Aluminosilicate minerals equilibria

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. 

Reactions M-O represent incongruent dissolution of Ca(OH)2s because the solubility of CaCOjS is much smaller than the solubility of Ca(OH)2s (Table 2.7). Therefore, introduction of Ca(OH)2s to water in equilibrium with atmospheric C02 leads to spontaneous formation of CaC03s. The well-known incongruent dissolution phenomena are those representing the dissolution of aluminosilicate minerals. For example, K-feldspars (orthoclase) undergo incongruent dissolution when exposed to water and carbonic acid to form kaolinite ... 

In general, dissolution rates of other silicate and aluminosilicate minerals also increase under acid and alkaline conditions and are relatively independent of pH in the near-neutral pH range. Drever (1994) summarizes this behavior with a general equation for the forward rate far from equilibrium... 

It has often been argued that clay minerals cannot be used as models for soils in surface chemistry because soil clays are too heterogeneous and impure. The work reported here shows that, for both equilibrium and kinetic studies, standard aluminosilicates are useful models for soils. 

Second, Feth et al. (3) observe that the waters gain much of their silica in a few feet of travel, showing that it is the action of the high C02 water that produces "kaolinite. The rock minerals react, forming "kaolinite continuously in the system, and the "kaolinite controls the water composition by its presence. If the aluminum analyses were not so low, and hence analytically suspect, an attempt could be made to calculate an equilibrium constant for the substance formed. All that can be said at the moment is that the values of Si02 and Al concentrations and of pH are reasonable for those controlled by an aluminosilicate of the approximate stability of kaolinite. 

The primary minerals of igneous rocks are all mildly basic compounds. When they react in excess with acids such as HC1 and CO2, they produce neutral or mildly alkaline solutions plus a set of altered aluminosilicate and carbonate reaction products. It is improbable that ocean water has changed through time from a solution approximately in equilibrium with these reaction products, which are clay minerals and carbonates. 

Our discussion of mineral surfaces will be restricted to simple oxide and hydroxide minerals that are widely used in adsorption studies (Table 9-1). Under strong weathering conditions, these minerals may comprise a substantial fraction of the available surface area in soils and aquifers. More complex minerals, including parent material and partially weathered products (especially aluminosilicates), are of equal or greater importance in most other subsurface environments. Excellent reviews of the equilibrium and reaction chemistry of aluminosilicate surfaces are available (Voudrias and Reinhard, 1986 Mortland, 1970). 

Concentrations of silica of around 2 ppm were reached in dilute salt solution with mica and kaolin and up to 15 ppm with montmorillonite (36). When seawater was enriched with soluble silica to 25 ppm SiOa, it remained at that level for a year in the absence of these minerals, but when the latter were then added, the silica was removed from solution down to the 2-15 ppm level that was reached when the minerals alone were added. Since many ocean waters contain 2-10 ppm SiOj, it is possible that this value is reached as the equilibrium solubility of colloidal aluminosilicate in suspension. The above experiment is consistent with the fact that in pure water, pure a morphous silica dissolves to give a concentration of monosilicic acid of 100-110 ppm, but in the presence of polyvalent metal cations such as iron, aluminum, and other metals, colloidal silicates are formed with a much lower solubility with respect to monosilicic acid. Her (37) has shown that soluble aluminum reduces the solubility of amorphous silica from about 110 to less than 10 ppm. 

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