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A new compound, heptamagnesium bis(arsenate) tetrakis(hydrogenarsenate), Mg
7(AsO
4)
2(HAsO
4)
4, was synthesized by a hydrothermal method. The structure is based on a three-dimensional framework of edge- and corner-sharing MgO
6, MgO
4(OH)
2, MgO
5, AsO
3(OH) and AsO
4 polyhedra. Average Mg-O and As-O bond lengths are in the ranges 2.056-2.154 and 1.680-1.688 Å, respectively. Each of the two non-equivalent OH groups is bonded to both an Mg and an As atom. One OH group is involved in a very short hydrogen bond [O
O = 2.468 (3) Å]. The formula unit is centrosymmetric, with all atoms in general positions except for one Mg atom, which has site symmetry
. The compound is isotypic with Mn
7(AsO
4)
2(HAsO
4)
4 and
M7(PO
4)
2(HPO
4)
4, where
M is Fe, Co or Mn.
Supporting information
The title compound was prepared by a hydrothermal method. A mixture of magnesium hydroxide (0.215 g), scandium(III) oxide (0.167 g) and arsenic(V) acid (0.390 g) was added to distilled water (1.5 ml), and the resultant mixture (pH 4.5) was sealed in a Teflon-lined stainless steel autoclave. The bomb was placed in an air oven at 493 K, held for 7 d, and then allowed to cool slowly to room temperature within the oven. The reaction products were washed with distilled water and slowly dried at room temperature in air. The large majority of the well crystallized reaction products consisted of previously unknown Mg3(AsO4)2·H2O, forming sprays of very small colourless acicular crystals. Although the crystal size of this phase was insufficient for X-ray data collection, an X-ray powder diffraction pattern clearly indicates that it is isostructural with its Co and Ni analogues (Palazzi et al., 1978), the crystal structures of which are not known. The minor component Mg2(As2O7)·2H2O (Wu et al., 1996) formed similar but larger sprays of acicular crystals. The title compound formed minor quantities of small colourless lenticular to tabular crystals. Their crystal habit is dominated by the pinacoids {102}, {102} (uncertain) and {010}; the first two are somewhat curved and show a weak striation parallel to [010]. A fourth reaction product, small colourless rounded prisms of adamite-type Mg2(AsO4)(OH) (Rojo et al., 1997), was only present in trace amounts.
The atomic coordinates of isotypic Fe7(PO4)2(HPO4)4 (Zhou et al., 2002) were used as starting values in the final step of the refinement. H atoms were freely refined, and the resulting distances and angles are listed in Table 2. The highest peak, 1.09 e Å−3, is at a distance of 1.59 Å from the O2 site. The deepest hole, −0.99 e Å−3, is at a distance of 0.54 Å from the As3 site.
Data collection: COLLECT (Nonius, 2003); cell refinement: HKL SCALEPACK (Otwinowski & Minor 1997); data reduction: HKL DENZO (Otwinowski & Minor, 1997) and SCALEPACK; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS (Dowty, 1999) and ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXL97.
