Compression of the perovskite-related mineral bernalite Fe(OH)_3 to 9 GPa and a reappraisal of its structure

摘要

The octahedral-framework mineral bernalite, Fe(OH)_3, provides a rare opportunity to examine directly the effects of a vacant A site upon the physical properties of perovskite-like structures. Here, we report the effect upon compressibility. Bernalite has been reported previously as having space group Immm (Birch et al., 1993), but numerous reflections violating I-centering were observed in the present study. A case is presented for bernalite having orthorhombic space group Pmmn. Lattice parameters were refined using the Le Bail method for a metrically tetragonal cell and their variation with pressure at room temperature was determined from 17 measurements at pressures from 10~(-4) to 9.3 GPa using synchrotron X-ray powder diffraction. No discontinuities in the compression curves of lattice parameters were observed. Fitting to a second-order Birch-Murnaghan equation-of-state (K_(T0') = 4) gives V_0 = 438.51+ -0.06 A~3 and K_(T0) = 78.2 + - 0.4 GPa. Second-order fits of (a/a_0)~3 and (c/c_0)~3 give elastic moduli K_(T0_a) = 82.0(6) GPa and K_(T0c) = 71.6(4) GPa: the shorter cation-cation distance is the more compressible. These values are very close to those of stottite, FeGe(OH)_6, which has tilt system a~+a~+c~-. The difference in the elastic moduli K_(T0a) and K_(T0c) of bernalite and their close similarity to the stottite values support the revised Pmmn structure (tilt system a~+b~+c~-) for bernalite proposed here. The compressional anisotropy observed in bernalite may reflect its highly anisotropic and directional H-bonding topology.