Vibrational spectra of dense, hydrous magnesium silicates at high pressure: Importance of the hydrogen bond angle

Changes in configuration and hydrogen bonding are inferred from the complex responses of peak parameters (frequency, width, height, and anal to compre ssion for various hydroxyl groups in dense, hydrous magnesium silicates (DH MS). Stretching Frequencies (nu(i),) depend not only on both O-H...O bond l ength but also bond angle, which suggests a need to re-examine frequency-st ructure relationships at 1 atm. For phase B [(Mg12SiSi3O19)-Si-VI-Si-IV(OH) (2)], nu of the most intense OH peak determined from IR spectra at nearly h ydrostatic pressures (P) decreases to a minimum at 5 GPa, and then rises to a broad maximum near 35 GPa, whereas nu of the other intense peak decrease s to a broad minimum near 30 GPa. This behavior shows that compression of p hase B initially moves O19 toward H2 on O21 and rotates H1 about O20 away f rom O19 and toward O21 inside the triangle of O atoms, thus increasing the H1-O20-O19 bond angle (from 12 to 21 degrees at 37 GPa). H-H repulsion rota tes H2 outside of the triangle by 11 degrees at 37 Cpa. Changes in O-O bond length and H-O-O angle calculated from the trends of OH frequency with inc reasing pressure are consistent with the relatively incompressible layer of SiO4 tetrahedra. Widths, areas. and heights of the hydroxyl peaks also inc rease with P (whereas areas of the lattice modes are constant), with H1 bei ng much more affected: This response suggests that the H1 bond bends furthe r with compression. Raman spectra of stoichiometric OH- modes in superhydro us phase B [(Mg10SiSi2O14)-Si-VI-Si-IV (OH)(4),] roughly parallel these tre nds, consistent with similarly paired hydroxyls with bent bonds. For phase A [(Mg7Si2O8OH)-Si-IV)(6)], the Raman OH- bands depend linearly on P, and w ith constant peak height with slopes similar to those observed for weak str etching modes of trace H in phase B. This behavior is common and is associa ted with linear hydrogen bonds and unpaired OH groups. Combining our data w ith previous results reveals systematic, linear relationships of d nu/dP \( 0) with nu that suggest high-polyhedral bulk moduli for DHMS. Most frequencies of 31 mid-IR lattice modes for phase B depend linearly on curvature is seen for several modes. Raman frequencies of lattice modes fro m superhydrous phase B (shyB) and phase A depend linearly on P.