Received 6 December 2012
The bjarebyite group of minerals, characterized by the general formula BaX2Y2(PO4)3(OH)3, with X = Mg, Fe2+ or Mn2+, and Y = Al or Fe3+, includes five members: bjarebyite BaMn2+2Al2(PO4)3(OH)3, johntomaite BaFe2+2Fe3+2(PO4)3(OH)3, kulanite BaFe2+2Al2(PO4)3(OH)3, penikisite BaMg2Al2(PO4)3(OH)3, and perloffite BaMn2+2Fe3+2(PO4)3(OH)3. Thus far, the crystal structures of all minerals in the group, but penikisite, have been determined. The present study reports the first structure determination of penikisite (barium dimagnesium dialuminium triphosphate trihydroxide) using single-crystal X-ray diffraction data of a crystal from the type locality, Mayo Mining District, Yukon Territory, Canada. Penikisite is isotypic with other members of the bjarebyite group with space group P21/m, rather than triclinic (P1 or P-1), as previously suggested. Its structure consists of edge-shared [AlO3(OH)3] octahedral dimers linking via corners to form chains along . These chains are decorated with PO4 tetrahedra (one of which has site symmetry m) and connected along  via edge-shared [MgO5(OH)] octahedral dimers and eleven-coordinated Ba2+ ions (site symmetry m), forming a complex three-dimensional network. O-HO hydrogen bonding provides additional linkage between chains. Microprobe analysis of the crystal used for data collection indicated that Mn substitutes for Mg at the 1.5% (apfu) level.
For penikisite, see: Mandarino et al. (1977). For other mineral members in the bjarebyite group, see: Moore & Araki (1974); Cooper & Hawthorne (1994); Kolitsch et al. (2000); Elliott & Willis (2011). For the definition of polyhedral distortion, see: Robinson et al. (1971).
Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XtalDraw (Downs & Hall-Wallace, 2003); software used to prepare material for publication: publCIF (Westrip, 2010).
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: HB7009 ).
The authors gratefully acknowledge support of this study by the Science Foundation Arizona.
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