Synthetic gyrolite and Z-phase were subjected to treatment with different amount of 0.1N-HCl at 25•Ž. The removal of interlayer calcium ions and shrinkage of basal spacing occur at the early stage of acid treatment, the basal spacing decreasing to 2.06 nm for synthetic gyrolite and 1.34 nm for Z-phase. For acid-treated products subjected to soak in water at 60•Ž for 20 h, recovery of basal spacing took place and it may be considered that calcium ions move from octahedral sheets, O and O into

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... erlayer space through the network of tetrahedral sheets, S2 and S2 by the aids of water. Z-phase is slightly superior to synthetic gyrolite as for the stability against acid.