heptamagnesium bis(arsenate) tetrakis(hydrogenarsenate)
top
Crystal data top
Mg7(AsO4)2(HAsO4)4 | Z = 1 |
Mr = 1007.72 | F(000) = 478 |
Triclinic, P1 | Dx = 3.652 Mg m−3 |
Hall symbol: -P 1 | Mo Kα radiation, λ = 0.71073 Å |
a = 6.644 (1) Å | Cell parameters from 2620 reflections |
b = 8.042 (2) Å | θ = 2.0–30.0° |
c = 9.750 (2) Å | µ = 11.19 mm−1 |
α = 104.70 (3)° | T = 293 K |
β = 107.89 (3)° | Plate, colourless |
γ = 101.16 (3)° | 0.12 × 0.07 × 0.04 mm |
V = 458.2 (2) Å3 | |
Data collection top
Nonius KappaCCD area-detector diffractometer | 2649 independent reflections |
Radiation source: fine-focus sealed tube | 2411 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.015 |
ψ and ω scans | θmax = 29.9°, θmin = 2.3° |
Absorption correction: multi-scan (HKL SCALEPACK; Otwinowski & Minor, 1997) | h = −9→9 |
Tmin = 0.347, Tmax = 0.663 | k = −11→11 |
5192 measured reflections | l = −13→13 |
Refinement top
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.025 | All H-atom parameters refined |
wR(F2) = 0.061 | w = 1/[σ2(Fo2) + (0.025P)2 + 1.44P] where P = (Fo2 + 2Fc2)/3 |
S = 1.09 | (Δ/σ)max = 0.001 |
2649 reflections | Δρmax = 1.09 e Å−3 |
178 parameters | Δρmin = −0.99 e Å−3 |
0 restraints | Extinction correction: SHELXL97 (Sheldrick, 1997), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
Primary atom site location: structure-invariant direct methods | Extinction coefficient: 0.0034 (6) |
Crystal data top
Mg7(AsO4)2(HAsO4)4 | γ = 101.16 (3)° |
Mr = 1007.72 | V = 458.2 (2) Å3 |
Triclinic, P1 | Z = 1 |
a = 6.644 (1) Å | Mo Kα radiation |
b = 8.042 (2) Å | µ = 11.19 mm−1 |
c = 9.750 (2) Å | T = 293 K |
α = 104.70 (3)° | 0.12 × 0.07 × 0.04 mm |
β = 107.89 (3)° | |
Data collection top
Nonius KappaCCD area-detector diffractometer | 2649 independent reflections |
Absorption correction: multi-scan (HKL SCALEPACK; Otwinowski & Minor, 1997) | 2411 reflections with I > 2σ(I) |
Tmin = 0.347, Tmax = 0.663 | Rint = 0.015 |
5192 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.025 | 0 restraints |
wR(F2) = 0.061 | All H-atom parameters refined |
S = 1.09 | Δρmax = 1.09 e Å−3 |
2649 reflections | Δρmin = −0.99 e Å−3 |
178 parameters | |
Special details top
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
Mg1 | 0.0000 | 0.0000 | 0.0000 | 0.0141 (3) | |
Mg2 | 0.38174 (17) | 0.45857 (14) | 0.10979 (11) | 0.0108 (2) | |
