Smectite-vermiculite

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). 

Giese et al. [48] have compiled a list of y+ and y values for a variety of clay minerals. Smectites, vermiculites, and micas tend to be very hydrophilic, with y > 28mJ/m2 and with a sizable electron acceptor parameter as well (y+ > 1 mJ/m2). Illites, on the other hand, were reported to be moderately hydrophobic (y varied from 14 to 19mJ/m2) and tended to have y+ — 0. 

An important feature of the smectites, vermiculites and other 2 1 layer silicates is that isomorphous substitutions can occur in both the tetrahedral and octahedral sheets. Thus, substitution of Si by A1 occurs in the tetrahedral sheet, together with replacement of A1 by Mg, Fe, Li or other small atoms in the octahedral sheet. The substitutions lead to a deficit of positive charge, which is compensated by the presence of exchangeable, interlayer cations. 

The amount of plagioclase (Na and Ca) and smectite-vermiculite clay minerals decreases downstream, to the benefit of quartz and kaolinite. 

The most widespread of the secondary minerals formed during the development of a laterite weathering profile are iron and aluminium ses-quioxides (Table 3.1). These may form either directly from the alteration of primary minerals, or else via a series of pathways involving the formation of intermediary sheet silicate minerals and clays (e.g. chlorite, illite, smectite, vermiculite and halloysite), which are then themselves broken down, stripped of their mobile ions and silica, and eventually converted to alumina and ferric oxyhydroxide residua (Figure 3.9). It is not possible to describe these mineral transformations in detail, but the key issue is that under tropical-type weathering conditions these transformation pathways lead to... 

In a survey of U.S. stream sediments, Kennedy (1965) concluded that the makeup and properties of the stream sediments essentially equaled that of local soils. In the eastern states (50 to 150 cm precipitation), dominant clays in the <4 jum (0.004 mm) fraction were illite, kaolinite, ver-miculite, and interlayered clays, with a CEC of 14 to 28 meq/lOOg. In central and west-central states (25 to 100 cm precipitation) Kennedy found dominant smectite, vermiculite, mixed-layer illite, kaolinite, quartz, and feldspar in the <4 /zm fraction, with a CEC range of 25 to 65 meq/100 g. In California and Oregon, because of the wide range of wet and dry conditions (<25 to >200 cm precipitation), clays were highly variable, and had a range of CEC s from 18 to 65 meq/l(X) g for the <4 /um fraction. 

Clays (for instance, smectite, vermiculite, kaolinite) Ditrigonal caverns of siloxan (=X ) Changeable metals (=XMe+) Hydroxides of facet surfaces (=XOH)... 

A nickel laterite from Piaui, Brazil was used in this study. The top size of the ore was limited to 12.5 mm. The nickel laterite was composed mainly of quartz, silico-femiginous plasma, and clay minerals. The nickel content is 0.9%. The main nickel-bearing minerals are chlorite, smectite, vermiculite, manganese oxide and silico-ferruginous plasma [6]. Since moisture content, acid concentration, retention time and drum rotation speed are operating factors in drum agglomeration, they were investigated in this study. 

Clay minerals of the smectite-vermiculite group are formed within the rock cycle by several mechanisms including ... 

The trace elements listed above may be coprecipitated with secondary minerals (see Section 17.2) that are involved in soil formation. Such secondary minerals include oxides of aluminum, iron, and manganese (precipitation of hydrated oxides of iron and manganese very efficiently removes many trace metal ions from solution) calcium and magnesium carbonates smectites vermiculites and illites. 

The foregoing discussion indicated that several phyllosihcate minerals, either naturally or as the result of chemical treatment, have molecular species inserted between the siUcate layers. Water is the most common interlayer sp>ecies in nature, and water is normally found in smectites, vermiculites and hydrated halloysites. The quantity of interlayer water is a function of relative humidity and the type of interlayer cation, in the case of smectites and vermiculites. 

In spite of the fact that both hydrophilic and hydrophobic clay and other minerals particles surfaces consist of metal oxides, with the oxygen atoms in all cases prominently exposed at the surface of the mineral, some are very hydrophobic. As shown above (Table 8.3), hydiophilicity is mainly determined by a high value of the electron donor parameter (7 ) of the polar component (7 ) of the surface teninon. This strong dectixm-donicity (Lewis basicity) is mainly due to the relatively loosely bound oxygen atoms at the surface of hydrophilic clay particles, such as smectites, vermiculites, micas or other metal oxide surfaces, such as silica (see Chapter 9 for the 7 -values of various clay and other mineral particles). 

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