Abstract
It is demonstrated that dioctahedral layer silicates, upon heating, transform into trioctahedral modifications. The mechanism of the transformation must be a redistribution of the sixcoordinated trivalent ions over all the octahedral positions; every site will be occupied by two-thirds Al-atom. In this way pyrophyllite, e. g., will aquire a lalc-like structure. Besides pyrophyllite, the studied minerals are hydrous mica, montmorillonite, and paragonite.

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