Mg3 | −0.27861 (18) | 0.19316 (14) | −0.28743 (12) | 0.0127 (2) | |
Mg4 | −0.05478 (17) | 0.28877 (14) | −0.51560 (12) | 0.0117 (2) | |
As1 | 0.22767 (5) | 0.15152 (4) | −0.22142 (3) | 0.00975 (8) | |
As2 | 0.09044 (5) | 0.57817 (4) | −0.17210 (3) | 0.00912 (8) | |
As3 | −0.59290 (5) | 0.23136 (4) | −0.62937 (3) | 0.01213 (9) | |
O1 | 0.0075 (3) | 0.1852 (3) | −0.3392 (2) | 0.0121 (4) | |
O2 | 0.2191 (4) | 0.1913 (3) | −0.0450 (2) | 0.0133 (4) | |
O3 | 0.5351 (4) | 0.7472 (3) | 0.2304 (2) | 0.0128 (4) | |
O4 | −0.1978 (4) | 0.0750 (3) | −0.7170 (3) | 0.0147 (4) | |
O5 | 0.3193 (3) | 0.5305 (3) | −0.0830 (2) | 0.0112 (4) | |
O6 | 0.0596 (4) | 0.5423 (3) | −0.3550 (2) | 0.0129 (4) | |
O7 | 0.1134 (4) | −0.2019 (3) | −0.0947 (3) | 0.0150 (4) | |
O8 | 0.1327 (3) | 0.5470 (3) | 0.1598 (2) | 0.0119 (4) | |
O9 | −0.3694 (4) | 0.2114 (3) | −0.5031 (3) | 0.0158 (4) | |
O10 | 0.4710 (4) | 0.3946 (3) | 0.3033 (3) | 0.0157 (4) | |
O11 | 0.2577 (4) | 0.2973 (3) | −0.5190 (3) | 0.0162 (5) | |
O12 | −0.2538 (4) | −0.0344 (3) | −0.2322 (3) | 0.0153 (4) | |
H1 | −0.176 (11) | 0.115 (9) | −0.789 (8) | 0.07 (2)* | |
H2 | 0.336 (9) | 0.369 (7) | −0.454 (6) | 0.033 (15)* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
Mg1 | 0.0189 (7) | 0.0109 (7) | 0.0159 (7) | 0.0048 (6) | 0.0109 (6) | 0.0042 (6) |
Mg2 | 0.0098 (5) | 0.0126 (5) | 0.0103 (5) | 0.0037 (4) | 0.0043 (4) | 0.0038 (4) |
Mg3 | 0.0126 (5) | 0.0126 (5) | 0.0145 (5) | 0.0046 (4) | 0.0065 (4) | 0.0048 (4) |
Mg4 | 0.0129 (5) | 0.0120 (5) | 0.0100 (5) | 0.0036 (4) | 0.0042 (4) | 0.0034 (4) |
As1 | 0.00943 (14) | 0.01028 (15) | 0.00964 (14) | 0.00273 (10) | 0.00383 (10) | 0.00332 (11) |
As2 | 0.00898 (14) | 0.00957 (14) | 0.00902 (14) | 0.00310 (10) | 0.00344 (10) | 0.00312 (10) |
As3 | 0.01151 (15) | 0.01236 (15) | 0.01174 (15) | 0.00341 (11) | 0.00371 (11) | 0.00365 (11) |
O1 | 0.0099 (9) | 0.0163 (11) | 0.0119 (10) | 0.0049 (8) | 0.0040 (8) | 0.0071 (8) |
O2 | 0.0158 (10) | 0.0133 (10) | 0.0110 (10) | 0.0025 (8) | 0.0066 (8) | 0.0038 (8) |
O3 | 0.0123 (10) | 0.0117 (10) | 0.0143 (10) | 0.0026 (8) | 0.0067 (8) | 0.0028 (8) |
O4 | 0.0192 (11) | 0.0105 (10) | 0.0139 (10) | 0.0038 (8) | 0.0070 (9) | 0.0030 (8) |
O5 | 0.0093 (9) | 0.0151 (10) | 0.0115 (10) | 0.0052 (8) | 0.0041 (8) | 0.0069 (8) |
O6 | 0.0185 (11) | 0.0114 (10) | 0.0087 (9) | 0.0049 (8) | 0.0050 (8) | 0.0031 (8) |
