Silicate smectite

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

Chem. Descrip. Colloidal magnesium aluminum silicate (smectite)... 

Figure 14.5 Scheme showing the formation of porous silica-clay nanoarchitectures (D) in the following steps (i) Elomoionic charged layered silicates (smectites and vermiculites)... 

Chem. Descrip. Colloidal magnesium aluminum silicate (smectite) (94-97%), opal CT (3-6%)... 

Bentonite is a rock rich in montmorillonite that has usually resulted from the alteration of volcanic dust (ash) of the intermediate (latitic) siliceous types. In general, reUcts of partially unaltered feldspar, quartz, or volcanic glass shards offer evidence of the parent rock. Most adsorbent clays, bleaching clays, and many clay catalysts are smectites, although some are palygorskite [1337-76 ]. 

Hydrated calcium silicate minerals such as xonotlite, truscottite, and gyrolite are rare but have been reported from several Au-Ag deposits. They do not coexist with Au-Ag minerals but instead are found with quartz, carbonates, and johannsenite. However, in the Keisen No. 3-2 vein in the Hishikari Au-Ag deposits, a close association of electrum with truscottite, smectite and calcite is observed (Imai and Uto, 2001). 

Main gangue minerals of the Se-type deposits comprise quartz, adularia, illite/ smectite interstratified mixed layer clay mineral, chlorite/smectite interstratified mixed layer clay mineral, smectite, calcite, Mn-carbonates, manganoan caleite, rhodoehrosite, Mn-silicates (inesite, johannsenite) and Ca-silicates (xonotlite, truscottite). 

Principal gangue minerals in base-metal vein-type deposits are quartz, chlorite, Mn-carbonates, calcite, siderite and sericite (Shikazono, 1985b). Barite is sometimes found. K-feldspar, Mn-silicates, interstratified mixed layer clay minerals (chlorite/smectite, sericite/smectite) are absent. Vuggy, comb, cockade, banding and brecciated textures are commonly observed in these veins. 

In the Se-type gangue minerals comprise quartz, adularia, illite/smectite inter-stratified mixed layer clay mineral, smectite, calcite, Mn carbonates (manganoan calcite, rhodochrosite), Mn silicates (inesite, johansenite) and Ca silicates (xonotlite, truscottite). In comparison, the Te-type contains fine-grained, chalcedonic quartz, sericite, barite, adularia and chlorite/smectite interstratified mixed layer clay mineral. Carbonates and Mn minerals are very poor in the Te-type and they do not coexist with Te minerals. Carbonates are abundant and barite is absent in the Se-type. Grain size of quartz in the Te-type is very fine, while large quartz crystals are common in the Se-type. 

The effect of pH on both clay swelling and fines production has been widely discussed(89-95). Little consensus is found in this literature. Suggested treatments range from application of fluoboric acid(96) to 15% KOH(92) solutions — both treatments are believed to create a protective silicate film that inhibits release of fines. Polyacrylate polymers can provide protection against swelling of smectite clays and shales(97-100). 

Smectite is the first secondary mineral to form upon rock weathering in the semi-arid to sub-humid tropics. Smectite clay retains most of the ions, notably Ca2+ and Mg2+, released from weathering primary silicates. Iron, present as Fe2+ in primary minerals, is preserved in the smectite crystal lattice as Fe3+. The smectites become unstable as weathering proceeds and basic cations and silica are removed by leaching. Fe3+-compounds however remain in the soil, lending it a reddish color aluminum is retained in kaolinite and A1-oxides. Leached soil components accumulate at poorly drained, lower terrain positions where they precipitate and form new smectitic clays that remain stable as long as the pH is above neutral. Additional circumstances for the dominance of clays are ... 

Smectites are 2 1 charged layered silicates from natural (montmorillonite, hectorite, beidellite, saponite etc.) or synthetic (synthetic fluorohectorites, such as... 

Laird DA, Barriuso E, Dowdy RH, Koskinen WC (1992) Adsorption of atrazine on smectites. Soil Sci Soc Am J 56 62-67 LeBaron PC, Wang Z, Pinnavaia TJ (1999) Polymer-layered silicate nanocomposites an overview. Appl Clay Sci 15 11-29 Lee J-F, Crum JR, Boyd SA (1989) Enhanced retention of organic contaminants by soil exchanged with organic cations. Environ Sci Technol 23 1365-1372 Lee J-F, Mortland MM, Boyd SA, Chiou CT (1989a) Shape-selective adsorption of aromatic molecules from water by tetramethylammonium-smectite. J Chem Soc Faraday Trans I 8 2953-2962... 

