Aluminosilicate imogolite

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. 

Liu, C., and Huang, P. M. (2002). Role of hydroxyl-aluminosilicate ions (proto-imogolite soil) in the formation of humic substances. Org. Geochem. 33, 295-305. 

Imogolite is a naturally occurring aluminosilicate mineral with a unique tunnel-like or tubular structure. The... 

The reactions of several other minerals which thermally decompose to form mullite have been studied by Si and Al NMR. These include the mica mineral muscovite, which also contained sufficient iron to permit a complementary Fe Mossbauer study (MacKenzie et al. 1987), the hydroxyfluoride mineral topaz (Day et al. 1995) and the semi-amorphous aluminosilicate minerals allophane (MacKenzie et al. 1991) and imogolite (MacKenzie et al. 1989). The same combination of NMR nuclei has been used to study the thermal decomposition of other aluminosilicates including an illite-rich clay (Roch et al. 1998), montmorillonite (Brown et al. 1987), and a related mineral, Fuller s Earth (Drachman et al. 1997). NMR has also been used to study the effect of water vapour on the thermal decomposition of montmorillonite clay compacts (Temuujin et al. 2000a). 

The proof of reversibility in primary mineral weathering would be instances where primary mineral structures have formed under earth-surface conditions. There are reports that secondary quartz can slowly precipitate at room temperature from solutions supersaturated with monosilicic acid. More typically, however, precipitated silica in soils is structurally disordered, in the form of chalcedony or opal. In fact, as long as alumina is present, silica does not precipitate as a separate phase, reacting instead to form aluminosilicates (layer silicates, imogolite, or allophane). 

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. ... 

The strain energy per atom follows the general trend I/R2 for all known NTs except for imogolite. When the tube is formed by a symmetric layer, equation 1 is valid. Imogolite is composed of nonsymmetrical aluminosilicate layer and a difference in the surface tensions Aa of outer and inner tube surfaces must be taken into account. As a result, an additional contribution is included to strain energy as can be seen in equation 2 and 3. 

Alvarez-Ramirez, F. (2007) Ab initio simulation of the structural and electronic properties of aluminosilicate and aluminogermanate natotubes with imogolite-like structure. Physical Review B, 76. 

Bursill, L.A., Peng, J.L. Bourgeois, L.N. (2000) Imogolite An aluminosilicate nanotube material. Philosophical Magazine A, Physics of Condensed Matter Structure Defects and Mechanical Properties, 80,105-117. 

Theoretical methods have been used since 2002 to investigate the electronic, structural and mechanical properties of imogolites aiming to contribute to the understanding of these systems at atomic level. Most of the theoretical investigations at that time were based on force fields specially developed for aluminosilicate or aluminogermanate systems . The large size of NT unit cell is still a limitation for the use of more sophisticated quantum mechanical methods. 

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