SI-29 AND H-1-NMR SPECTROSCOPY OF HIGH-PRESSURE HYDROUS MAGNESIUM SILICATES

We present NMR spectroscopic data, obtained by H-1 MAS, H-1 static spi n-echo, and Si-29 {H-1} CP-MAS techniques, for a series of hydrous mag nesium silicate samples synthesized at high pressure. This series incl udes chondrodite, beta-Mg2SiO4, and phases A, B, superhydrous B, and E . Phases B and superhydrous B give very narrow Si-29 NMR peaks and dis play the most deshielded Si-VI chemical shifts yet reported: -170.4 pp m for B and -166.6 for superhydrous B. The H-1 NMR spectra of B and su perhydrous B confirm the presence of paired hydroxyls, as determined f rom refinement of the H positions from X-ray diffraction data. The H-1 MAS NMR spectra of phase B contain peaks for the two distinct hydroge n positions, with chemical shifts of +4.7 and +3.3 ppm. The static H-1 spectrum contains a powder pattern characteristic of a strongly coupl ed hydrogen pair, from which a dipolar coupling constant of 18.6(4) kH z and inter-hydrogen distance of d(H-H)=1.86(2) Angstrom were obtained . Superhydrous B appears to give two poorly resolved H-1 MAS peaks, co nsistent with the presence of two distinct hydrogen pairs in the P2(1) mit crystal structure. Analysis of its spin-echo spectrum gives d (H - H)= 1.83 (3) Angstrom, slightly shorter than for phase B. beta-Mg2Si O4, coexisting with phases B and superhydrous B, appears to give Si-29 {H-1} CP-MAS signal, indicating that it contains significant H concen tration. The Si-29 chemical shifts for phases B, superhydrous B, and c hondrodite, together with those reported previously for other Mg-silic ates, show a good correlation with structural parameters.