Hydrogen bonding interactions in phase A [Mg7Si2O8(OH)6] at ambient and high pressure


H. Kagi

Laboratory for Earthquake Chemistry, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan

J. B. Parise

Center for High Pressure Research, (an NSF-funded Science and Technology Center) Department of Geosciences
State University of New York at Stony Brook, Stony Brook, New York 11794-2100, USA

H. Cho

Environmental Molecular Sciences Laboratory, MS K8-98
Pacific Northwest National Laboratory, Richland, Washington 99352, USA

G. R. Rossman

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA

J. S. Loveday

Department of Physics and Astronomy, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, UK


Physics and Chemistry of Minerals 27, 225-233


Neutron powder diffraction data of phase A (Mg7Si2O8(OH)6) were collected at ambient pressure and 3.2 GPa (calculated from the compressibility of phase A) from the deuterated compound, and the structure was refined using the Rietveld method. The derived crystal structure implies that hydrogen atoms occupy two distinct sites in phase A, both forming hydrogen bonds of different lengths with the same oxygen atom. This picture is supported by IR spectra, which exhibit two absorption bands at 3400 and 3513 cm-1 corresponding to OH stretching vibrations, and proton NMR spectra, which display two peaks with equal intensities and isotropic chemical shifts of 3.7 and 5 ppm. The D-D distance [D(1)-D(2) distance] at ambient pressure was found to be 2.09 0.02 from the neutron diffraction data and 2.09 0.05 from the NMR spectra. At 3.2 GPa, there is no statistically significant increase in the O-D interatomic distance while the hydrogen bonding interaction DO appears to increase for one of the hydrogen sites, D(1), which has the stronger hydrogen bonding interaction compared with the other hydrogen, D(2), at ambient pressure. The O-D bond valences, determined indirectly from the DO distances were 0.86 and 0.91 at ambient pressure, and 0.83 and 0.90 at 3.2 GPa, for D(1) and D(2), respectively.