Saturation state silica minerals

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. 

Two principal factors drive reaction in the evaporating fluid. First, the loss of solvent concentrates the species in solution, causing the saturation states of many minerals to increase. The precipitation of amorphous silica, for example,... 

The production of copper from sulphide minerals is accomplished with a preliminary partial roast of die sulphides before reaction widr air in the liquid state, known as mattes, to form copper metal (conversion). The principal sources of copper are minerals such as chalcopyrite, CuFeSa and bornite CuaFeSa, and hence the conversion process must accomplish the preferential oxidation of non, in the form of FeO, before the copper metal appears. As mentioned before, tire FeO-SiOa liquid system is practically Raoultian, and so it is relatively easy to calculate the amount of iron oxidation which can be canned out to form this liquid slag as a function of the FeO/SiOa ratio before copper oxidation occurs. The liquid slag has a maximum mole fraction of FeO at the matte blowing temperatures of about 0.3, at solid silica saturation. 

Clay minerals are present in almost all surface-water and ground-water systems, and in many instances may be controlling the concentration of aluminum, silica, iron, magnesium, or other cations in solution. The thermodynamic data necessary to evaluate the state of reaction (saturation) are not available for some clay minerals, and for those minerals with published values, the data are in disagreement by as much as 10 kilocalories per mole for the same clay mineral. A critical review of the available data for kaolinite and sepiolite, incorporating both the most recent thermodynamic data for the components in the reaction schemes and a more complete computation for the solubility data, yields the values of -907.7 +1.3 and 1105.6 +0.4 kilocalories per mole for the free energy of formation of kaolinite and sepiolite, respectively. 

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