Bl. Phillips et al., SI-29 AND H-1-NMR SPECTROSCOPY OF HIGH-PRESSURE HYDROUS MAGNESIUM SILICATES, Physics and chemistry of minerals, 24(3), 1997, pp. 179-190
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.