Inorganic solid phases in soils can generally be described as minerals. In soils, rocks provide the raw (i.e. ‘parent’) materials for minerals in soils. Minerals may derive directly from rocks, with little or no chemical or structural changes, although physical changes, e.g. comminution, commonly occur. In this case they are referred to as primary minerals. However, many of the minerals that are of most importance for soil properties are secondary. While these have formed from rock minerals under the influence of soil-forming processes, principally weathering, they usually comprise different phases from those present in the rocks.
Table 3.1 comprises a compilation of (a) the characteristics and properties of inorganic solid phases that occur most commonly in soils and (b) the nature of their processes of formation and transformation and their occurrence in soils.
The information in Table 3.1 is extracted from Dixon and Weed (1989), Churchman and Burke (1991), Churchman et al. (1993, 1994), Churchman (2000), Olson et al. (2000), and Dixon and Schulze (2002), and is discussed as follows under the various categories in the table.
Characteristics and properties
Primary and secondary minerals
Although there is little doubt that most occurrences of those designated as primary minerals in Table 3.1 have a direct origin as the residue of minerals that formerly constituted rocks, some – for example quartz, micas, calcite and the zeolite analcime – may also form pedogenically.