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 (KT0′ = 4) gives V0 = 438.51 ± 0.06 Å3 and KT0 = 78.2 ± 0.4 GPa. Second-order fits of (a/a0)3 and (c/c0)3 give elastic moduli KT0a = 82.0(6) GPa and KT0c = 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 KT0a and KT0c 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.

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