Alteration, Formation, and Occurrence of Minerals in Soils

Secondary minerals are the most reactive inorganic materials in soils. Furthermore, they occur commonly in association with the most reactive organic materials in soils. It has oen been inferred that knowledge of the properties of these reactive materials should enable close predictions of the useful soil properties, whether for growing plants, for ltering and partitioning water ¥ow, for

immobilizing contaminants, for supporting human-made structures, or for other purposes. However, most of the information about the formation of secondary minerals through the alteration of primary minerals derives from studies of largely inorganic processes taking place in relatively “clean” environments. ¤e soil, by contrast, is a heterogeneous milieu that is a dynamic part of the biosphere continually changing in response to climatic variations over all scales of time and space. It is not at all surprising that the minerals in the soil can be quite di¦erent in their chemical and physical characteristics from those of the “type” minerals formed in more “geological” environments with which they may share a name and ideal crystalline structure (Churchman, 2010). Chadwick and Chorover (2001) made the point that the crisp boundaries in stability diagrams are, in the real world of natural soils, not so clear-cut. However, the e¦ort involved in understanding the real nature of minerals in soils is worthwhile in view of the potential reward of being able to explain and predict properties across di¦erent sorts of soils from the nature of their constituents

20.1 Introduction ...........................................................................................................................20-1 20.2 Alteration of Primary Minerals ...........................................................................................20-2

Acknowledgments .......................................................................................................................... 20-54 References ........................................................................................................................................ 20-54

and that of their associations. As discussed in Section 20.4, soil mineralogy has to some extent been in decline as a subdiscipline of soil science and it may be that this is due, at least in part, to the oen-unjustied assumption that soil minerals are similar in their properties to those of their well-characterized “types” formed in nonsoil environments. ¤e implications of this erroneous or only partly true assumption, and new directions for soil clay mineralogy, are also discussed in Section 20.4.