A Weathering Example

Many of the above observations can be illustrated through discussion of an example of weathering taken from Goldich (1938) (see also Krauskopf 1967). The parent rock in this humid, temperate climate example is a quartz-feldspar-biotite gneiss. The mineralogic and oxide composition of this rock and its weathered product soil is given in Table 7.2. 

Of total silica in the rock, 22% has been lost to the soil solution, chiefly from weathering of the plagioclase feldspar, biotite, and hornblende. All or most of the quartz has evidently been unaffected by weathering. 

TABLE 7.2 The composition of a granitic-type rock and of its weathering products  

Weathering of the feldspars, biotite, and hornblende has also released their cations to the soil solution, as apparent from the high percentage losses of MgO, CaO, Na20, and K2O. 

Weathering has led to oxidative loss of FeO in the biotite and hornblende and a relative increase in Fc203 in the soil. A comparison of the total iron as Fe in the parent rock and the soil shows that a 44% loss of Fe has occurred on weathering. 

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