29Si and 1H NMR spectroscopy of high-pressure hydrous magnesium silicates

We present NMR spectroscopic data, obtained by 1H MAS, 1H static spin-echo, and 29Si{1H} CP-MAS techniques, for a series of hydrous magnesium silicate samples synthesized at high pressure. This series includes chondrodite, β-Mg2SiO4, and phases A, B, superhydrous B, and E. Phases B and superhydrous B give very narrow 29Si NMR peaks and display the most de-shielded SiVI chemical shifts yet reported: −170.4 ppm for B and −166.6 for superhydrous B. The 1H NMR spectra of B and superhydrous B confirm the presence of paired hydroxyls, as determined from refinement of the H positions from X-ray diffraction data. The 1H MAS NMR spectra of phase B contain peaks for the two distinct hydrogen positions, with chemical shifts of +4.7 and +3.3 ppm. The static 1H spectrum contains a powder pattern characteristic of a strongly coupled hydrogen pair, from which a dipolar coupling constant of 18.6(4) kHz and inter-hydrogen distance of d(H–H)=1.86(2) Å were obtained. Superhydrous B appears to give two poorly resolved 1H MAS peaks, consistent with the presence of two distinct hydrogen pairs in the P21mn crystal structure. Analysis of its spin-echo spectrum gives d(H–H)=1.83(3) Å, slightly shorter than for phase B. β-Mg2SiO4, coexisting with phases B and superhydrous B, appears to give 29Si{1H} CP-MAS signal, indicating that it contains significant H concentration. The 29Si chemical shifts for phases B, superhydrous B, and chondrodite, together with those reported previously for other Mg-silicates, show a good correlation with structural parameters.