Rietveld refinement of a natural cobaltian mansfieldite from synchrotron data

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—H⋯O hydrogen bonding.

Related literature

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).

Experimental

Crystal data

• AlAsO₄·2H₂O

• M_r = 203.53

• Orthorhombic, P b c a

• a = 8.79263 (11) Å

• b = 9.79795 (10) Å

• c = 10.08393 (11) Å

• V = 868.73 (2) ų

• Z = 8

• Synchrotron radiation

• λ = 0.68780 Å

• μ = 7.25 mm⁻¹

• T = 298 K

• Specimen shape: flat sheet

• 5.0 × 5.0 × 0.4 mm

• Particle morphology: powder, light pink

Data collection

• ESRF BM08 beamline

• Specimen mounted in transmission mode

• Scan method: fixed

• Absorption correction: for a cylinder mounted on the φ axis T_min = 0.072, T_max = 0.095

• 2θ_min = 6.0, 2θ_max = 53.0°

• Increment in 2θ = 0.01°

Refinement

• R_p = 0.039

• R_wp = 0.050

• R_exp = 0.039

• R_B = 0.034

• S = 1.31

• Excluded region(s): none

• Profile function: CW pseudo-Voigt

• 60 parameters

• No restraints

• H-atom parameters not refined

• Preferred orientation correction: Spherical harmonics ODF

Table 1
Selected bond lengths (Å)

Al1—O1	1.908 (4)
Al1—O2^i	1.906 (4)
Al1—O3^ii	1.850 (4)
Al1—O4^iii	1.848 (4)
Al1—O5w	1.914 (4)
Al1—O6w	1.988 (4)
As2—O1	1.676 (3)
As2—O2	1.654 (3)
As2—O3	1.704 (4)
As2—O4	1.682 (4)
Symmetry codes: (i) ; (ii) ; (iii) -x, -y, -z.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O5w—H51⋯O4^i	0.83	1.78	2.607 (5)	168 (1)
O5w—H52⋯O1^iv	0.71	2.03	2.614 (5)	161 (1)
O6w—H62⋯O3	0.93	1.71	2.587 (5)	156 (1)
Symmetry codes: (i) ; (iv) .

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).

Supporting information

S1. Comment

The mineral mansfieldite belongs to the general group of hydrous arsenates, a structure subgroup of variscite (AlPO₄.2H₂O), and has been known since the discovery of Allen et al. (1948). It is of white to pale gray colour and has a vitreous luster. It often develops encrustations, crust-like or rounded aggregates on the matrix, and individual crystals are rarely observed. While the structural data of synthetic mansfieldite were reported recently by Harrison (2000), no data regarding natural samples have been provided in literature up to now, probably because of the rare occurrence of crystalline material suitable for structural investigations. Rietveld refinement of a natural sample of a Co-bearing mansfieldite has now been carried out using synchrotron powder diffraction data (Fig. 1).

Mansfieldite crystallizes in the Pbca space group and is isostructural with the the arsenate minerals scorodite [FeAsO₄.2H₂O] (Xu et al., 2007; Hawthorne, 1976; Kitahama et al., 1975), yanomamite [InAsO₄.2H₂O] (Tang et al., 2001; Botelho et al., 1994), TlAsO₄.2H₂O (Mooney-Slater, 1961), and the phosphate minerals variscite [AlPO₄.2H₂O] (Kniep et al., 1977), strengite [FePO₄.2H₂O] (Taxer & Bartl, 2004), InPO₄.2H₂O (Tang et al., 2001; Mooney-Slater, 1961), GaPO₄.2H₂O (Loiseau et al., 1998) and TlPO₄.2H₂O (Mooney-Slater, 1961). Correspondingly to the mentioned arsenates, the structure of mansfieldite is composed of AsO₄ tetrahedra and AlO₄(H₂O)₂ octahedra, each tetrahedron being connected to four octahedra and each octahedron being connected to four tetrahedra, as shown in Fig. 2. The interatomic distances Al—O and As—O resulting from the refinement of the structure are reported in Table 1. The two water molecules are located in cis position, but the O atoms O5W and O6W do not participate in the linkage with the As—O₄ tetrahedra, while three of the hydrogen atoms (H1, H2 and H3) take part in D—H···A bonds that link O5W to O4, O5W to O1, and O6W to O3, respectively (Table 2). Such a structure framework displays channels along b. With respect to the synthetic mansfieldite (Harrison, 2000), the natural sample shows a slightly smaller unit cell volume, which is the effect of slightly smaller octahedral (9.110 versus 9.187 ų) and tetrahedral (2.428 versus 2.450 ų) volumes. Also the distortion index of both the polyhedra, 0.019 and 0.008 for the octahedral and tetrahedral site, respectively, is larger than that calculated for the synthetic material, viz. 0.014 and 0.002. A slight distortion of the structure is probably due to the small amount of incorporated Co and other elements present in the structure of the natural sample. Bond valence calculations show slightly overbonded values of 3.036 and 5.079 valence units for the cation in the octahedral site and in the tetrahedral site, respectively. The isotropic thermal parameters for O5W and O6W, 0.0182 (12) and 0.0164 (12) Ų, respectively, are slightly larger than those of the other oxygen atoms and reveal a certain degree of disorder of the water molecules along the channels, since they are not taking part in the metal-oxygen-metal chains of the structure.

