Allophane and Imogolite

Imogolite is a mineral with better defined composition, having the unit formula Al2Si03(0H)4. It displays short-, medium-, and long-range order. A1 is present only 

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Allophane and Imogolite. AUophane is an amorphous clay that is essentially an amorphous soHd solution of sUica, alumina, and water (82). In allophane less than one-half of the aluminum is held in tetrahedral coordinations and the Si02 to AI2O2 ratio typically varies between 1.3 and 2.0, but values as low as 0.83 have been reported. The typical morphology of allophane is cylindrical (37). AUophane may be associated with haUoysite, smectite minerals, or it may occur as a homogeneous mixture with evansite, an amorphous soHd solution of phosphoms, alumina, and water. Its composition, hydration, and properties vary. Chemical analyses of two allophane samples are given in Table 5. 

Secondary minerals. As weathering of primary minerals proceeds, ions are released into solution, and new minerals are formed. These new minerals, called secondary minerals, include layer silicate clay minerals, carbonates, phosphates, sulfates and sulfides, different hydroxides and oxyhydroxides of Al, Fe, Mn, Ti, and Si, and non-crystalline minerals such as allophane and imogolite. Secondary minerals, such as the clay minerals, may have a specific surface area in the range of 20-800 m /g and up to 1000 m /g in the case of imogolite (Wada, 1985). Surface area is very important because most chemical reactions in soil are surface reactions occurring at the interface of solids and the soil solution. Layer-silicate clays, oxides, and carbonates are the most widespread secondary minerals. 

J. P. Boudot, Relative efficiency of complexed aluminium, non-crystalline A1 hy-droxyde, allophane and imogolite in retarding the biodegradation of citric acid. Geoderma 52 29 (1992). 

Amorphous Aluminosilicates. These occur in soils influenced by volcanic activity and are associated with very high moisture retention and anion fixation, and low to very high pH-dependent CEC. They may also bind organic matter tightly, protecting it against decomposition. Examples are allophane and imogolite. 

Clark, C.J. and McBride, M.B., Cation and anion retention by natural and synthetic allophane and imogolite, Clays Clay Miner, 32, 291, 1984. 

Table 7-4 Selected properties of phyllosilicates, allophane, and imogolite"... 

Wada, K. (1977). Allophane and imogolite. In "Minerals in Soil Environments" (J. B. Dixon and S. B. Weed, eds.), pp. 603-638. Soil Science Society of America, Madison, Wisconsin. 

Secondary silicates form as clay minerals in soils after weathering of the primary silicates in igneous minerals. The secondary silicates include amorphous silica (opal) at high soluble silica concentrations and the very important aluminosilicate clay minerals kaolinite, smectite (montmorillonite), vermiculite, hydrous mica (il-lite), and others. Kaolinite tends to form at the low silicate concentrations of humid soils, whereas smectite forms at the higher silicate and Ca concentrations of arid and semiarid soils. The clay fraction of soils usually contains a mixture of these day minerals, plus considerable amorphous silicate material, such as allophane and imogolite, which may not be identifiable by x-ray diffraction. 

A more pronounced structural disorder often exists in freshly precipitated silica or metal hydroxides in soils, since these compounds typically are amorphous. Structurally disordered aluminosilicates, known collectively as allophane and imogolite, are common in the clay fractions of soils formed on volcanic ash parent material. ... 

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