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Mineralogical Magazine; June 2005; v. 69; no. 3; p. 309-315; DOI: 10.1180/0026461056930252
© 2005 Mineralogical Society of Great Britain and Ireland
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Compression of the perovskite-related mineral bernalite Fe(OH)3 to 9 GPa and a reappraisal of its structure

M. D. Welch1,*, W. A. Crichton2 and N. L. Ross3

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 (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.

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




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