29‐Si and 27‐Al Magic‐Angle Spinning Nuclear Magnetic Resonance Study of the Thermal Transformations of Pyrophyllite

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Abstract
The thermal transformation of pyrophyllite was analyzed using 27‐Al and 29‐Si MAS‐NMR spectroscopies, thermal analysis (DTA‐TG), dilatometry, and X‐ray diffraction techniques. Above 800°C, dehydroxylation of pyrophyllite is the origin of a broad component in the 27‐Al NMR spectra which is associated with a distorted pentahedral coordination of Al. At 1000°C, tetrahedral sheet breakdown takes place and a partial segregation of amorphous SiO2 is detected by NMR spectroscopy. This process permits rearrangements of aluminum ions and favors the formation of small disordered nuclei of mullite. The 27‐Al NMR signals observed at 59.5, 40.7, and 1 ppm have been assigned to 4‐ and 6‐coordinated Al in mullite, and the 29‐Si NMR signals observed at −110 and −88 ppm are assigned to well‐crystallized mullite and cristobalite, respectively, formed during thermal decomposition of pyrophyllite.