Stability of Parent Material Minerals

Another example of weathering reversal is salt accumulation due to impeded soil drainage or seawater inundation. Weathering under these conditions reverses in the sense that the secondary minerals formed are chemically similar to igneous and sedimentary rock minerals and are unstable under well-drained, oxidative conditions. The chemistry of soil development deals with the degradation of parent minerals and the formation of secondary minerals over a wide range of chemical conditions. 

Metamorphic rocks contain the above minerals and chemically similar valiants. Metamorphic rocks are occasionally rich in Mg minerals, such as the pyroxenes (approximate composition Ca(Mg, Fe)Si20g) and the chemically similar but more complex augites. Serpentine metamorphic rocks are even richer in Mg because of antigorite (Mg3Si205(0H)4). 

The mineral structures, of igneous minerals are varying organizations of silicon-oxygen tetrahedra and aluminium-oxygen tetrahedra and octahedra. The stability of igneous minerals also applies to the weathering of chemically and structurally similar minerals in metamorphic mid sedimentary rocks. 

FIGURE 7.2. Stability to weathering of some minerals in igneous and metamorphic rocks. (Adapted from S. S. Goldich. 1938. J. Geol. 46 38.) 

A second chemical factor affecting mineral weatherability is the position of ions in the structure. The tetrahedra of Ca feldspars contain half Al3+ and half Si4+. At room temperature, Al3+ is more stable in octahedral coordination, The charge deficit created by the Al3+ substitution is made up by Ca2+ ions between the tetrahedra. The structural strain, the charge deficit in the tetrahedra, and concentrated Ca2+ counter charge weaken the anorthite feldspar structure with respect to weathering relative to Na and K feldspars. In Na and K feldspars, only one-quarter of the tetrahedral positions are occupied by A1 and that charge deficit can be locally neutralized by Na+ or K+. Calcium feldspars are, therefore, the least stable feldspars under soil conditions. Potassium feldspars are more stable than Na feldspars, because K fits better between adjacent tetrahedra. 

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