SOLID-STATE AL-27 AND SI-29 NMR AND H-1 CRAMPS STUDIES OF THE THERMALTRANSFORMATIONS OF THE 2 1-PHYLLOSILICATE PYROPHYLLITE/

Solid-state multinuclear magic-angle spinning (MAS) NMR studies of the thermal transformations of the 2:1 phyllosilicate mineral, pyrophylli te, 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 p rogress of dehydroxylation between 150 and 550 degrees C. At 550 degre es C, pyrophyllite is completely dehydroxylated in 7 days to pyrophyll ite dehydroxylate, an aluminosilicate intermediate containing 5-coordi nate 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) S i-29 NMR results indicate that the dehydroxylate is formed prior to th e separation of the silica-alumina layer. At 950 degrees C, the therma lly induced transformation of pyrophyllite anhydride results in separa tion of the silica-alumina layer. A transition-alumina-type phase, con taining 4- and 6-coordinate aluminum, is formed between 950 and 1050 d egrees 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 incorporati on of aluminums into an amorphous Si/Al-containing phase. The mechanis m of dehydroxylation and of the higher temperature transformations of pyrophyllite dehydroxylate are discussed in light of these multinuclea r solid-state MAS NMR results.