O7 | 0.0211 (11) | 0.0095 (10) | 0.0166 (11) | 0.0062 (8) | 0.0098 (9) | 0.0036 (8) |
O8 | 0.0102 (9) | 0.0119 (10) | 0.0140 (10) | 0.0021 (8) | 0.0065 (8) | 0.0036 (8) |
O9 | 0.0127 (10) | 0.0229 (12) | 0.0135 (10) | 0.0080 (9) | 0.0043 (8) | 0.0076 (9) |
O10 | 0.0178 (11) | 0.0142 (11) | 0.0147 (10) | 0.0020 (9) | 0.0049 (9) | 0.0076 (8) |
O11 | 0.0148 (11) | 0.0204 (12) | 0.0132 (11) | 0.0066 (9) | 0.0074 (9) | 0.0016 (9) |
O12 | 0.0162 (11) | 0.0108 (10) | 0.0162 (11) | 0.0030 (8) | 0.0041 (8) | 0.0036 (8) |
Geometric parameters (Å, º) top
Mg1—O7i | 2.049 (2) | Mg4—O1 | 2.061 (2) |
Mg1—O7 | 2.049 (2) | Mg4—O6iv | 2.074 (2) |
Mg1—O2i | 2.132 (2) | Mg4—O11 | 2.074 (3) |
Mg1—O2 | 2.132 (2) | Mg4—O9 | 2.113 (3) |
Mg1—O12i | 2.281 (2) | As1—O3ii | 1.667 (2) |
Mg1—O12 | 2.281 (2) | As1—O1 | 1.677 (2) |
Mg2—O10 | 2.024 (2) | As1—O2 | 1.689 (2) |
Mg2—O5 | 2.050 (2) | As1—O4v | 1.717 (2) |
Mg2—O8 | 2.063 (2) | As2—O6 | 1.673 (2) |
Mg2—O5ii | 2.070 (2) | As2—O5 | 1.673 (2) |
Mg2—O2 | 2.125 (3) | As2—O8iii | 1.679 (2) |
Mg2—O3 | 2.185 (3) | As2—O7vi | 1.693 (2) |
Mg3—O8iii | 2.002 (3) | As3—O10vii | 1.652 (2) |
Mg3—O3iii | 2.051 (2) | As3—O12viii | 1.677 (2) |
Mg3—O9 | 2.054 (2) | As3—O9 | 1.678 (2) |
Mg3—O12 | 2.056 (3) | As3—O11ix | 1.731 (2) |
Mg3—O1 | 2.119 (2) | O4—H1 | 0.88 (7) |
Mg4—O4 | 2.055 (3) | O11—H2 | 0.71 (5) |
Mg4—O6 | 2.058 (3) | | |
| | | |
O7i—Mg1—O7 | 180.00 (13) | O3iii—Mg3—O1 | 168.96 (10) |
O7i—Mg1—O2i | 90.91 (9) | O9—Mg3—O1 | 77.21 (9) |
O7—Mg1—O2i | 89.09 (9) | O12—Mg3—O1 | 85.80 (10) |
O7i—Mg1—O2 | 89.09 (9) | O4—Mg4—O6 | 163.74 (11) |
O7—Mg1—O2 | 90.91 (9) | O4—Mg4—O1 | 107.41 (10) |
O2i—Mg1—O2 | 180.00 (12) | O6—Mg4—O1 | 88.53 (10) |
O7i—Mg1—O12i | 92.13 (9) | O4—Mg4—O6iv | 87.99 (10) |
O7—Mg1—O12i | 87.87 (9) | O6—Mg4—O6iv | 76.36 (10) |
O2i—Mg1—O12i | 86.10 (9) | O1—Mg4—O6iv | 164.15 (10) |
O2—Mg1—O12i | 93.90 (9) | O4—Mg4—O11 | 90.80 (11) |
O7i—Mg1—O12 | 87.87 (9) | O6—Mg4—O11 | 92.22 (11) |
O7—Mg1—O12 | 92.13 (9) | O1—Mg4—O11 | 90.73 (10) |
O2i—Mg1—O12 | 93.90 (9) | O6iv—Mg4—O11 | 85.20 (11) |
O2—Mg1—O12 | 86.10 (9) | O4—Mg4—O9 | 83.07 (11) |
O12i—Mg1—O12 | 180.00 (12) | O6—Mg4—O9 | 97.77 (11) |
O10—Mg2—O5 | 175.14 (10) | O1—Mg4—O9 | 77.22 (10) |
O10—Mg2—O8 | 91.09 (10) | O6iv—Mg4—O9 | 109.16 (10) |
O5—Mg2—O8 | 92.74 (10) | O11—Mg4—O9 | 164.07 (11) |
O10—Mg2—O5ii | 94.84 (10) | O3ii—As1—O1 | 112.64 (11) |
