Solid-State 27Al and 29Si NMR and H CRAMPS Studies of the Thermal Transformations of the 2:1 Phyllosilicate Pyrophyllite

Abstract

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 °C are reported. 27Al and 29Si NMR and 1H CRAMPS techniques have been used to follow the progress of dehydroxylation between 150 and 550 °C. At 550 °C, pyrophyllite is completely dehydroxylated in 7 days to pyrophyllite dehydroxylate, an aluminosilicate intermediate containing 5-coordinate aluminum, on the basis of the MAS 27Al NMR measurements at 14 T. MAS 27Al and CP/MAS (cross-polarization) and SP/MAS (single-pulse) 29Si NMR results indicate that the dehydroxylate is formed prior to the separation of the silica−alumina layer. At 950 °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 °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 °C. At 1250−1350 °C, the 29Si NMR shows that this glassy silica is converted to cristobalite, while the 27Al 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.