Aluminosilicate minerals, weathering

Silicate and aluminosilicate mineral-weathering reactions involve both H -ion attack on the mineral and hydration or hydrolysis of the product minerals (clays and metal oxyhydroxides) or product aqueous species. If the primary mineral contains aluminum, then clays may result from the weathering. If the primary mineral contains Fe(II) or other reduced metal ions, weathering may involve oxidation as well as hydrolysis. 

Allophane is present on the surface of primary aluminosilicate minerals weathering in the soil, and the external sites abundant on the surface of allophane resemble those of less abundant surface sites linked to the structures of weathering primary aluminosilicate minerals. Consideration of the behavior of individual sites of this type near specific atoms appears necessary for progress in the understanding of the chemistry of solid-fluid interfaces in the soil. 

Humus/SOM enter into a wide variety of physical and chemical interactions, including sorption, ion exchange, free radical reactions, and solubilization. The water holding capacity and buffering capacity of solid surfaces and the availability of nutrients to plants are controlled to a large extent by the amount of humus in the solids. Humus also interacts with solid minerals to aid in the weathering and decomposition of silicate and aluminosilicate minerals. It is also adsorbed by some minerals. 

Isomorphic substitution The replacement of some of the aluminum and silicon in aluminosilicate minerals by cations of similar ionic charge and radius. This usually occurs as a result of chemical weathering. 

Notice that in step 2, the weathering of an aluminosilicate mineral by the aggressive attack of "H2S04" waters may result in the release of dissolved silica, as well as cations. The ratio of silica to cations released depends on the composition of the primary mineral weathered and its alteration product. The weathering of a carbonate mineral gives cations and bicarbonate. 

In this chapter, the focus is on weathering of feldspars, aluminosilicate minerals, which are the most abundant mineral species in the earth crust (Banfield Hamers, 1997). Feldspars contain aluminium and silicon, which are arranged in a tetrahedral structure, with other cations in the voids of this structure. The common feldspars have compositions ranging between albite (NaAlSisOg) and K-feldspar (KAlSigOg) (alkali feldspars) and between albite and anorthite (CaAl2Si20g) (plagioclase feldspars). 

Aquifer solids are rarely composed of discrete mineral phases such as quartz, feldspar, or clay minerals. Over time, weathering reactions produce complex mixtures of different minerals of varying degrees of crystallinity. Since adsorption implies a surface or near surface phenomena, it is the mineral surfaces of aquifer solids that are of principal concern. These surfaces can consist of mixtures and coatings of various oxides, aluminosilicate minerals, carbonates, and organic matter (Coston et al., 1995 Farmer et al., 1991 Jeime, 1976 Taylor, 1987). 

In recent years rapid advances have been made in understanding mechanisms and controls on the dissolution and precipitation rates of silicate and aluminosilicate minerals in general. Much of this work has focused on rates of chemical weathering (cf. Steefel and Van Cappellen 1990 Lasaga... 

Figure 2.10 Schematic relationship between pH and dissolution rate for silicate and aluminosilicate minerals. Reprinted from Geochim. et Cosmochim. Acta, 58(10), J.T. Drever, The effect of land plants on weathering rates of silicate materials, 2325-32, 1994, with permission from Elsevier Science, Ltd, The Boulevard, Langford Lane, Kidlington, 0X5 1GB, U K.

As noted above, a major fraction of the silica released to natural waters from the weathering of silicate and aluminosilicate minerals, such as the feldspars, remains in solution, where it occurs chiefly... 

High aluminum (and silica) concentrations in acid streams and in sulfidic-waste tailings and coal-storage piles derive from the weathering of aluminosilicate minerals. Kaolinite clay, for example, is common in the underclays beneath Pennsylvania coal beds (Gang and Langmuir 1974). Above... 

The proof of reversibility in primary mineral weathering would be instances where primary mineral structures have formed under earth-surface conditions. There are reports that secondary quartz can slowly precipitate at room temperature from solutions supersaturated with monosilicic acid. More typically, however, precipitated silica in soils is structurally disordered, in the form of chalcedony or opal. In fact, as long as alumina is present, silica does not precipitate as a separate phase, reacting instead to form aluminosilicates (layer silicates, imogolite, or allophane). 

Suggest mechanisms to explain the more rapid weathering of aluminosilicate minerals at low and at high pH. 

The Isolation of Aluminum Aluminum, the most abundant metal in Earth s crust by mass, is found in numerous aluminosilicate minerals. Through eons of weathering, certain of these became bauxite, a mixed oxide-hydroxide that is the major ore of aluminum. In general terms, the isolation of aluminum is a two-step process that combines several physical and chemical separations. In the first, the mineral oxide, AI2O3, is separated from bauxite in the second, which we focus on here, the oxide is converted to the metal. 

The geochemical balance of a 103 acre watershed underlain by silicate bedrock was investigated. Base flow composition of the stream water was essentially constanty but flood flows showed a decrease in concentration of silica, bicarbonate, and sodium and an increase in sulfate, magnesium, calcium, and potassium. Laboratory experiments indicate that fresh rock or soil reacts rapidly with distilled water and achieves a composition similar to the stream water, suggesting control of water composition by reaction with the silicate minerals. The aluminosilicate minerals react with CO charged water to form kaolinite, releasing cations and silica to solution. The products of weathering are removed as particulate matter (0,28 metric tons per year) and dissolved material (1,5 metric tons per year). 

Dissolution of most aluminosilicate minerals also consinnes H ions and contributes base cations (Ca, Mg, Fe(II)), alkali elements (Na, K) and dissolved Si and Al to the tailings pore water (Blowes and Ptacek, 1994). Though, dissolution of almninosilicate minerals is slower than of metal hydroxides and much slower than that of carbonates. Feldspar weathering is mainly controlled by pH, silica, Na, K and Ca concentrations. One possible reactions path is ... 

Chapter 1 provides an extensive discussion of chemical equilibria as they pertain to minerals. This covers in detail chemical equilibria of solids and aqueous solutions, the solubility of various types of minerals, the weathering of silicate and aluminosilicate minerals, the composition of geothermal and groimd water, hydro-thermal alteration, oxidation and reduction reactions, and the partitioning of elements between aqueous solutions and crystals. 

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