Smectite surface coating

Chlorite replaces kaolinite, clay pseudomatrix, infiltrated clays, micas and heavy minerals. It occurs as rims composed of platelets oriented perpendicularly to grain surfaces. The rims were formed by replacing infiltrated smectitic clay coatings, which were originally oriented tangentially to grain surfaces (Fig. 14A) (see Moraes De Ros, 1990). These infiltrated clays were presumably introduced into the vadose zone of alluvial continental sediments under semi-arid conditions by episodic floods (Walker et al., 1978 Moraes De Ros,... 

Surfactant-coated interlayer sites in smectite have similar properties to detergents. A hydrophobic molecule or compound such as dioxin or other chlorinated phenol is attracted to the hydrophobic hydrocarbon tail projecting from the interlayer surface and immobilized. 

Short-range-ordered or noncrystalline Al precipitation products are ubiquitous in soil environments (Parfitt, 1980 Kampf et al., 2000). They dominate the chemical reactions in soils because of their extremely small particle sizes and highly reactive surfaces. In pure form they are not stable, but in the presence of chelating anions they may remain unchanged indefinitely. Short-range-ordered Al precipitates and soluble OH-Al species often coat crystalline minerals in soils or may be interlayered into the interlamellar spaces of vermiculites or smectites, altering the surface properties of these phyllosificates. OH-Al interlayered vermiculites and smectites are particularly abundant in Ultisols and Alfisols (Bamhisel and Bertsch, 1989). 

Figure 1.11. Some effects of pedochemical weathering on phyllosilicate surfaces. Weathering produces interlayer hydroxy-polymers, interstratification, and external-surface organic and inorganic polymer coatings on smectite. On kaolinite, organic and iron oxide coatings are produced by weathering. After Jenne ...

Surface layers may be calcium carbonate, hydroxide or, in freshly broken crystals, possibly calcium oxide. It is now well established that coccoliths in chalk are coated with very thin aluminosilicates (smectites and other clay minerals) and organic matter (humates, etc) [6]. During comminution, compaction or metamorphosis, these coatings may be disrupted or may still adhere to the calcite surface. Aluminium silicates have been identified in pure limestones and pure marbles, and have been shown to be unattached to calcite fragments. 

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