Thermal Reactions of Pyrophyllite Studied by High‐Resolution Solid‐state 27 Al and 29 Si Nuclear Magnetic Resonance Spectroscopy

Water is lost in two overlapping steps from well‐crystallized pyrophyllite from Coromandel, New Zealand. The pyrophyllite structure survives the loss of the first 30% of the total water content, but the loss of a further 60% water in the second step results in the formation of pyrophyllite dehydroxylate, with corresponding changes in both the 29 Si and 27 Al solid‐state NMR spectra. Detailed analysis of the 29 Si chemical shift of the dehydroxylate has allowed the silicate layer structure of this phase to be refined. A similarly detailed interpretation of the 27 A1 spectra is not possible because of electric field gradient effects which result in the loss of ∼90% of the A1 spectral intensity due to the formation of five‐coordinate A1 on dehydroxylation. The loss of further water from the dehydroxylate on further heating results in the formation of mullite and cristobalite and is accompanied by changes in the 29 Si and 27 Al spectra which can be accounted for in terms of coordination changes in the structural regions which contained the residual hydroxyls.