Mineral Formation in Soils

The formation of secondary minerals in soils generally results from the combination and addition of ions and molecules from the soil solution to the solid phase. This mechanism was originally given little consideration, because aluminium and silicon in solution did not appear to combine during laboratory experiments. Only relatively recently has the slow kinetics of such reactions been appreciated. Experiments that take slow reactivity into account and provide nucleation centers for crystal formation have shown that secondary minerals can precipitate from solutions containing the proper constituent ions and Si(OH)4. 

Despite the unlikelihood of secondary mineral formation by ion substitution into or movement within an existing solid, some secondary 2 1 layer silicates apparently are formed by solid-phase changes of mica fragments inherited from the parent material. Hydrous mica, for example, is a product of chemical weathering as well as mechanical breakdown of mica. Hydrous mica, in turn, can be modified directly to vermiculite, montmorillonite, or chlorite. The process is not completely understood, but seemingly involves the outward diffusion of K+ from between the layer lattices and a subsequent or simultaneous reduction of charge within the layer lattice. 

Name Mineral Class Environment Ubiquity in Soils Importance 

Pyrite Sulfides Tidal marshes (reducing conditions and hard-rock mine tailings (coal and shale beds) C Primary mineral (oxidizing conditions) but secondary phase forms in reducing environments large metal and acidity input to surface waters during weathering 

Dolomite Carbonates Shallow, young soils formed in limestone R Major constituent of limestone parent material fertilizer source 

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Figure 9.7 Clay mineral composition of the surface layers of residual soils formed on (a) quartz- and feldspar-rich rocks, and (b) Fe- and Mg-rich igneous rocks in California. After I Barshad. The effect of variation in precipitation on the nature of clay mineral formation in soils from acid and basic igneous rocks, Prac. Int. Clay Conf., 19 by the Geological Survey of Norway. Used by permission.

Barshad, I. 1966. The effect of variation in precipitation on the nature of clay mineral formation in soils from acid and basic igneous rocks. Proc. int. clayconf. Jerusalem 1, pp. 167-73. 

Violante A, Gianfreda L (2000) Role of biomolecules in the formation and reactivity towards nutrients and organics of variable charge minerals and organo-mineral complexes in soil environment. In Bollag J-M, Stotzky G (eds) Soil biochemistry, vol 10. Marcel Dekker, New York, USA, pp 207-270... 

Adsorption may influence precipitation by means other than the processes mentioned above. Davies (Chapter 23) discusses the role of the surface as a catalyst for oxidation of adsorbed Mnz+. Redox reactions may contribute substantially to the formation of manganese oxide coatings on mineral surfaces in soils and sediments. 

The laboratory derived model of hematite formation in soils via ferrihydrite has received general acceptance. So far, it is the only way to produce hematite at ambient temperatures and in the pH range of soils. Support from soil analysis, however, is meagre. Hematite is usually associated with other Fe oxides, mainly with goethite but not with ferrihydrite. There seems to be only one report of a ferrihydrite-hema-tite association (based on XRD and Mossbauer spectra) viz. in several andisols formed from basalt in the warm and moist climate of Hawaii (Parfitt et al., 1988). In this case, in addition to the low age of the soils, high release of Si may retard the transformation of ferrihydrite to hematite, whereas normally, the rate of transformation of ferrihydrite seems to be higher than that of ferrihydrite formation, so that this mineral does not persist. 

An analysis of the thermodynamic stability models of various nickel minerals and solution species indicates that nickel ferrite is the solid species that will most likely precipitate in soils (Sadiq and Enfield 1984a). Experiments on 21 mineral soils supported its formation in soil suspensions following nickel adsorption (Sadiq and Enfield 1984b). The formation of nickel aluminate, phosphate, or silicate was not significant. Ni and Ni(OHX are major components of the soil solution in alkaline soils. In acid soils, the predominant solution species will probably be NE, NiS04°, and NiHP04° (Sadiq and Enfield 1984a). 

In spite of these caveats, we will find that phase diagrams involving clays, micas, and related phases provide us with useful insights regarding such processes as weathering and mineral alteration and formation in soils and sedimentary rocks. 

Adsorption of Metal Ions and Ligands. The sohd—solution interface is of greatest importance in regulating the concentration of aquatic solutes and pollutants. Suspended inorganic and organic particles and biomass, sediments, soils, and minerals, eg, in aquifers and infiltration systems, act as adsorbents. The reactions occurring at interfaces can be described with the help of surface-chemical theories (surface complex formation) (25). The adsorption of polar substances, eg, metal cations, M, anions. A, and weak acids, HA, on hydrous oxide, clay, or organically coated surfaces may be described in terms of surface-coordination reactions ... 

Soil reaction (pH) The relationship between the environment and development of acid or alkaline conditions in soil has been discussed with respect to formation of soils from the parent rock materials. Soil acidity comes in part by the formation of carbonic acid from carbon dioxide of biological origin and water. Other acidic development may come from acid residues of weathering, shifts in mineral types, loss of alkaline or basic earth elements by leaching, formation of organic or inorganic acids by microbial activity, plant root secretions, and man-made pollution of the soil, especially by industrial wastes. 

The release of ions through weathering is also considered an input to soils. Elements that were bound in mineral crystals are released into the soil solution. These ions can be involved in soil processes and the formation of new organic or inorganic materials, or leached from the soil into the groundwater. 

There are several environmentally significant mercury species. In the lithosphere, mercury is present primarily in the +II oxidation state as the very insoluble mineral cirmabar (HgS), as a minor constituent in other sulfide ores, bound to the surfaces of other minerals such as oxides, or bound to organic matter. In soil, biological reduction apparently is primarily responsible for the formation of mercury metal, which can then be volatilized. Metallic mercury is also thought to be the primary form emitted in high-temperature industrial processes. The insolubility of cinnabar probably limits the direct mobilization of mercury where this mineral occurs, but oxidation of the sulfide in oxygenated water can allow mercury to become available and participate in other reactions, including bacterial transformations. 

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