Solid-State^{27Al and29Si NMR and1H CRAMPSStudies of the Thermal Transformations of the 2:1Phyllosilicate Pyrophyllite}

Solid-state multinuclear magic-angle spinning (MAS) NMR studies of the thermal transformations of the 2:1 phyllosilicate mineral, pyrophyllite, over the temperature range 150-1350 degrees C are reported. Al-27 and Si-29 NMR and H-1 CRAMPS techniques have been used to follow the progress of dehydroxylation between 150 and 550 degrees C. At 550 degrees C, pyrophyllite is completely dehydroxylated in 7 days to pyrophyllite dehydroxylate, an aluminosilicate intermediate containing 5-coordinate aluminum, on the basis of the MAS Al-27 NMR measurements at 14 T. MAS Al-27 and CP/MAS (cross-polarization) and SP/MAS (single-pulse) Si-29 NMR results indicate that the dehydroxylate is formed prior to the separation of the silica-alumina layer. At 950 degrees C, the thermally induced transformation of pyrophyllite anhydride results in separation of the silica-alumina layer. A transition-alumina-type phase, containing 4- and 6-coordinate aluminum, is formed between 950 and 1050 degrees C, In addition, a high content of amorphous silica glass and a small amount of a poorly ordered Si/Al-containing mullite phase forms between 950 and 1050 degrees C, At 1250-1350 degrees C, the Si-29 NMR shows that this glassy silica is converted to cristobalite, while the Al-27 NMR indicates that this process is accompanied by conversion of octahedral aluminums to tetrahedral aluminums, possibly by incorporation of aluminums into an amorphous Si/Al-containing phase. The mechanism of dehydroxylation and of the higher temperature transformations of pyrophyllite dehydroxylate are discussed in light of these multinuclear solid-state MAS NMR results.