Cation exchange in the interlayers of expansible layer silicates

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Abstract
The relation between counter ion distribution, ion mobility and degree of contraction has been studied on hydrobiotite (2–5 µ size), equilibrated with mixed RbCl-SrCl2 solutions. From X-ray data, isotopic exchange and chemical extraction, it appears that the counter ions of the contracted portions of the interlayers are not in equilibrium with the solution phase. The distribution of counter ions in the interlayer space follows a gradient from the margin of the mineral to the centre. This distribution is the result of the interruption of a diffusion process, caused by the contraction of layers during exchange. Isotopic exchange is rapid on the external surfaces and in the expanded portions of the interlayers, but very slow in the contracted part, where the rate of exchange amounts to only 0.002–0.02 m-eq/100 g/day, and decreases with time. On exchange with MgCl2 solutions, secondary expansion of the interlayer space occurs and some of the counter ions trapped in the contracted regions are released.

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