High-pressure single crystal X-ray diffraction and FT-IR observation of natural chondrodite and synthetic OH-chondrodite

High-pressure single crystal X-ray diffraction studies of a natural chondrodite, Mg4.76Fe0.22Ti0.02Si1.99O8 (OH1.26F0.74), and a synthetic OH-chondrodite, Mg4.98Si2.01H2.00O10, were performed using a diamond anvil cell (DAC) up to 7.3 and 5.9 GPa, respectively and at room temperature. FT-IR spectra of the natural chondrodite under high-pressure conditions up to 9.9 GPa was also observed using a DAC. The axial linear compressibilities of these samples are calculated as β a = 1.69(4) × 10, β b = 2.98(4) × 10 and β c = 2.74(5) × 10 (GPa) for the natural chondrodite and β a = 2.11(18) × 10, β b = 2.83(18) × 10 and β c = 3.04(38) × 10 (GPa) for the synthetic OH-chondrodite. The isothermal bulk moduli of these samples were calculated as KT = 124.1(4) GPa for the natural chondrodite and KT = 117(2) GPa for the OH-chondrodite, by using the Birch-Murnaghan equation of state assuming K′ = 4. The bulk moduli of total void space in each sample, assuming K′ = 4, were calculated to be K□ = 116(2) GPa for the natural chondrodite and K□ = 113(4) GPa for the OH-chondrodite. The plots of bulk modulus versus the summation of the filling-factor of polyhedral sites show a good correlation between the humite minerals. This relationship can be explained by the replacement of 4O + Si + ⇔ 4(F, OH) + □ generated in the humite homologous series. In the FT-IR spectra of the natural chondrodite, four OH-stretching vibrational peaks were observed at 3688, 3566, 3558 and 3383 cm under ambient conditions. The pressure dependences of the frequency of these peaks up to 9.9 GPa are 2.8(3), 3.9(3), 4.0(3) and −2.1(2) (cmGPa), respectively. With increasing pressure up to 9.9 GPa, the 3383 cm peak shifts to lowerfrequency positions, whereas the other peaks shift to higher-frequency positions. The shortening of the O5⋅ ⋅ ⋅ O5 distance, which is not the shared edge between M3 octahedra, related to hydrogen bonding with increasing pressure causes the negative pressure-dependence of the 3383 cm peak. The positive pressure-dependence of the remaining IR-peaks is due to the compression of the O5-H bond, and not related to hydrogen bonding with increasing pressure.