O5—Mg2—O5ii | 80.54 (10) | O3ii—As1—O2 | 116.39 (11) |
O8—Mg2—O5ii | 159.14 (10) | O1—As1—O2 | 110.34 (11) |
O10—Mg2—O2 | 96.81 (10) | O3ii—As1—O4v | 105.97 (11) |
O5—Mg2—O2 | 85.13 (10) | O1—As1—O4v | 107.12 (12) |
O8—Mg2—O2 | 104.31 (10) | O2—As1—O4v | 103.44 (11) |
O5ii—Mg2—O2 | 94.83 (10) | O6—As2—O5 | 108.85 (11) |
O10—Mg2—O3 | 93.86 (10) | O6—As2—O8iii | 110.71 (11) |
O5—Mg2—O3 | 84.18 (10) | O5—As2—O8iii | 110.96 (11) |
O8—Mg2—O3 | 76.03 (9) | O6—As2—O7vi | 106.88 (11) |
O5ii—Mg2—O3 | 83.62 (10) | O5—As2—O7vi | 110.86 (12) |
O2—Mg2—O3 | 169.31 (10) | O8iii—As2—O7vi | 108.50 (11) |
O8iii—Mg3—O3iii | 80.46 (10) | O10vii—As3—O12viii | 112.96 (12) |
O8iii—Mg3—O9 | 100.55 (11) | O10vii—As3—O9 | 112.29 (12) |
O3iii—Mg3—O9 | 99.96 (10) | O12viii—As3—O9 | 112.01 (12) |
O8iii—Mg3—O12 | 130.04 (11) | O10vii—As3—O11ix | 106.97 (12) |
O3iii—Mg3—O12 | 104.17 (10) | O12viii—As3—O11ix | 108.92 (12) |
O9—Mg3—O12 | 126.44 (11) | O9—As3—O11ix | 103.01 (12) |
O8iii—Mg3—O1 | 89.52 (10) | | |
Symmetry codes: (i) −x, −y, −z; (ii) −x+1, −y+1, −z; (iii) −x, −y+1, −z; (iv) −x, −y+1, −z−1; (v) −x, −y, −z−1; (vi) x, y+1, z; (vii) x−1, y, z−1; (viii) −x−1, −y, −z−1; (ix) x−1, y, z. |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O11—H2···O10ii | 0.71 (5) | 1.97 (5) | 2.666 (4) | 166 (6) |
O4—H1···O7v | 0.88 (7) | 1.59 (7) | 2.468 (3) | 174 (7) |
Symmetry codes: (ii) −x+1, −y+1, −z; (v) −x, −y, −z−1. |
Experimental details
Crystal data |
Chemical formula | Mg7(AsO4)2(HAsO4)4 |
Mr | 1007.72 |
Crystal system, space group | Triclinic, P1 |
Temperature (K) | 293 |
a, b, c (Å) | 6.644 (1), 8.042 (2), 9.750 (2) |
α, β, γ (°) | 104.70 (3), 107.89 (3), 101.16 (3) |
V (Å3) | 458.2 (2) |
Z | 1 |
Radiation type | Mo Kα |
µ (mm−1) | 11.19 |
Crystal size (mm) | 0.12 × 0.07 × 0.04 |
|
Data collection |
Diffractometer | Nonius KappaCCD area-detector diffractometer |
Absorption correction | Multi-scan (HKL SCALEPACK; Otwinowski & Minor, 1997) |
Tmin, Tmax | 0.347, 0.663 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 5192, 2649, 2411 |
Rint | 0.015 |
(sin θ/λ)max (Å−1) | 0.702 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.025, 0.061, 1.09 |
No. of reflections | 2649 |
No. of parameters | 178 |
H-atom treatment | All H-atom parameters refined |
Δρmax, Δρmin (e Å−3) | 1.09, −0.99 |
Selected bond lengths (Å) topMg1—O7 | 2.049 (2) | Mg4—O1 | 2.061 (2) |
