Quartz amorphous silica

SYNS ACCUSAND amorphous quartz AMORPHOUS SILICA BORSIL P CRYPTOCRYSTALLINE QUARTZ DENKA F 90 DENKA FB 44 ... 

AQUEOUS SILICA SPECIES AND THE SOLUBILITIES OF QUARTZ, AMORPHOUS SILICA, AND OTHER SILICA POLYMORPHS... 

Sverjensky (2001) showed that the values for capacitance Ci obtained for a wide variety of oxides and electrolyte types within the framework of the triple layer model (Sahai and Sverjensky, 1997a) fell into two groups. For rutile, anatase, and magnetite, values of Ci increased with decreasing crystallographic radius of the electrolyte cation from Cs+ to Li+. For quartz, amorphous silica, goethite, hematite, and alumina, values of Ci increased with decreasing hydrated electrolyte cation radius from Li+ to Cs+. 

SYNONYMS amorphous quartz, amorphous silica, cryptocrystalline quartz, fused quartz, fused silica, fuselex, microcrystalline quartz, quartz, glass, quartz sand, silicon dioxide, silicone dioxide, suprasil, vitreous silica. 

CAS 60676-86-0 EINECS/ELINCS 231-545-4 Synonyms Amorphous quartz Amorphous silica Cryptocrystalline quartz Fused quartz Fused silica... 

According to Fieldes [1952], silica gel crystallizes to cryptocrystalline chalcedonite and perhaps eventually to secondary quartz. Amorphous silica is not restricted to immature soils... 

A crystalline form of free silica, extremely hard and inert chemically very resistant to heat. Quartz in refractory bricks and amorphous silica in diatomaceous earth are altered to cristobalite when exposed to high temperatures (calcined). Cristobalite is extensively used in precision casting by the hot wax process, dental laboratory work, and certain speciality ceramics. 

A regenerative absorbent consisting of the amorphous silica manufactured by the action of MCI on sodium silicate. Hard, glossy, quartz-like in appearance. Used in dehydrating and in drying and as a catalyst carrier. 

Amorphous silica The noncrystalline forms of silica or quartz. 

Dominant gangue minerals in Kuroko deposits are quartz, barite, anhydrite, gypsum, chlorite, sericite, and sericite/smectite. Morphology of quartz changes from euhedral in the centre to the irregular in the margin of the deposits (Urabe, 1978). No amorphous silica and cristobalite have been found. 

Solubilities of quartz and amorphous silica in aqueous solutions increase with increasing of temperature (Holland and Malinin, 1979). Solubility of barite depends on salinity and temperature (Blount, 1977). The solubility of barite in hydrothermal solution having more than 1 molal NaCl concentration increases with increasing temperature, while a solubility maximum exists in the solution with NaCl concentration less than ca. 0.2 molal (Blount, 1977). 

Quartz or amorphous silica tends to precipitate from the solution having relatively high temperature and low flow rate and under high A/M condition. 

If fluids initially in equilibrium with quartz ascend rapidly, some metastable minerals (amorphous silica, cristobalite, wairakite) may precipitate because of supersaturation with respect to Si02 (e.g., Wolery, 1978 Bird and Norton, 1981). Important processes for the supersaturation and deviation from the equilibrium between fluids and rocks are adiabatic boiling, mixing of fluids and conductive cooling of fluids (Giggenbach, 1984). 

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. 

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... 

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.

Each mineral in the calculation dissolves and precipitates according to the kinetic rate law (Eqn. 26.1) used in the previous examples and the rate constants listed in Table 26.1. We take the same specific surface areas for quartz and cristobalite as we did in our calculations in Section 26.1, and assume a value of 20 000 cm2 g-1 for the amorphous silica, consistent with measurements of Leamnson el al. (1969). The procedure in react is... 

In the calculation results (Fig. 26.6), the initial segment of the path is marked by the disappearance of the amorphous silica as it reacts to form cristobalite. The amorphous silica is almost completely consumed after about 10000 years of reaction. The mineral s mass approaches zero asymptotically because (as can be seen in Equation 26.1) as its surface area As decreases, the dissolution rate slows proportionately. During the initial period, only a small amount of quartz forms. 

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. 26.6. Variation in mineral volumes over a kinetic reaction path designed to illustrate Ostwald s step sequence. The calculation traces the reaction at 25 °C among the minerals amorphous silica (tine line), cristobalite (medium line), and quartz (bold line). The top diagram shows results plotted against time on a linear scale the time scale on the bottom diagram is logarithmic. The decrease in total volume with time reflects the differing molar volumes of the three minerals.

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.

Silicon dioxide, 22 380. See also Amorphous silica Quartz in ceramics, 5 582... 

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