The clay minerals in soil are in the form of layer-lattice minerals, and are made up of sheets of hydroxyl ions or oxygen. The clay minerals fall into two groups: (1) those with three groups of ions lying in a plane (the 1: 1 group of minerals), and (2) those with four groups of ions lying in a plane (the 2: 1 group of minerals). The type mineral of the 1: 1 group is kaolinite, which typically has a very low charge on it. In contrast, the 2: 1 type mineral, for example illite, carries an appreciably higher negative charge per unit weight than the kaolin group. More detailed information on the clay particles, their composition, and charges upon them is given in Theng (1979) and Oades et al. (1989).
A key concern to the soil ecologist is the extremely high surface area found per gram of clay mineral. Surface areas can range from 50 to 100 square meters per gram for kaolinitic clays, from 300 to 500 square meters per gram for vermiculites, and from 700 to 800 square meters per gram for well-dispersed smectites (Russell, 1973). These impressively large surface areas can play a pivotal role in adsorbing and desorbing inorganic and organic constituents in soils, and have only recently been treated in an appropriately analytical fashion as an integral part of the soil nutrient system (Tisdall and Oades, 1982; Oades et al, 1989).
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