Mg1—O2 | 2.132 (2) | Mg4—O6iii | 2.074 (2) |
Mg1—O12 | 2.281 (2) | Mg4—O11 | 2.074 (3) |
Mg2—O10 | 2.024 (2) | Mg4—O9 | 2.113 (3) |
Mg2—O5 | 2.050 (2) | As1—O3i | 1.667 (2) |
Mg2—O8 | 2.063 (2) | As1—O1 | 1.677 (2) |
Mg2—O5i | 2.070 (2) | As1—O2 | 1.689 (2) |
Mg2—O2 | 2.125 (3) | As1—O4iv | 1.717 (2) |
Mg2—O3 | 2.185 (3) | As2—O6 | 1.673 (2) |
Mg3—O8ii | 2.002 (3) | As2—O5 | 1.673 (2) |
Mg3—O3ii | 2.051 (2) | As2—O8ii | 1.679 (2) |
Mg3—O9 | 2.054 (2) | As2—O7v | 1.693 (2) |
Mg3—O12 | 2.056 (3) | As3—O10vi | 1.652 (2) |
Mg3—O1 | 2.119 (2) | As3—O12vii | 1.677 (2) |
Mg4—O4 | 2.055 (3) | As3—O9 | 1.678 (2) |
Mg4—O6 | 2.058 (3) | As3—O11viii | 1.731 (2) |
Symmetry codes: (i) −x+1, −y+1, −z; (ii) −x, −y+1, −z; (iii) −x, −y+1, −z−1; (iv) −x, −y, −z−1; (v) x, y+1, z; (vi) x−1, y, z−1; (vii) −x−1, −y, −z−1; (viii) x−1, y, z. |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
O11—H2···O10i | 0.71 (5) | 1.97 (5) | 2.666 (4) | 166 (6) |
O4—H1···O7iv | 0.88 (7) | 1.59 (7) | 2.468 (3) | 174 (7) |
Symmetry codes: (i) −x+1, −y+1, −z; (iv) −x, −y, −z−1. |
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Mg7(AsO4)2(HAsO4)4 is a new magnesium arsenate hydrogenarsenate, and was synthesized by a hydrothermal method. It is the sixth simple magnesium arsenate containing protonated AsO4 tetrahedra. The other compounds known are the two minerals brassite (MgHAsO4·4H2O; Brasse & Remy, 1970; Fontan et al., 1973; Protas & Gindt, 1976) and rösslerite (MgHAsO4·7H2O; Ferraris & Franchini-Angela, 1973; Street & Whitaker, 1973; Whitaker, 1973), and the synthetic compounds MgHAsO4·H2O (Guerin & Mattrat, 1958; Mattrat & Guerin, 1960; Stahl-Brasse, 1972), MgHAsO4·2.5H2O (Guerin & Mattrat, 1958; Brasse et al., 1971; Stahl-Brasse, 1972), Mg(H2AsO4)2 (Grunze & Thilo, 1964) and Mg(H2AsO4)2·H2O (Grunze & Thilo, 1964). The crystal structures of the last four have not been determined.
The crystal structure of Mg7(AsO4)2(HAsO4)4 is based on a three-dimensional framework of edge- and corner-sharing MgO6, MgO4(OH)2, MgO5, AsO3(OH) and AsO4 polyhedra (Figs. 1 and 2). The asymmetric unit contains four crystallographically distinct Mg sites, three As sites, twelve O sites and two H sites. The formula unit is centrosymmetric, with all atoms in general positions except atom Mg1, which has site symmetry 1. Each of the two non-equivalent OH groups is bonded to both an Mg and an As atom.
