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A structural refinement of a natural sample of a Co-bearing mansfieldite, AlAsO4·2H2O [aluminium orthoarsenate(V) dihydrate], has been performed based on synchrotron powder diffraction data, with 5% of the octahedral Al sites replaced by Co. Mansfieldite is the aluminium analogue and an isotype of the mineral scorodite (FeAsO4·2H2O), with which it forms a solid solution. The framework structure is based on AsO4 tetrahedra sharing their vertices with AlO4(H2O)2 octahedra. Three of the four H atoms belonging to the two water molecules in cis positions take part in O—HO hydrogen bonding.
Mansfieldite (AlAsO4·2H2O) was first described by Allen et al. (1948 ) and the synthetic analogue was structurally characterised by Harrison (2000 ). For the structures of isotypic minerals and synthetic compounds, see: Botelho et al. (1994 ), Tang et al. (2001 ) (yanomamite, InAsO4·2H2O); Kniep et al. (1977 ) (variscite, AlPO4·2H2O); Hawthorne (1976 ), Kitahama et al. (1975 ), Xu et al. (2007 ) (scorodite, FeAsO4·2H2O); Taxer & Bartl (2004 ) (strengite, FePO4·2H2O); Loiseau et al. (1998 ) (synthetic GaPO4·2H2O); Mooney-Slater (1961 ) (synthetic InPO4·2H2O and TlPO4·2H2O).
Data collection: local image plate reading software; cell refinement: GSAS (Larson & Von Dreele, 2004 ) and EXPGUI (Toby, 2001 ); data reduction: FIT2D (Hammersley, 1997 ); program(s) used to solve structure: atomic coordinates from Harrison (2000 ); program(s) used to refine structure: GSAS and EXPGUI; molecular graphics: VICS (Izumi & Dilanian, 2005 ); software used to prepare material for publication: publCIF (Westrip, 2009 ).
Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536808044127/wm2209sup1.cif
Rietveld powder data: contains datablocks I. DOI: 10.1107/S1600536808044127/wm2209Isup2.rtv
This work was funded by the research grant No. 21403(296) of the University of Florence. We thank Steve Sorrel for supplying the specimen.
The specimen used in this study is from the locality of Mt. Cobalt, Cloncurry District, Queensland, Australia. Preliminarily, some transparent single crystals were selected from massive, light purple coloured, mansfieldite associated with smolianinovite [(Co,Ni,Mg,Ca)3(Fe3+,Al)2(AsO4)4.11H2O]. The average elemental chemical composition, determined using electron microprobe analyses, yielded the empirical chemical formula, calculated on a total of two cations per formula unit, (Al0.944Co3+0.046Cu2+0.005 Fe3+0.003 Zn2+0.002)Σ=1 (As0.972Al0.022P0.006)Σ=1O3.975.2H2O resulting in the simplified formula AlAsO4.2H2O. The excess Al resulting from the calculation has been arbitrarily assigned to the tetrahedral site. X-ray data collections of some single crystals, with a CCD equipped diffractometer, revealed that all the samples were actually polycrystalline aggregates and showed irregular and broadened spots typical of materials with high mosaicity. Refinements from single-crystal X-ray diffraction data yielded, in the best case, a not satisfactorily RF index of 6.54%. Fragments of pure mansfieldite were then ground and used for synchrotron X-ray data collection.
Structural data were refined employing the Rietveld method and starting from the atomic coordinates provided by Harrison (2000), except for the H atom parameters that were not refined but included in the model. The site occupancies were assigned according to the composition of the idealised chemical formula (Al0.95Co3+0.05)AsO4.2H2O, with 5% Co at the octahedral Al sites.
