Compensating interlayer

In the case of subsurface cation exchange, charge compensation cations are held in the solid phase within crystals in interlayer positions, structural holes, or surface... 

Early work by Boyd et al. (1947), performed on zeohtes, showed that the ion exchange process is diffusion controlled and the reaction rate is limited by mass transfer phenomena that are either film-diffusion (ED) or particle-diffusion (PD) dependent. Under natural conditions, the charge compensation cations are held on a representative subsurface solid phase as follows within crystals in interlayer... 

Montmorillonite, one of the most commonly encountered smectites, is similar to pyrophyllite (2 1) but has some interlayer cations and extra water. In pyrophyllite the layers are neutral because Si " in the tetrahedral sheet is not replaced by Al. In the smectites there is substitution of Al for Si " in the tetrahedral sheets, and occasionally Al appears in octahedral locations as well (for the names assigned to the end members, see Brindley and Brown, 1980, pp. 169-170.) The charge imbalances of the substitutions are compensated by interlayer cations, usually Na or Ca. These cations are easily exchangeable. The hydration level of the smectites is also variable. These minerals are very responsive to changes in water content as well as to the salt contents of the water. Other liquids that might be associated with the minerals and temperature can also effect changes in the chemical and crystal structure. 

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