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, 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 compressibili- ties 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 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 correla- tion 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 condi- tions. 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 −1 GPa −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.