Structural talc-saponite

Table XXXVIII). Brindley (1955) has suggested that stevensite is a mixed-layer talc-saponite however, Faust et al. (1959) considered it to be a defect structure with a random distribution of vacant sites in the octahedral sheets. A small proportion of domains with few or no vacancies would then be present having characteristics of talc. The layer charge in stevensite is due to an incompletely filled octahedral sheet (Faust and Murata, 1953). This deficiency is minor (0.05—0.10) and the resulting cation exchange capacity is only about one-third that of the dioctahedral montmorillonites (100 mequiv./lOO g.).

This particle is naturally occurring and found around the world. It is easily mined and purified. The reactor for the particle was a volcano. The ash from many volcanoes was spread around the earth during an intense period of activity many millions of years ago. This ash was transformed into clay (montmorillonoids or smectites) by natural processes, into uncharged species (talc and pyrophyllite) and charged species through isomorphic substitution of the crystal structure (hectorite, montmorillon-ite, saponite, suconite, volchonskoite, vermiculite, and nontronite). 

Smectic type clays are characterized by a 2 1 structure depicted for the mineral montmorillonite in Figure 2.1. The structure has a central layer of octahedrally coordinated metal ions sandwiched between two layers of tetrahedrally coordinated silicons. This structural motif is shared by a series of minerals that includes talc, pyrophyllite, montmorillonite, non-tronite, hectorite, vermiculite, saponite, Beidellite, and the various forms of mica. The differences between these minerals are mainly in structural... 

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. 

At the other end of the smectite series are the high-magnesium members, hectorite and saponite. These clays possess a talc structure, with a trioctahedral magnesia l er sandwiched between the silica layers. Swellability results from minor substitution of aluminum for silicon in saponite or Hthimn for magnesium in hectorite. As with montmorillonite, the type of exchangeable cation determines the degree of swelling. Hectorite also has partial substitution of lattice hydroxyls by fluorine. 

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