Chemical weathering aluminosilicate

Clay mineral A layered aluminosilicate, such as kaolinite, dUte, chlorite, and montmordlonite. Most are formed by chemical weathering of rocks on land. 

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. 

Under transport-limited weathering, the amount of fresh rock available to weathering is limited. Chemical weathering is faster then physical weathering and available minerals ultimately contribute to the solute load in proportion to their abundance in the bedrock. Such is the case for the nearly complete destruction of aluminosilicates from old laterites and saprolites in which base cations are all effectively removed from the regolith. 

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... 

The rates of dissolution of carbonates and aluminosilicates as a function of pH are generalized in Fig. 2.11. Calcite and dolomite dissolution rates are generally 10 to 1 O -fold faster than rates for the silicates and decrease with pH up to saturation with the carbonates, usually between pH 8 and 10. Dissolution rates among the silicates range widely and are greatest for rapidly weathered minerals such as nepheline and olivine and slowest for quartz, muscovite (illite) and kaolinite, important products of chemical weathering in soils, discussed in more detail in Chap. 7. 

Hydrogen carbonates get into water via chemical weathering of aluminosilicates by CO2 and HjO and the reaction between carbonate minerals and CO2. 

At the surface of the raised bog, the pore waters are derived primarily from recharging precipitation, and are high in dissolved organic acids from the oxidation of plant material. pH is low (<5), and dissolved aluminum is high, whereas dissolved silicon and alkaline earth cation concentrations are low. Examination of silt grains from the peat show that aluminosilicate minerals are chemically weathered but quartz is not. In this type of water, aluminum is mobilized as organic-aluminum complexes at acidic pH, and... 

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. 

Soil is the product formed when the rocks of the earth s crust are exposed at the surface and are subjected to various physical, chemical, and, eventually, biological weathering processes. The minerals in these rocks are predominantly silicates, which dominate the characteristics of most soils. Table 1 shows those elements that are found in the crust above an average concentration of 1% and their corresponding soil content. The importance of aluminosilicates in soil is clear from the enrichment factors of approximately 1 for O, Si, and Al. Some loss occurs of K, Fe, Ca, Na, and Mg as a result of soil processes. But two elements, C and N, show considerable enrichment in soil because of the crucial role played by organic matter. 

The rate at which these elements will leach from the ash sample is dependent on the form in which the element is present and the location of the element within the ash matrix or adsorbed onto the ash particle surface. The ash spheres are chemically stable in the environment and are resistant to weathering due to the aluminosilicate matrix. Any element present in this matrix will not be readily avaiable for leaching. Elements adsorbed onto the surface of the ash spheres will be more readily leached. Uncombusted mineral matter may account for presence of high concentrations of certain elements in the whole ash analyses. However, leachates generated from these ashes may not reflect the high concentrations because the extraneous material associated with the ash is not in a form that is susceptible to leaching within the time span of these experiments. 

The formation of such minerals and authigenic feldspar under highly saline conditions is sometimes termed reversed weathering. This is appropriate in the sense that ions weathered in other locations are thus incorporated into new minerals instead of flowing to the sea. The minerals formed, however, are considerably different from the original igneous aluminosilicates, although their chemical compositions may be similar. 

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. 

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