|AlAsO4·2H2O||F(000) = 789|
|Mr = 203.53||Dx = 3.112 Mg m−3|
|Orthorhombic, Pbca||Synchrotron radiation, λ = 0.68780 Å|
|Hall symbol: -P 2ac 2ab||µ = 7.25 mm−1|
|a = 8.79263 (11) Å||T = 298 K|
|b = 9.79795 (10) Å||Particle morphology: powder|
|c = 10.08393 (11) Å||light pink|
|V = 868.73 (2) Å3||flat sheet, 5.0 × 5.0 mm|
|Z = 8|
|ESRF BM08 Beamline diffractometer||Absorption correction: for a cylinder mounted on the axis Debye-Scherrer, Term (= MU.r/wave) = 2.4540. Correction is not refined.|
|Data collection mode: transmission||Tmin = 0.072, Tmax = 0.095|
|Scan method: Stationary detector|
|Refinement on Inet||Excluded region(s): none|
|Least-squares matrix: full||Profile function: CW Pseudo-Voigt|
|Rp = 0.039||60 parameters|
|Rwp = 0.050||no restraints|
|Rexp = 0.039||H-atom parameters not refined|
|RBragg = 0.034||w = 1/[Yi]|
|R(F2) = 0.03400||(Δ/σ)max = 0.01|
|χ2 = 1.716||Background function: GSAS Background function number 1 with 14 terms. Shifted Chebyshev function of 1st kind|
|? data points||Preferred orientation correction: Spherical Harmonics ODF|
|Al1||0.1478 (3)||0.18068 (16)||0.12682 (18)||0.0087 (5)*||0.95|
|Co1||0.1478 (3)||0.18068 (16)||0.12682 (18)||0.0087 (5)*||0.05|
|As2||0.03632 (9)||−0.13857 (6)||0.15042 (6)||0.00875 (18)*|
|O1||0.0106 (4)||0.0308 (3)||0.1440 (3)||0.0055 (8)*|
|O2||0.0003 (5)||−0.1986 (3)||0.3005 (3)||0.0077 (12)*|
|O3||0.2208 (5)||−0.1729 (4)||0.1105 (3)||0.0086 (12)*|
|O4||−0.0815 (5)||−0.2171 (3)||0.0434 (4)||0.0063 (11)*|
|O5w||0.2296 (5)||0.1244 (4)||0.2939 (3)||0.0182 (12)*|
|O6w||0.3186 (4)||0.0696 (4)||0.0559 (3)||0.0164 (12)*|
|Al1—O1||1.9080 (35)||As2—O4||1.682 (4)|
|O1—Al1—O2vi||90.64 (19)||O1—As2—O3||108.35 (18)|
|O1—Al1—O3ii||179.4743 (18)||O1—As2—O4||110.18 (18)|
|O1—Al1—O4iii||91.93 (18)||O2—As2—O3||109.15 (21)|
|O1—Al1—O5w||86.31 (15)||O2—As2—O4||107.83 (19)|
|O1—Al1—O6w||95.09 (18)||O3—As2—O4||110.13 (18)|
|O2vi—Al1—O3ii||88.84 (18)||Al1—O1—As2||132.87 (23)|
|O2vi—Al1—O4iii||91.28 (18)||Al1vii—O2—As2||134.72 (23)|
|O2vi—Al1—O5w||95.56 (17)||Al1viii—O3—As2||136.63 (23)|
|O2vi—Al1—O6w||173.93 (23)||Al1iii—O4—As2||134.59 (22)|
|O3ii—Al1—O4iii||87.99 (19)||Al1—O5w—H51||109.26 (34)|
|O3ii—Al1—O5w||93.84 (18)||Al1—O5w—H52||120.00 (32)|
|O4iii—Al1—O5w||172.94 (19)||Al1—O6w—H61||113.93 (35)|
|O4iii—Al1—O6w||90.54 (17)||Al1—O6w—H62||112.12 (32)|
Symmetry codes: (i) −x, y+1/2, −z+1/2; (ii) −x+1/2, y+1/2, z; (iii) −x, −y, −z; (iv) x−1/2, y, −z+1/2; (v) −x, y−1/2, −z+1/2; (vi) −x, y+3/2, −z+3/2; (vii) −x−1, y+1/2, −z+1/2; (viii) −x+1/2, y−1/2, z.
|O5w—H51···O4i||0.8295||1.784||2.607 (5)||168.19 (28)|
|O5w—H52···O1ix||0.709||2.028||2.614 (5)||160.98 (27)|
|O6w—H62···O3||0.931||1.709||2.587 (5)||155.80 (26)|
Symmetry codes: (i) −x, y+1/2, −z+1/2; (ix) x+3/2, y, −z+3/2.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2209).