The structure of antimonian dussertite and the role of antimony in oxysalt minerals

Abstract

<jats:title>Abstract</jats:title> <jats:p>The structure of antimonian dussertite, <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00012652_inline01.png" />(AsO<jats:sub>4</jats:sub>)<jats:sub>2</jats:sub>(OH,H<jats:sub>2</jats:sub>O)<jats:sub>6</jats:sub>, has been refined in space group R3̄<jats:italic>m</jats:italic> with <jats:italic>a</jats:italic> 7.410(3), <jats:italic>c</jats:italic> 17.484(4) Å, <jats:italic>Z</jats:italic> = 3, to <jats:italic>R</jats:italic> = 3.2 % and <jats:italic>R</jats:italic><jats:sub>w</jats:sub> = 3.7 % using 377 observed reflections with <jats:italic>I</jats:italic> ≥ 3 σ(<jats:italic>I</jats:italic>). The structure is of the alunite-type and consists of sheets of corner-sharing (Fe<jats:sup>3+</jats:sup>,Sb<jats:sup>5+</jats:sup>)O<jats:sub>6</jats:sub> octahedra parallel to (0001). The substitution of Sb<jats:sup>5+</jats:sup> for Fe<jats:sup>3+</jats:sup>, and not for As<jats:sup>5+</jats:sup>, is unambiguously demonstrated not only by the structure refinement but also by electron microprobe analyses and crystal-chemical considerations. The icosahedrally coordinated Ba cations occupy cavities between pairs of octahedral sheets and are surrounded by six O atoms from the AsO<jats:sub>4</jats:sub> tetrahedra and six O atoms from the (Fe<jats:sup>3+</jats:sup>,Sb<jats:sup>5+</jats:sup>)O<jats:sub>6</jats:sub> octahedra. The mean bond lengths for the various coordination polyhedra are As-O 1.684(3) Å, (Fe,Sb)-O 2.004(1) Å, and Ba-O 2.872(2) Å. A hydrogen-bonding network is modelled using bond-valence calculations. The dussertite sample investigated is the first member of the crandallite group found to contain substantial Sb.</jats:p>