Crystal structure of norbergite under high-pressures up to 8.2 GPa

High-pressure single crystal X-ray diffraction study of norbergite, Mg₃SiO₄(OH,F)₂, was performed up to 8.2 GPa with a diamond anvil cell using synchrotron radiation. The sample of single crystal was from the Sterling Hill Mine, New Jersey, U.S.A.. The chemical formula of this sample was Mg_2.984(10)Fe_0.014(2)Ti_0.011(1)Si_0.988(5)(OH_{0.31(5)}F_{1.69(5)}) from the average of 11 data points measured by EPMA (JEOL, JXA-8800M). The hydrogen contents were calculated based on OH+F=2. The X-ray diffraction intensity data were measured using synchrotron radiation by an automated four-circle X-ray diffractometer installed at the BL-10A beam line, Photon Factory, High-Energy Accelerator Research Organization, Tsukuba, Japan. The wavelength of synchrotron radiation was calibrated by the unit cell constant of a ruby standard crystal. The lattice parameters of this sample were measured at ambient, 0.5, 3.1, 4.2, 4.7, 5.4, 6.3, 7.6 and 8.2 GPa. No discontinuity of the lattice parameters was observed in this pressure range. From the data of these lattice parameters, the axial liner compressibilities of this sample was calculated as {\beta$_a}=2.18(4){×}10^-3, β _b=2.93(7){×}10^-3 and β _c=2.77(6){×}10^-3 (GPa^-1). The unit cell was based on Pbnm setting. The isothermal bulk modulus of the norbergite, calculated from the Birch-Murnaghun equation of state, was K_T=113(2) GPa assuming K'=4. The structural parameters were refined at ambient, 4.7, 6.3 and 8.2 GPa conditions. R and R_w values of these refinements were 2.2{%} and 4.8{%} for 0.0001 GPa, 4.6{%} and 6.7{%} for 4.7 GPa, 5.3{%} and 7.0{%} for 6.3 GPa, 5.3{%} and 7.1{%} for 8.2 GPa, respectively. In the norbergite structure, there are three types of (OH,F)-(OH,F) distances, which related hydrogen bonding. Unshared (OH,F)-(OH,F) distances were decreased 2.9{%} and 1.8{%} up to 8.2 GPa, respectively. Shared (OH,F)-(OH,F) distance was decreased 0.8{%} in the pressure range investigated. The volume of polyhedral site at 8.2 GPa were 94.0{%} for M1 site, 93.5{%} for M2 site and 97.4{%} for T site, compared to those at ambient conditions.