Smectite group mineral

SMECTITE GROUP MINERALS. The structuTe of Water adsorbed by smectite group minerals has been studied extensively in both its static (D structure) and dynamic aspects. As with water molecules on vermiculite, the behavior of water on smectite surfaces is conditioned sensitively on the type of exchangeable cation and on the location of isomorphic cation substitutions in the layer structure. In many respects, a discussion of the configuration of water molecules hydrating smectites is parallel to that for vermiculite. 

Smectite group minerals, when present in the petroleum reservoirs, play an important role in the migration of hydrocarbons. At the shallow level of reservoir rocks smectite can exist, but at deeper level the increase of temperatiue transforms it to other minerals. Generally dioctahedral smectites are transformed to illite and trioctahedral smectites are transformed to chlorite, releasing the interlayer water molecules in both cases. That released water increases the pore fluid pressure that may lead to migration of the hydrocarbons. 

Bentonite, which is largely composed of the smectite-group mineral montmorillonite, has the following fimctions in the drilling fluid. 

Smectites are stmcturaUy similar to pyrophylUte [12269-78-2] or talc [14807-96-6], but differ by substitutions mainly in the octahedral layers. Some substitution may occur for Si in the tetrahedral layer, and by F for OH in the stmcture. Deficit charges in smectite are compensated by cations (usually Na, Ca, K) sorbed between the three-layer (two tetrahedral and one octahedral, hence 2 1) clay mineral sandwiches. These are held relatively loosely, although stoichiometricaUy, and give rise to the significant cation exchange properties of the smectite. Representative analyses of smectite minerals are given in Table 3. The deterrnination of a complete set of optical constants of the smectite group is usually not possible because the individual crystals are too small. Representative optical measurements may, however, be found in the Uterature (42,107). 

Products of this type seem to protect the humus from rapid incorporation into new biological processes. Additional factors that appear to be associated with the accumulation of organic matter in Mollisols are high exchange capacities, saturation with calcium, an abundance of mineral colloids and a high content of minerals of the smectite group (Fenton, 1983). 

Figure 4.7 Structure of one of the smectite group of clay minerals montmorillonite, Al2Si40io(OH)2.nH20. (After Evans, 1966 Figure 11.08b, by permission of Cambridge University Press.)...

The smectite group of clay minerals is also poorly crystalline but perhaps better known because of their cation exchange capacity and their occurrence in the bentonite clays. A general formula for montmorillonite, which is one of the dioctahedral smectites is... 

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. 

The amount of substitution in the tetrahedral and octahedral sheets and the ratio of octahedral to tetrahedral sheets are the primary differentiating characteristics between the many clay minerals (Fig. 3.6). For example, clays that have one tetrahedral sheet and one octahedral sheet are known as 1 1 clay minerals (e.g., kaolin group) (Fig. 3.7) clays that have two tetrahedral sheets and one octahedral sheet are known as 2 1 clay minerals (e.g., smectite group) (Fig. 3.8) or mica and vermiculite (Fig. 3.9), while clays that have two tetrahedral sheets and two octahedral sheets are known as 2 2 clay minerals (e.g., chlorite) (Fig. 3.10). These sheet arrangements give rise to various mineral surface identities such as magnitude (specific surface), functional groups, and interactions with solution species. 

The montmorillonite (Chapter 1, Table 1.2) clay mineral can be used as a model substance in the study of the interfacial processes of rocks and soils. It is a layer silicate, a member of the smectite group. Its structure is appropriate for modeling the most important interfacial processes in geological formations. Besides, it is a fairly widespread mineral in rocks and soils, and plays an important role in the nutrient cycle of soils. In addition, it has many agricultural, industrial, and environmental applications. 

The three-layer phyllosilicates include talc and pyrophyllite, illite and the smectite group clays, various mixed-layer clays, vermiculite, and the micas (e.g., muscovite, phlogopite, and biotite). We will limit ourselves to a discussion of the more environmentally important of these minerals, which include the micas, the smectites and illites, interlayered (mixed-layer) smectite-illites and vermiculite. 

The different classes of clay minerals, namely, 1/1,2/1, and so on, have a differenf arrangemenf of tetrahedral and octahedral layers. Structural units of clays, fherefore, consist of either (1) alternating tetrahedral sheets (OT or 1/1 structure e.g., the kaolinite group) (2) a sandwich of one ocfahedral sheet between two tetrahedral sheets (TOT or 2/1 structure e.g., smectite clay minerals, of which fhe mosf common member is monf moril-lonite) or (3) an arrangement in which three TOT units alternate with a brucite layer (2/1/1 structure e.g., chlorite). 

The definition of adsorbed water adopted in Sec. 2.3 requires an arrangement of water molecules that differs significantly from that in an appropriate reference aqueous phase. For water on the surfaces of kaolinite group minerals the reference phase is bulk liquid water, whereas for water on vermiculite and smectite surfaces the reference phase is an aqueous solution because of the presence of exchangeable cations on the 2 1 layer silicates. On the ba,si.s of this definition, the consensus developed in Sec. 2.3 is that the spatial extent of adsorbed water on a phyllosilicate... 

How the clay minerals of smectite group play an important role in the migration of hydrocarbons in petroleum reservoirs ... 

The basal spacing of illite is 10 A. The K, Ca or Mg interlayer cations prevent the entrance of H2O into the structure. For this reason the basal spacing in illite is constant. Thus, the illite group minerals cannot expand by absorption of water like halloysite, vermiculite and smectite. 

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