Citation: Churchman, G.J.; Lowe, D.J. 2012. Alteration, formation, and occurrence of minerals in soils. In: Huang, P.M.; Li, Y; Sumner, M.E. (editors) “Handbook of Soil Sciences. 2nd edition. Vol. 1: Properties and Processes”. CRC Press (Taylor & Francis), Boca Raton, FL, pp.20.1-20.72. Alteration, Formation, and Occurrence of Minerals in Soils G.J. Churchman, University of Adelaide, Australia and D.J. Lowe, University of Waikato, New Zealand 20.1 Introduction............................................................................................................................20-1 20.2 Alteration of Primary Minerals............................................................................................ 20-2 Common Primary Minerals in Soils • Observations of Relative Stabilities of Primary Minerals in Soils • Thermodynamic and Structural Explanations of Relative Stabilities • Processes and Products of Alteration of the Main Types of Primary Minerals by Weathering • Peculiarities of Processes and Products of Alteration by Weathering in Soils • Processes of Mineral Alteration and Formation by Weathering: Summary 20.3 Occurrence of Clay Minerals in Soils................................................................................ 20-31 Occurrence of Kaolinite in Soils • Occurrence of Halloysite in Soils • Occurrence of Illite in Soils • Occurrence of Vermiculite in Soils • Occurrence of Smectite in Soils • Occurrence of Palygorskite and Sepiolite in Soils • Occurrence of Iron Oxides in Soils • Occurrence of Manganese Oxides in Soils • Occurrence of Aluminum Hydroxides, Oxyhydroxides, and Oxides in Soils • Occurrence of Phosphate, Sulfide, and Sulfate Minerals in Soils • Occurrence of Pyrophyllite, Talc, and Zeolites in Soils • Occurrence of Neogenetic Silica in Soils • Occurrence of Titanium and Zirconium Minerals in Soils • Occurrence of Highly Soluble Minerals in Soils • Occurrence of Secondary Minerals in Soils: Summary 20.4 Influence of Mode of Formation upon Predictions of Properties of Soils from Their Clay Mineralogy.............................................................................................. 20-48 Introduction: The Role of Mineralogy in Soil Science • Contributions of Classical Clay Mineralogy toward Explanations of Soil Properties • Potential of Classical Clay Mineralogy for Explaining Soil Properties • Nature of Soil Minerals and Relationship to Their Mode of Formation Acknowledgments...........................................................................................................................20-54 References.........................................................................................................................................20-54 Tables are on pp. 159 and following pages For the definitive version of this article please see the Handbook of Soil Science 2nd ed (citation at top of page) 1 20.1 Introduction This chapter, like Churchman (2000), seeks to bring readers up to date with information and understanding about the alteration of minerals and the nature of their products in the context of the formation and development of soils. It complements various articles by Bergaya et al. (2006), and the recent books by Velde and Meunier (2008) and Velde and Barré (2010). This current chapter differs from Churchman (2000) in that it discusses the manner in which minerals, and especially secondary minerals, actually occur in soils, i.e., it deals (in Section 20.3) with the occurrence, as well as the alteration and formation 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 often 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 filtering and partitioning water flow, for immobilising 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. The 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 different 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 effort 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 different sorts of soils from the nature of their constituents and that of their associations. As is discussed in Section 20.4, soil mineralogy has to some extent been in decline as a sub-discipline of soil science and it may be that this is due, at least in part, to the often unjustified assumption that soil minerals are similar in their properties to those of their well-characterized ‘types’ formed in nonsoil environments. The implications of this erroneous or only partly true assumption, and new directions for soil clay mineralogy, are also discussed in Section 20.4. 2 This chapter deals with minerals, both primary and secondary, with the processes of weathering and associated clay formation, and with soils. We define each of these terms in Table 20.1, which also includes some alternative definitions, and some issues involved in these definitions. Insert Table 20.1 here (see p. 159) 20.2Alteration of primary minerals 20.2.1 Common primary minerals in soils The most common primary minerals found in soils are listed in Table 20.2 within their chemical (compositional) types and their mineralogical groupings together with the types of (crystal) structures for these groupings. Table 20.2 also lists chemical formulae for the minerals or groups of minerals and summarizes the general sorts of soils in which they occur and often also their relative abundance in these soils. Other properties of the minerals listed are given in Churchman (2006, Table 3.1). In soils, primary minerals, as residuals of physical and chemical weathering processes, most commonly occur in coarser particles (of sand- and silt-size), although the phyllosilicates such as the micas, chlorite, vermiculite, and talc, and some oxides, such as those of titanium and also apatite, and volcanic glass, can also be found in reasonable proportions in the clay-size factions of some soils. (Insert Table 20.2) 20.2.2 Observations of relative stabilities of primary minerals in soils In most cases, weathering involves the reaction of minerals (or mineraloids) with water, or, at least, an aqueous solution. There are exceptions where alteration of primary minerals occurs but where liquid water is either absent or is not the primary agent effecting alteration. This is the case in cold (and polar) and also hot desert zones on Earth (Shoji et al., 2006). It is probably also the case in some extraterrestrial environments, such as the Moon, but not in others, such as Mars, where there is abundant evidence for liquid water, at least in the past 3 (Certini and Scalenghe, 2006). In environments where liquid water does not provide the main weathering milieu, the dominant agents of alteration nonetheless involve climatic factors, hence they constitute weathering. These include mainly physical actions such as frost wedging and successive freezing and thawing in cold desert zones on Earth (Shoji et al., 2006), processes such as wind ablation in both hot and cold terrestrial desert zones, and such processes as solar-wind irradiation on Moon (Certini and Scalenghe, 2006), and perhaps also on other dry extraterrestrial bodies and regions. Other agents besides the purely climatic can also alter primary minerals, given enough time. Among these are some resulting from various human activities (Certini and Scalenghe, 2006), including agriculture (Velde and Meunier, 2008), mining (Murakami et al., 1996; Banfield and Murakami, 1998), landfills (Batchelder et al., 1998), industrial pollution (Rampazzo and Blum, 1992; Yaron et al., 2010), and probably also global climate change (Berg and Banwart, 2000; Amundson, 2001; Leifeld, 2006). Most of these agents affect mineral alteration via aqueous solutions, although in some cases, for example, the oxidation of sulfides, particularly iron pyrite, alteration of minerals occurs when water is absent (Dent and Pons, 1995). Nevertheless, alteration by weathering most often occurs in an aqueous environment and the most important determinant of the relative stability of primary minerals to weathering is their relative solubility in water. All minerals have some solubility in water. Only rarely, however, for example, for simple salts like halite (NaCl) found, for example, in arid and saline soils, do they dissolve congruently in water. Instead, hydrolysis commonly occurs, with the more soluble components being removed selectively from the mineral. This removal may leave a solid residue which differs in composition from the original mineral, or a solid alteration product precipitates out