The most significant class of inorganic supports, which is used for the direct ion exchange of positively charged transition-metal complexes, are smectite clays. Pin-navaia has introduced the use of these swelling, layered silicate clays for catalysis. Other clays include montmorillonite, bentonite, and laponite. As shown by Pinna-vaia, cationic transition-metal complexes can be readily exchanged (intercalated) into the solvated interlayers of these silicates (Eq. (1)) [117] ... 

Clays composed of layers are called layered silicates. The most common sheets are tetrahedral silicon and octahedral aluminum (see Figure 3.2, Figure 3.3, and Figure 3.4). Three common representative clays in soil are 1 1 kalo-inite, 2 1 fine-grained micas, and 2 1 smectites that is, kaolinites have one sheet of silicon tetrahedra and one sheet of aluminum octahedra. The finegrained mica and smectites have two sheets of silicon tetrahedra and one sheet... 

In terms of soil development and the development of soil horizons, the smectites and fine-grained micas are found in younger, less weathered soils. Kaolinite and amorphous clays are found in highly weathered soils. Considering a time sequence, at the beginning of formation, soil will contain more complex clays that weather to simpler forms over time. However, it is convenient to start with a description of the simpler layer silicate clays and then describe the more complex clays. 

Sorption depends on Sorption Sites. The sorption of alkaline and earth-alkaline cations on expandable three layer clays - smectites (montmorillonites) - can usually be interpreted as stoichiometric exchange of interlayer ions. Heavy metals however are sorbed by surface complex formation to the OH-functional groups of the outer surface (the so-called broken bonds). The non-swellable three-layer silicates, micas such as illite, can usually not exchange their interlayer ions but the outside of these minerals and the weathered crystal edges ("frayed edges") participate in ion exchange reactions. 

There are no unequivocal weathering reactions for the silicate minerals. Depending on the nature of parent rocks and hydraulic regimes, various secondary minerals like gibbsite, kaolinite, smectites, and illites are formed as reaction products. Some important dissolution processes of silicates are given, for example, by the following reactions ... 

In view of the problems associated with the expanding 2 1 clays, the smectites and vermiculites, it seemed desirable to use a different clay mineral system, one in which the interactions of surface adsorbed water are more easily studied. An obvious candidate is the hydrated form of halloysite, but studies of this mineral have shown that halloysites also suffer from an equally intractable set of difficulties (JO.). These are principally the poor crystallinity, the necessity to maintain the clay in liquid water in order to prevent loss of the surface adsorbed (intercalated) water, and the highly variable morphology of the crystallites. It seemed to us preferable to start with a chemically pure, well-crystallized, and well-known clay mineral (kaolinite) and to increase the normally small surface area by inserting water molecules between the layers through chemical treatment. Thus, the water would be in contact with both surfaces of every clay layer in the crystallites resulting in an effective surface area for water adsorption of approximately 1000 tor g. The synthetic kaolinite hydrates that resulted from this work are nearly ideal materials for studies of water adsorbed on silicate surfaces. 

Our model for the adsorption of water on silicates was developed for a system with few if any interlayer cations. However, it strongly resembles the model proposed by Mamy (12.) for smectites with monovalent interlayer cations. The presence of divalent interlayer cations, as shown by studies of smectites and vermiculites, should result in a strong structuring of their primary hydration sphere and probably the next nearest neighbor water molecules as well. If the concentration of the divalent cations is low, then the water in interlayer space between the divalent cations will correspond to the present model. On the other hand, if the concentration of divalent cations approaches the number of ditrigonal sites, this model will not be applicable. Such a situation would only be found in concentrated electrolyte solutions. 

For layer silicates with low structural charge (i. . smectites), this expansion is limited to about four molecular layers of water if the exchangeable cation has a charge of +2. Since the silicate platelet is about 0.96 nm thick, the repeat spacing along the c-axis is then approximately 0.96 + (4x.26) = 2.0 nm. 

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