Abstract
Unpublished work carried out in this laboratory several years ago had shown that the first-order basal X-ray reflections of illite from South Wales occurred at a spacing of about 10Å both with potassium- and calcium-saturated samples. Recently Jackson and others observed that hydrated mica samples having a 10Å basal spacing when saturated with potassium, showed a 12 to 13Å. spacing after saturation with calcium. They interpreted their results by ascribing to illite a limited capacity for reversible lattice shrinkage and expansion, identical in type with that known for montmorillonite.In order to study the problem further three illite and two glauconite samples were treated with concentrated calcium chloride solutions for five weeks under frequent renewal of the solution.

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