Related literature

Mansfieldite (AlAsO₄.2H₂O) was first described by Allen et al. (1948) and the synthetic analogue 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).

S2. Experimental

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, (Al_0.944Co³⁺_0.046Cu²⁺_0.005 Fe³⁺_0.003 Zn²⁺_0.002)_Σ=1 (As_0.972Al_0.022P_0.006)_Σ=1O_3.975.2H₂O 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 R_F index of 6.54%. Fragments of pure mansfieldite were then ground and used for synchrotron X-ray data collection.

S3. Refinement

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 (Al_0.95Co³⁺_0.05)AsO₄.2H₂O, with 5% Co at the octahedral Al sites.

Computing details

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 (Larson & Von Dreele, 2004) and EXPGUI (Toby, 2001); molecular graphics: VICS (Izumi & Dilanian, 2005); software used to prepare material for publication: publCIF (Westrip, 2009).

Figures

	Fig. 1. The observed, calculated, background and difference X-ray diffraction profile for natural mansfieldite. Bragg reflection positions are shown at the bottom.
	Fig. 2. The crystal structure of mansfieldite, viewed along the b axis. The unit cell is outlined and the hydrogen bonds are represented by dashed lines.

aluminium orthoarsenate(V) dihydrate

Crystal data

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) Å^3	flat sheet, 5.0 × 5.0 mm
Z = 8

Data collection

ESRF BM08 Beamline diffractometer	Absorption correction: for a cylinder mounted on the ϕ 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

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(F^2) = 0.03400	(Δ/σ)max = 0.01
χ^2 = 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

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
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)*
H51	0.19	0.17	0.354	0.08*
H52	0.309	0.114	0.301	0.036*
H61	0.347	0.093	−0.01	0.033*
H62	0.295	−0.023	0.053	0.047*

Geometric parameters (Å, º)

Al1—O1	1.908 (4)	As2—O4	1.682 (4)
Al1—O2^i	1.906 (4)	O5W—H51	0.8295
Al1—O3^ii	1.850 (4)	O5W—H52	0.709
Al1—O4^iii	1.848 (4)	O6W—H61	0.746
Al1—O5w	1.914 (4)	O6W—H62	0.931
Al1—O6w	1.988 (4)	O1—H52^iv	2.028
As2—O1	1.676 (3)	O3—H62	1.709
As2—O2	1.654 (3)	O4—H51^v	1.790
As2—O3	1.704 (4)
O1—Al1—O2^vi	90.64 (19)	O1—As2—O3	108.35 (18)
O1—Al1—O3^ii	179.4743 (18)	O1—As2—O4	110.18 (18)
O1—Al1—O4^iii	91.93 (18)	O2—As2—O3	109.2 (2)
O1—Al1—O5w	86.31 (15)	O2—As2—O4	107.83 (19)
O1—Al1—O6w	95.09 (18)	O3—As2—O4	110.13 (18)
O2^vi—Al1—O3^ii	88.84 (18)	Al1—O1—As2	132.9 (2)
O2^vi—Al1—O4^iii	91.28 (18)	Al1^vii—O2—As2	134.7 (2)
O2^vi—Al1—O5w	95.56 (17)	Al1^viii—O3—As2	136.6 (2)
O2^vi—Al1—O6w	173.9 (2)	Al1^iii—O4—As2	134.6 (2)
O3^ii—Al1—O4^iii	87.99 (19)	Al1—O5w—H51	109.3 (3)
O3^ii—Al1—O5w	93.84 (18)	Al1—O5w—H52	120.0 (3)
O3^ii—Al1—O6w	85.4 (2)	H51—O5w—H52	114.6
O4^iii—Al1—O5w	172.94 (19)	Al1—O6w—H61	113.9 (4)
O4^iii—Al1—O6w	90.54 (17)	Al1—O6w—H62	112.1 (3)
O5w—Al1—O6w	82.82 (17)	H61—O6w—H62	110.3
O1—As2—O2	111.20 (17)

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.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O5w—H51···O4^i	0.83	1.78	2.607 (5)	168 (1)
O5w—H52···O1^ix	0.71	2.03	2.614 (5)	161 (1)
O6w—H62···O3	0.93	1.71	2.587 (5)	156 (1)

Symmetry codes: (i) −x, y+1/2, −z+1/2; (ix) x+3/2, y, −z+3/2.