Weathering rock-forming minerals

Feldspars are the most abundant minerals of igneous and metamorphic rocks 0 2). Being the most abundant rock-forming minerals of the earth s crust, they have received a proportionately large share of attention from students of weathering, and most of the major analytical advances and conceptual models which have been applied to other mineral groups have also been applied to feldspars. 

For interactions between solid and liquid phases, two cases have to be distinguished weathering of rock-forming minerals and the weathering of trace minerals. 

For the weathering of rock-forming minerals, the solution kinetics is determined by the solubility product and transport in the vicinity of the solid-water-interface. If the dissolution rate of a mineral is higher than the diffusive transport from the solid-water interface, saturation of the boundary layer and an exponential decrease with increasing distance from the boundary layer results. In the following text this kind of solution is referred to as solubility-product controlled. If the dissolution rate of the mineral is lower than diffusive transport, no saturation is attained. This process is called diffusion-controlled solution (Fig. 23 right). 

Pottery is produced by the conversion of sedimentary clay (produced by the weathering of rocks) into hard rocklike objects. The clay minerals, which were formed by the chemical decomposition of certain rock-forming minerals, contain trace elements. The sediments in which these clays are found, however, also contain fragments of the primary minerals from the parent rock (including grains of silica sand). These detrital components, which result from the physical and chemical breakdown of minerals, are often accompanied by authigenic minerals that are chemically precipitated from aqueous solutions. In some ceramics, additional components were added as temper during production. 

Rock-forming minerals and igneous rocks react with water and the species contained in it, and with ambient oxygen, thereby weakening rocks through solubilization (e.g., hydrolysis of silicates), redox, and precipitation reactions the rocks are transformed in this manner into soils and sediments that are further broken up through physical weathering that favors the transport processes. 

To help understand the discussion on weathering, a veiy brief review of the names and chemical formulae for some of the important rock-forming minerals is presented here. The silicate discussion is mainly from Drever (1982). 

Sec. 7.2 Weathering Rates of Some Rock-Forming Minerals... 

WEATHERING RATES OF SOME ROCK-FORMING MINERALS... 

Wilson, M.J. 1975. Chemical weathering of some primary rock-forming minerals. Soil Sci. 119 349-355. 

Feldspars are the most abundant rock-forming minerals in the earth s crust, warranting a great deal of study in order to understand the process by which they weather. It is believed that feldspar weathering proceeds by the following steps ... 

A flow diagram of the weathering sequences that can be expected for the most important rock-forming minerals is depicted in Figure 6.9. This diagram incorporates the processes of alteration, neoformation by addition (silica added to Fe and A1 oxides in confined environments), and neoformation by subtraction (silica removed... 

Figure 6,9. Weathering pathways for the common rock-forming minerals of the earth s crust. (Adapted from B. Mason. 1966. Principles of Geochemistry. 3rd ed. New York Wiley.)...

A higher content of magnesium is characteristic of basic and ultrabasic magmatic and metamorphic rocks (e.g., basalt and serpen-tinite) and their weathering products. Mg-rich rock-forming minerals are biotite and other dark-colored silicate minerals, as well as serpentine. In the case of the serpentinite soils, a Mg-adapted natural vegetation has developed. In the case of sediments, dolomite is a Mg-rich limestone young sediments deposited from seawater are also Mg-rich, as can be seen in marsh soils. 

Thus, the transformation extent of hypogene rocks depends first of all on the abundance and acidity of water. The more intense the water-exchange and the lower pH value in water, the greater the amount of silicon is removed, and the faster and deeper is the process of chemical weathering. In the localities of more alkaline water from hypogene rock-forming minerals form mostly montmorillonites, wherever neutral weakly-acidic water dominates, form kaolinites, and in the areas of abimdant acid water - gibbsite. 

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