Influence of Grinding in Pyrophyllite−Mullite Thermal Transformation Assessed by 29Si and 27Al MAS NMR Spectroscopies

The influence of grinding on the thermal transformation of pyrophyllite to mullite is analyzed by 27 Al and 29 Si MAS NMR spectroscopies, using X-ray powder diffraction and thermal analysis (DTA-TG) as complementary techniques. Mechanical treatments by dry grinding produce partial destruction of pyrophyllite and rearrangement of Al ions in tetra- and pentahedral environments. Tetrahedral sheet breakdown favors Al atomic diffusion and formation of Si-poor nuclei of mullite at 1000 °C. In particular, the exothermic peak detected at 1000 °C in DTA curves is ascribed to transformation of pentahedral coordination of Al in ground samples into tetra- and octahedral aluminum of mullite. During thermal decomposition of pyrophyllite, segregation of amorphous silica is produced. These results are compared with those obtained in kaolinite−mullite transformation, which were previously analyzed.