ABSTRACT
The inorganic fraction of soils is composed of rock fragments and minerals of varying size and composition. They are sometimes distinguished into primary and secondary minerals. Primary minerals are, by denition, rock-forming minerals that are present in soils chemically unchanged, whereas secondary minerals are minerals that have been formed by weathering of primary minerals. But this distinction creates problems, because secondary minerals may well be regarded as primary on a pedological basis. On the basis of size, three major inorganic soil fractions are usually recognized: (1) the coarse fraction (2 to 0.050 mm) called sand, (2) the ne fraction (0.050 to 0.002 mm) called silt, and (3) the very ne fraction (<0.002 mm) referred to as clay (Soil Survey Staff, 1975, 2006). In soil science, we are used to considering clay as a colloid, although strictly speaking only the ne clay fraction <0.2 μm is colloidal clay. Materials coarser than sand, like rocks and gravel, may also be present in soils, but they are usually not considered soil constituents, except iron concretions. However, they may form sand, silt, and clay upon weathering. Despite the variability in composition, the inorganic fractions are predominantly silicates and oxides. Six types of soil silicates are usually recognized on the basis of the arrangement of the SiO4 tetrahedra in their structure:
1. Cyclosilicates-Closed rings or double rings of tetrahedra (SiO3, Si2O5) 2. Inosilicates-Single or double chains of tetrahedra (SiO3, Si4O11) 3. Nesosilicates-Separate SiO4 tetrahedra 4. Phyllosilicates-Sheets of tetrahedra (Si2O5) 5. Sorosilicates-Two or more linked tetrahedra (Si2O7, Si5O16) 6. Tectosilicates-Framework of tetrahedra (SiO2)
The minerals are listed above by alphabetical order, but on the basis of molecular structure, nesosilicates, composed of single tetrahedra, are the simplest in structure, followed by sorosilicates, where two or more tetrahedra are linked together to form the mineral. The structure becomes increasingly more complex in cyclo-, phyllo-, and tectosilicates (Figure 6.1). Examples of mineral species belonging to the six groups of silicates are listed in Table 6.1. The sand and a major part of the silt fraction belong to the cyclo-, ino-, neso-, soro-, or tectosilicates. They consist of primary minerals and make up the skeletal framework of the soil. Because they are coarse in size, they have low specic surface area and do not exhibit colloidal properties. Although not really chemically active in chemical reactions, they participate in a number of reactions and exhibit some adsorption. They play an important role in soil texture and soil permeability. Many of the sand and silt minerals are also of importance for formation of clays. Most of the clays are phyllosilicates. Clays are of special importance in soil chemistry, because they have a surface chemistry different from that of the larger mineral grains. Clays also exhibit bulk physical properties different from gravel, sand, or silt. Many of the minerals in soil clays are crystalline in structure, whereas others may poorly exhibit crystals or are structurally disordered. Some of the clays may be amorphous (e.g., allophane,
silica, alumina, and iron oxide). Many prefer to call them noncrystalline or paracrystalline clays. The name short-range-order (SRO) minerals is becoming popular. They were formed by rapid pedogenic processes, are unstable, and may eventually crystallize in time. The crystalline clays are then sometimes referred to as long-range-order minerals (Chesworth, 2008; Huang et al., 2005; Lal, 2006; Tan, 2003). Silica, alumina, and iron oxide may occur in soils as discrete, independent minerals, or as coatings around crystalline clay particles and other inorganic soil constituents. Not
TABLE 6.1 Examples of Mineral Species Classified According to the Six Types of Soil Silicates
all of the clays belong to the phyllosilicate group, or layer-lattice silicates. The soil’s clay fraction also contains other minerals, such as the palygorskite-sepiolite minerals, quartz in particle sizes of <2 μm, sesquioxides, titanium oxides, pyrophyllite, talc, suldes, sulfates, and phosphates. The major types of phyllosilicates are listed in Table 6.2.
