Montmorillonite quasicrystal

Unit layers of smectites, notably montmorillonite, can associate in stacked, roughly parallel alignment to form a quasicrystal [52]. This particle structure is stabilized by attractive interactions between the basal planes of unit layers, as mediated by adsorbed cations and water molecules. The prototypical example of a montmorillonite quasicrystal is that comprising stacks of four to seven unit layers, with Ca2+ adsorbed in outer-sphere complexes on the siloxane surfaces to serve as molecular cross-links binding the unit layers together through electrostatic forces. This kind of quasicrystal appears to form with any bivalent cation and for any smectite [21,54]. 

The evidence for quasicrystal formation in suspensions of mono-valent-ion-saturated montmorillonites [23,58] indicates that wav for Na-montmorillonite should lie between 1.0 and 2.0. The viscosity data in Fig. 11 in fact lead to wav = 1.3 if the ratio of hD-values [from Eq. (30)] for Na-montmorillonite and Li-montmorillonite is equated to their ratio of av values, with av = 1.0 for Li-montmorillonite. Similarly, if the ratio of mp-values taken from Eq. (31) is equated to a ratio of av values (i.e., mp = wo wav, where wo is the mass of a unit-layer particle), then av = 1.4 0.3 on the basis of the data in Fig. 11, along with other published data on light transmission by dilute Li- and Na-montmorillonite suspensions [23]. 

QUASICRYSTAL FORMATION. The mixing together of Na- and Ca-montmorillonite suspensions to produce an overall charge fraction of Na" on the clay particles below 0.1 results in a very rapid (less than 1 min) formation of quasicrystals from conversion of the Na-montmorillonite particles.This rapid conversion is necessarily mediated by a redistribu- tion of the exchangeable cations such that Na ions are relocated, as required, to the external surfaces of already-formed quasicrystals that contain Ca ions on their internal surfaces. The relocation probably involves replacement by Na of Ca " already on external surfaces since the latter ions are likely to have a higher mobility than Ca " adsorbed inside a quasicrystal. 

These characteristics of quasicrystal formation and breakdown are consistent with the trends in Fig. 6.2. When Enh is less than 0.1, for example, the properties of a mixed Na/Ca-montmorillonite do not differ much from those of Ca-montmorillonite and the quasicrystal should remain a stable entity. When Ens is larger than 0.6, however, the mixed Na/Ca-montmorillonite exhibits properties that are indistinguishable from those of a Na-montmorillonite and a quasicrystal should be an inherently unstable structural unit. 

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