Atom Mg1 lies at the centre of an Mg1O6 octahedron with considerable bond-length distortion (Table 1). In contrast, Mg2O6 and Mg4O4(OH)2 represent less distorted octahedra, and atom Mg3 is surrounded by five O ligands, forming a distorted Mg3O5 trigonal bipyramid. Five-coordinate Mg is relatively rare but has also been observed in the arsenate BaMgAs2O7 (Mihajlović et al., 2004), in other oxysalts, such as BiMg2VO6 (Huang & Sleight, 1992), LiMgBO3 (Norrestam, 1989), SrMgP2O7 (Tahiri et al., 2002), Mg5Na2(PO4)4 (Yamakawa et al., 1994), Mg3Ti4P6O24 (Benmoussa et al., 1990), and in the minerals farringtonite [Mg3(PO4)2; Nord & Kierkegaard, 1968] and grandidierite [(Mg,Fe)Al3SiBO9; Stephenson & Moore, 1968]. The Mg2-, Mg3- and Mg4-centred polyhedra share common edges to form zigzag chains nearly parallel to [101]. In contrast, the Mg1O6 octahedron only shares corners with adjacent polyhedra. Average Mg—O bond lengths are 2.154 for Mg1, 2.086 for Mg2, 2.056 for Mg3 and 2.073 Å for Mg4. These values are reasonably close to the average in oxidic Mg compounds (2.085 Å; Baur, 1981). As expected from the distortion theorem (Brown & Shannon, 1973; Brown, 1981), the largest average bond-length is shown by the most distorted octahedron, viz. Mg1O6.
The three non-equivalent As atoms all show tetrahedral coordination. The AsO4 groups are relatively regular, although the two protonated groups, centred by As1 and As3, show a stronger angular and bond-length distortion in comparison with the unprotonated As2O4 tetrahedron. The two As—OH bonds, As1—O4iv and As3—O11viii [symmetry codes: (iv) −x, −y, −1 − z; (viii) x − 1, y, z], are distinctly longer than the As—O bonds (Table 1), which is in good agreement with earlier observations that protonated XO4 (X is P or As) oxyanion groups generally show an elongated X—OH distance (Ferraris, 1970; Ferraris & Ivaldi, 1984). Average As—O bond lengths are within a close range, with values of 1.688 for As1, 1.680 for As2 and 1.685 Å for As3. A very short hydrogen bond [2.468 (3) Å] occurs via O4—H1···O7iv (Table 2), in contrast with a second, longer, hydrogen bond via O11—H2···O10i [2.666 (4) Å; symmetry code: (i) 1 − x, 1 − y, −z]. Please check added symmetry code.
A bond-valence analysis of the structure was performed using parameters from Brese & O'Keeffe (1991). This confirms the divalent character of all four Mg atoms, with bond-valence sums of 1.78 (Mg1), 2.10 (Mg2), 1.88 (Mg3) and 2.15 (Mg4) v.u. (valence units). Bond-valence sums for the three As atoms amount to 4.97 (As1), 5.07 (As2) and 5.01 (As3) v.u. The O ligands have sums of 1.96 (O1), 1.85 (O2), 1.96 (O3), 1.52 (O4 = OH), 2.03 (O5), 2.02 (O6), 1.60 (O7), 2.07 (O8), 1.97 (O9), 1.77 (O10), 1.46 (O11 = OH) and 1.85 (O12) v.u. Thus, the most underbonded O ligand (O7) represents the acceptor of the very strong hydrogen bond (see above), whereas the second most underbonded O ligand (O10) accepts the less strong second hydrogen bond.
The title compound is isotypic with Mn7(AsO4)2(HAsO4)4 (Rojo et al., 2002) and M7(PO4)2(HPO4)4, where M is FeII (Zhou et al., 2002), a mixture of FeII and FeIII (Vencato et al., 1994), CoII (Lightfoot & Cheetham, 1988; Rojo et al., 2001), and MnII (Cudennec et al., 1986; Riou et al., 1987; Rojo et al., 2002). The Zn—P, Co—As and Zn—As analogues are apparently not known. The unit-cell volume of the title compound is distinctly smaller than that of its MnII analogue [491.0 (3) Å3; Rojo et al., 2002], as would be expected from the effective ionic radii of MgII and (high-spin) MnII (0.720 and 0.830 Å, respectively; Shannon, 1976).