiar agate, car-nelian, sard, amethyst, chalcedony, flint, and chert. All are composed of Si02 with only small or trace amounts of other elements or compounds included during crystallization. In many cases it is the additional components that impart the peculiar color, optical, or physical properties to these minerals. The names are familiar because since ancient times these minerals were used or commonly set in jewelry. All of the names are varieties of the mineral quartz, the stable form of SiOj, formed at ordinary temperatures and pressures. Si02 also forms several polymorphs, which are mentioned below. 

Chalcedony is a variety of quartz composed of fibrils that are not separable. The fibrils may have variable thickness, and lengths, from a few to hundreds of microns, and may be twisted (Fig. 2.19C). The fiber axis, usually perpendicular to the free or growing surface, is easily detected in the crystalline material consisting of tightly packed, parallel or subparallel fibrils. The range of color, and the mechanisms of inclusion of impurities, and especially of water and (OH) in chalcedony, have been described and discussed by many authors, most recently by Frondel (1978, 1982). 

The optical character of chalcedony is distinct from that expected for the normally uniaxial mineral, quartz, and signals the fibrous nature of a particular sample. The direction of fiber elongation is often parallel to the [1120] crystallographic direction of the quartz structure (Fig. 2.19A), but other fiber directions have also been determined within a single sample (Frondel, 1985). The presence of helically twisted fibers are suspected from the variations in extinction and birefringence noted along the fiber length (Fig. 2.19C). More detailed information on the optical or other physical and chemical properties of quartz and its many varieties can be found in volume 3 of Palache et al. (1962) and in Frondel (1985). 

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