Hole defects in the crystal structure of synthetic lipscombite (Fe 4.7 3+ Fe 2.3 2+ )[PO4]4O2.7(OH)1.3 and genetic crystal chemistry of minerals of the lipscombite-barbosalite series |
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| Authors: | O. V. Yakubovich I. M. Steele V. S. Rusakov V. S. Urusov |
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| Affiliation: | (1) Moscow State University, Vorob’evy gory, Moscow, 119992, Russia;(2) University of Chicago, Chicago, USA |
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| Abstract: | The crystal structure of a synthetic analog of the mineral lipscombite (Fe 2.3 2+ Fe 4.7 3+ )[PO4]4O2.7(OH)1.3 obtained under hydrothermal conditions in the LiF-Fe2O3-(NH4)2HPO4-H2O system is resolved (R = 0.040) by X-ray diffraction analysis (Bruker Smart diffractometer with a highly sensitive CCD detector, MoK α radiation): a = 14.776(3) Å, b = 14.959(3) Å, c = 7.394(1) Å, β = 119.188(4)°, sp. gr. C2/c, Z = 4, ρexp = 3.8 g/cm3, ρcalcd = 3.9 g/cm3. Fe2+ and Fe3+ cations are statistically distributed in each of four crystallographically independent positions, while occupying the corresponding octahedra with probabilities of 60, 90, 100, and 91%. The ratio Fe2+/Fe3+ in the composition of the crystals was established by Mössbauer spectroscopy. Lipscombite is interpreted as a mineral of variable composition described by the formula (Fe x 2+ Fe n?x 3+ )[PO4]4Oy(OH)4?y . The field of stability is determined as a function of the iron content and the ratio Fe2+/Fe3+. It is shown that at n = 6 iron cations are ordered in octahedra and barbosalite structure is formed. An interpretation of genetically and structurally related members of the lipscombite family within a unified polysomatic series is proposed. |
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