ABSTRACT
This chapter considers the physico-chemical characteristics of magmatic and metamorphic–sedimentary rocks, the magmatic types will show high chemical instability at the surface, while metamorphic and sedimentary rocks will form new phases but will be dominated by minerals from the transformation of the initial phyllosilicate mineralogy. Most of the outcropping, and hence soil-forming, rocks on continents are of sedimentary origin and have retained much of the mineralogical characteristics of the materials that formed them. Non-silicates may occur in soil-forming rocks either as major components but only occasionally, as minor components that are widespread, or also as rare components. The higher-grade metamorphic materials can contain aluminous micas, but the chlorite tends to be destabilised in favour of other non-phyllosilicate minerals such as garnet. The non-phyllosilicate minerals formed by metamorphism are eventually transformed into different types of smectite minerals or kaolinite under weathering conditions.
