Quartz hydrothermal

Ray, A., CantiU, E., Stevens, M. G., and Aldridge, L., Use of DTA to Determine the Effect of Mineralizers on the Cement-Quartz Hydrothermal Reactions, Part 2, Clay Addition, Thermochimica Acta, 250 189-195 (1995)... 

Other Industrial Applications. High pressures are used industrially for many other specialized appHcations. Apart from mechanical uses in which hydrauhc pressure is used to supply power or to generate Hquid jets for mining minerals or cutting metal sheets and fabrics, most of these other operations are batch processes. Eor example, metallurgical appHcations include isostatic compaction, hot isostatic compaction (HIP), and the hydrostatic extmsion of metals. Other appHcations such as the hydrothermal synthesis of quartz (see Silica, synthetic quartz crystals), or the synthesis of industrial diamonds involve changing the phase of a substance under pressure. In the case of the synthesis of diamonds, conditions of 6 GPa (870,000 psi) and 1500°C are used (see Carbon, diamond, synthetic). 

Hydrothermal crystallisation processes occur widely in nature and are responsible for the formation of many crystalline minerals. The most widely used commercial appHcation of hydrothermal crystallization is for the production of synthetic quartz (see Silica, synthetic quartz crystals). Piezoelectric quartz crystals weighing up to several pounds can be produced for use in electronic equipment. Hydrothermal crystallization takes place in near- or supercritical water solutions (see Supercritical fluids). Near and above the critical point of water, the viscosity (300-1400 mPa s(=cP) at 374°C) decreases significantly, allowing for relatively rapid diffusion and growth processes to occur. 

Solubility. An important aspect of sihca chemistry concerns the sihca— water system. The interaction of the various forms of sihca with water has geological significance and is apphed in steam-power engineering where the volatilization of sihca and its deposition on turbine blades may occur (see Power generation), in the production of synthetic quartz crystals by hydrothermal processes (qv), and in the preparation of commercially important soluble sihcates, coUoidal sihca, and sihca gel. 

Equipment. A typical commercial quartz-growing autoclave is Hlustrated in Figure 1. The material of constmction for use at 17 MPa (25,000 psi) and 400°C can be a low carbon steel, such as 4140, or various types of low aHoy steel. The closure, a modified Bridgeman closure, is based on the unsupported area principle (12). That is, the pressure in the vessel is transmitted through the plunger to the steel surfaces which initially are nearly line contacts. Thus, the pressure in the seal surface gready exceeds the pressure in the vessel because most of the area of the plunger is unsupported. Hydrothermal equipment has been further discussed (10). 

Figure 1.34. Frequency histogram for MgO/FeO ratios (in wt%) of chlorite from the basalt studied (A) and MORE (B). Data sources are Shikazono and Kawahata (1987), Humphris and Thompson (1978) (M Mid-Atlantic Ridge) and Kawahata (1984) (C Costa Rica Rift, Galapagos Spreading Centre). The data on chlorite from MORE are taken from typical metabasalt and not from quartz-chlorite breccia and veins which formed in a hydrothermal upflow zone (Shikazono et al., 1987).

Precipitation of barite and quartz. Barite and quartz are the most common gangue minerals in the submarine hydrothermal ore deposits such as Kuroko deposits and back-arc basin deposits (e.g., Okinawa, Mariana deposits) (Halbach et al., 1989 Shikazono, 1994 Shikazono and Kusakabe, 1999). These minerals are also common in midoceanic ridge deposits. 

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). 

However, as already noted, the barite content in Kuroko ore inversely correlates to the quartz content and the occurrences of barite and quartz in the submarine hydrothermal ore deposits are different. The discrepancy between the results of thermochemical equilibrium calculations based on the mixing model and the mode of occurrences of barite and quartz in the submarine hydrothermal ore deposits clearly indicate that barite and quartz precipitated from supersaturated solutions under non-equilibrium conditions. Thus, it is considered that the flow rate and precipitation kinetics affect the precipitations of barite and quartz. 

A comparison of the calculated results (Figs. 1.53 and 1.54) with the mode of occurrences of quartz and barite in the submarine hydrothermal ore deposits indicates... 

The results of calculations are in agreement with the occurrences of barite and silica and chemical features of discharging fluids in the submarine hydrothermal ore deposits namely, quartz is inferred to precipitate in subseafloor environment and barite in seabottom environment. 

Figure 1.86. Variation in chemical compositions (in molal unit) of hydrothermal solution with temperature. Thermochemical data used for the calculations are from Helgeson (1969). Calculation method is given in Shikazono (1978a). Chloride concentration in hydrothermal solution is assumed to be 1 mol/kg H2O. A-B Na concentration in solution in equilibrium with low albite and adularia, C-D K concentration in solution in equilibrium with low albite and adularia, E-F HaSiOa concentration in equilibrium with quartz, G-H Ca + concentration in equilibrium with albite and anorthite (Shikazono, 1978a, 1988b).

Supergroup rocks in the Hishikari district suffered hydrothermal alteration. Chlorite, quartz and sericite occur abundantly near the veins. The other constituents are pyrite, albite, calcite and organic matter. 

Oxygen isotopic fractionation factors used for the calculation were taken from Taylor (1997). Initial 8 0 value of hydrothermal solution (0%o) was estimated from 8 0 values of K-feldspar and quartz in the veins and homogenization temperatures (Shikazono and Nagayama, 1993), and that of groundwater (—7%c) was estimated from meteoric water value of the south Kyushu district (—7%c) (Matsubaya et al., 1975). 

The dependence of concentration of K+, Na+, Ca + and H4Si04 in equilibrium with common alteration minerals (K-feldspar, Na-feldspar, quartz) on temperature is shown in Fig. 1.140 (Shikazono, 1988b). This figure demonstrates that (1) chemical compositions of hydrothermal solution depend on alteration minerals, temperature and chloride concentration, and K" " and HaSiOa concentrations increase and Ca + concentration decrease with increasing of temperature. In this case, it is considered that potassic alteration adjacent to the gold-quartz veins occurs when hydrothermal solution initially in... 

Therefore, it is thought that the mixing of acidic solution with hydrothermal solution occurred and andesite near the gold-quartz veins suffered superimposed potassic and advanced argillic alterations. 

Figure 1.142 shows the dependence of solubility of Si02 minerals (quartz, cristobalite) on temperature. As described already, cristobalite occurs in peripheral and shallower part of hydrothermal alteration zone. Quartz is present in zones occurring in deeper and closer to the gold-quartz veins. Such zoning from quartz to cristobalite is also common in main active geothermal systems (Hayashi, 1973 Takeno et al., 2000). 

Figure 1.142. The computed result of the relationship between dissolved silica (H4Si04) concentration of mixed fluid and temperature based on four reservoirs model (Shikazono et al, 2002). Open triangle solubility curve for quartz, Open square solubility curve for a-cristabalite, Solid triangle Hishikari Lower Andesite lava (drilling core), Cross Relatively fresh Hishikari Lower Andesite lava (drilling core). H.S. hydrothermal solution G.W. ground water.

The temperature of the initial hydrothermal solution is assumed to be 250°C from homogenization temperature of fluid inclusions in vein quartz (Shikazono and Nagayama, 1993). 

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