Abstract: Single crystal X-ray and electron diffraction studies were carried out to examine the thermal transformation sequences of pyrophyllite and talc, respectively. The results showed that both minerals transformed topotactically. No dehydroxylated talc phase was observed, while an anhydride of pyrophyllite was obtained at about 900°C. The presence of a distorted anhydride lattice of pyrophyllite tends to indicate a homogeneous mechanism for the dehydroxylation of pyrophyllite. In contrast, the diffraction features of heated talc together with the cell volume relationship between talc and enstatite suggest an inhomogeneous mechanism for this reaction.