Dehydroxylation pyrophyllite

Under mild conditions (cooling in steam at 1 atm pressure), dehydroxylated pyrophyllite recovers some of its hydroxyl groups, but the resultant material differs from the original (Heller et ah [1962]). There is no evidence for a similar occurrence with dehydroxylated talc. 

It is clear from the curves for both the pyrophyllite-talc group and the montmorillonite-saponite group that dehydroxylation occurs at a lower temperature for the dioctahedral (aluminian) minerals (cf the data of Caillere and H nin [1963, pp. 129-131]) and that these also show greater variability in thermal characteristics than the trioctahedral (magnesian) minerals. 

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