Compression of the perovskite-related mineral bernalite Fe(OH) 3 to 9 GPa and a reappraisal of its structure

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 T 0 ′ = 4) gives V 0 = 438.51 ± 0.06 A 3 and K T 0 = 78.2 ± 0.4 GPa. Second-order fits of ( a / a 0 ) 3 and ( c / c 0 ) 3 give elastic moduli K T 0a = 82.0(6) GPa and K T 0 c = 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 T 0 a and K T 0 c 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.