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

¹ Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
² European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, 38043 Grenoble, France
³ Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA

^* E-mail: m.welch{at}nhm.ac.uk

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 V₀ = 438.51 ± 0.06 ų and K_T0 = 78.2 ± 0.4 GPa. Second-order fits of (a/a₀)³ and (c/c₀)³ give elastic moduli K_T0a = 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.

KEYWORDS: bernalite, perovskite, equation of state, compression, hydrogen-bonding

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