Magnesium smectite

Greathouse, J.A., K. Refson, and G. Sposito. 2000. Molecular dynamics simulation of water mobility in magnesium-smectite hydrates. J. Am. Chem. Soc. 122 11459-11464. 

Saponite, a high magnesium smectite, is similar in strueture to talc but with limited substitution of tetrahedral Sf by while heetorite has the talc structure but with limited substitution of Li for octahedral Mg and F for OH". As with montmorillonite, the resulting charge imbalanee is eompensated by Na or Ca residing with oriented water in the interlaminar spaces. Saponite and heetorite have swelling, ion exchange, and absorbent properties similar to those of montmorillonite. 

Magnesium silicate smectite clay, 22 475 Magnesium silicide, 75 364 Magnesium statistics, U.S. historical, 75 324 Magnesium stearate... 

The Smectite Clays. The smectite-type clays are distinctive in that they expand and cause significant destruction to synthetic (human-made) structures. In this type of 2 1 clay, isomorphous substitution occurs in the aluminum sheet. If there is substitution of lower-oxidation-state metal such as magnesium, there will be an unsatisfied pair of bonding electrons in the interior of the crystal and there will be no noticeable change in the surface. Because the charge is in the interior of the crystal, its attraction for cations is diminished by distance. Thus, smectite crystals are not held together strongly by cations and are able to incorporate more water and ions between sheets when the environment is wet and less when it is dry. 

The layer silicates comprise tetrahedral sheets of silica and octahedral sheets of aluminium and magnesium hydroxide, with varying amounts of the Si, Al and Mg replaced by cations of lower valence giving the lattice a net negative charge. Two basic combinations occur 1 tetrahedral sheet with 1 octahedral (e.g. kaoUnite, halloysite), and 2 tetrahedral with 1 octahedral (e.g. smectite, vermiculite, illite). 

HARDER (H.), 1972. The role of magnesium in the formation of smectite minerals. Chem. Geol. H), 31-9. 

The three-sheet or 2 1 layer lattice silicates consist of two silica tetrahedral sheets between which is an octahedral sheet. These three sheets form a layer approximately 10 A thick. The oxygens at the tips of the tetrahedra point towards the center octahedral sheet and substitute for two-thirds of the octahedrally coordinated hydroxyls. The 2 1 clay minerals include the mica and smectite groups which are by far the most abundant of the clay minerals. The pure end members of this type are talc, a hydrous magnesium silicate pyrophyllite, a hydrous aluminum silicate and minnesotaite, a hydrous iron silicate. 

Clay mineral diagenesis also may play a role in dolomite formation during burial. The commonly observed conversion of smectites to illite can result in the release of the magnesium necessary for dolomite formation (e.g., McHargue and Price, 1982). Dolomite formation is observed near and within shale beds however, this process again appears to be a localized mechanism and probably is incapable of producing large quantities of dolomite. 

Figure 2. Structure of a typical chlorite-like hydroxy-interlayered clay in which the galleries of a 2 1 smectite structure are filled, or nearly so, with brucite-like sheets of mainly edge-shared Mg(OH) octahedra. Aluminum occasionally substitutes for magnesium in the brucite sheet to provide the charge balance necessary for electrical neutrality.

The transformation of smectite to mixed layer smectite-illite, and ultimately to illite, with increasing temperature is an extremely important reaction in many sedimentary basins, including the northern Gulf of Mexico Basin (Hower et al., 1976 Boles and Franks, 1979 Kharaka and Thordsen, 1992). The water and solutes released and consumed by this transformation are major factors in the hydrogeochemistry of these basins, because of the enormous quantities of clays involved. Several reactions conserving aluminum or maintaining a constant volume have been proposed for this transformation (Hower et al., 1976 Boles and Franks, 1979). The reaction proposed below (Equation (4)) conserves aluminum and magnesium, and is probably a closer approximation based on the composition of formation waters in these systems ... 

Palygorskite and sepiolite are magnesium-rich fibrous aluminosilicates that have been identified in basal deep-sea sediments (Table 4 e.g., Hathaway and Sachs, 1965 Bowles et al., 1971 Bonatti and Joensuu, 1968 Church and Velde, 1979 Jones and Galan, 1988 Velde, 1985). These phases are commonly associated with smectite, and it has been suggested that they originate by alteration of montmorillonite by low-temperature, magnesium-rich, hydrothermal solutions (Bonatti and Joensuu, 1968), e.g.. 

Alternative constructions of this reaction are conservative with respect to aluminum (Boles and Eranks, 1979b Land et ai, 1987). Such reactions require less input of potassium from minerals other than smectite, but result in a diminution of the total amount of clay by —25% and the generation of excess silicon, iron, and magnesium in amounts (near 15% of the total shale volume) that are not readily sequestered in known authigenic phases in either shales or sandstones (Awwiller, 1993). 

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