Weathering of complex silicate minerals

So far we have considered the weathering of monomer silicates (e.g. olivine), which dissolve completely (congruent solution). This has simplified the chemical reactions. The presence of altered mineral residues during weathering, however, suggests that incomplete dissolution is more usual. Upper crustal rocks have an average composition similar to the rock granodiorite (Table 4.4). This rock is composed of the framework silicates, plagioclase feldspar, potassium feldspar and quartz (Table 4.4), plagioclase feldspar being most abundant. Thus, a simplified weathering reaction for plagioclase feldspar might best represent average chemical weathering. We can illustrate this using the calcium (Ca)-rich plagioclase feldspar, anorthite.

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