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

Abstract

High-pressure single crystal X-ray diffraction studies of a natural chondrodite, Mg 4.76Fe 0.22Ti 0.02Si 1.99O 8 (OH 1.26F 0.74), and a synthetic OH-chondrodite, Mg 4.98Si 2.01H 2.00O 10, 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 -3, β b = 2.98(4) × 10 -3 and β c = 2.74(5) × 10 -3(GPa-1) for the natural chondrodite and β a = 2.11(18) × 10 -3, β b = 2.83(18) × 10 -3 and β c = 3.04(38) × 10 -3 (GPa -1) for the synthetic OH-chondrodite. The isothermal bulk moduli of these samples were calculated as K T = 124.1(4) GPa for the natural chondrodite and K T = 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 2- + Si 4 + ⇔ 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 -1 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) (cm -1GPa -1), respectively. With increasing pressure up to 9.9 GPa, the 3383 cm -1 peak shifts to lower-frequency 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 -1 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.