supplementary materials


Acta Cryst. (2013). E69, i29    [ doi:10.1107/S1600536813010714 ]

Mg6.75(OH)3(H0.166AsO4)3(HAsO4), a member of the M1-xM'6(OH)3(H2x/3AsO4)3(HAsO4) family (M,M' = Co; Ni)

M. Weil

Abstract top

In the structure of the title compound, magnesium hydroxide hydrogenarsenate (6.75/3/4), two different Mg2+ ions, one located on a site with symmetry 3m. (occupancy 3/4) and one on a general position, as well as two different AsO3(OH) tetrahedra (symmetry .m. with partial occupancy for the H atom for one, and symmetry 3m. with full occupancy for the H atom for the other) and one OH- ion (site symmetry .m.) are present. Both Mg2+ ions are octahedrally surrounded by O atoms. The MgO6 octahedra belonging to the partially occupied Mg2+ sites share faces, forming chains along [001]. The other type of MgO6 octahedra share corners and faces under formation of strands parallel to [001] whereby individual strands are linked through common corner atoms. The two types of AsO3(OH) tetrahedra interlink the strands and the chains, building up a three-dimensional framework resembling that of the mineral dumortierite. The OH groups were assigned on basis of bond-valence calculations and crystal chemical considerations.

Comment top

Crystals of the title compound, Mg6.75(OH)3(H0.166AsO4)3(HAsO4), were obtained serendipitously during crystal growth experiments intended to grow a possible descloizite-type phase 'MgHg(AsO4)(OH)' under hydrothermal formation conditions, similar to the formation of ZnHg(AsO4)(OH) (Weil, 2004a).

Mg6.75(OH)3(H0.166AsO4)3(HAsO4) belongs to the isotypic series M1 - xM'6(OH)3(H2x/3AsO4)3(HAsO4) for which the Co and Ni members have been structurally determined by Hughes et al. (2003). The crystal structures contain two M2+ ions, one (M1) located on a site with symmetry 3m. and with varying partial occupancy, and the other (M2) located on a general position with full occupancy. Two different AsO3(OH) tetrahedra, one (As1) with symmetry .m. and partial occupancy for its H atom, and the other (As2) with symmetry 3m. and full occupancy for the H atom, as well as one OH- ion (site symmetry .m.; full occupancy for the H atom) are also present.

The two metal ions are octahedrally surrounded by O atoms, with Mg—O distances in the range 2.018 (3) to 2.225 (3) Å. The Mg1O6 octahedra share faces forming chains parallel to [001]. Mg2O6 octahedra share faces and edges forming strands parallel to [001] (Fig. 1). Individual strands are linked with neighbouring strands through common corner atoms. The AsO3(OH) tetrahedra flank the chains and strands and link both motifs into a three-dimensional framework (Fig. 2).

Since the protons required for charge balance could not be located from difference maps, the assignment of OH groups was made both from crystal chemical considerations and calculation of bond valence sums (Brown, 2002). The valence sums for Mg1 (1.99 v.u.), Mg2 (2.13), As1 (5.00) and As2 (5.34) are near the expected values of 2 and 5, respectively. The values of 1.90 for O1, 2.09 for O2, 2.00 for O3, 1.07 for O4, 2.19 for O5 and 1.17 for O6 suggest that O1, O4 and O6 belong to hydroxide groups; the occupancy of the attached H atom sites of O1 is 0.166 and is dependent on the occupancy of the Mg1 site to which the As1O3(OH) group is attached. H atoms attached to O4 and O6 are fully occupied. O4 is the OH group bonded to four Mg22+ cations. For this bridging µ4 group the longest Mg—O distances of 2.139 (3) and 2.225 (3) Å are observed. O6 is the OH group of an As2O3(OH) tetrahedron; it is solely bonded to As2 and is a much longer (As2—O6(H) = 1.710 (8) Å) than the As2—O5 bonds (1.644 (3) Å) which is typical for AsO3(OH) units.

An interesting feature of this structure type is the short M···M contact within the chains of face-sharing M1O6 octahedra running along [001]. The observed Mg···Mg distance of 2.5422 (1) Å corresponds to c/2 and lies between the respective distances of 2.5460 (1) Å for the Co and of 2.4843 (5) Å for the Ni member (Hughes et al., 2003).

The topological similarities between the framework structure of the title compound and that of the minerals dumortierite (Alexander et al., 1986) and cancrinite has been discussed in detail by Hughes et al. (2003).

Related literature top

For the isotypic Co and Ni members of the M1-xM'6(OH)3(H2x/3AsO4)3(HAsO4) series, see: Hughes et al. (2003). For other reaction products obtained under the given or similar hydrothermal conditions, see: Weil (2004a,b). For the crystal structure of dumortierite, see: Alexander et al. (1986). The bond-valence method has been described by Brown (2002).

Experimental top

200 mg of an amorphous precipitation product obtained by reacting MgCO3 and arsenic acid (ca 20%wt) was mixed with 300 mg HgO and placed in a Teflon container (volume 10 ml) that was filled up with two-thirds of its volume with water. The inlay was placed in a steel autoclave and heated at 493 K for two weeks. Besides colourless needle-shaped crystals of the title compound, recrystallized HgO and α-(Hg2)3(AsO4)2 (Weil, 2004b) were also present, as determined by single-crystal X-ray diffraction of selected crystals.

Refinement top

The atomic coordinates of the isotypic Co compound (Hughes et al., 2003) were used as starting parameters for refinement. The site occupation factor of Mg1 was refined freely; no significant deviation from full occupancy for Mg2 was observed. Hydrogen atoms could not be located reliably from difference maps and hence were not included in the refinement. According to the occupancy of Mg1, the overall number of H atoms was calculated as 4.5 for charge compensation. The maximum and minimum remaining electron densities were found 1.14 and 1.21 Å away from atom As2.

Computing details top

Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS for Windows (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures top
[Figure 1] Fig. 1. Strands of face- and edge-sharing Mg2O6 octahedra extending parallel to [001]. Atoms are displayed with anisotropic displacement parameters at the 74% level. The O atom (O4) of the OH group is given in green.
[Figure 2] Fig. 2. The crystal structure of the title compound in a projection along [001]. MgO6 octahedra are blue, AsO4 tetrahedra red. O atoms are displayed with anisotropic displacement parameters at the 74% level. O atoms belongig to OH groups (with fully or partly occupied H atoms) are given in green.
Magnesium hydroxide hydrogenarsenate (6.75/3/4) top
Crystal data top
Mg6.75(OH)3(H0.166AsO4)3(HAsO4)Dx = 3.575 Mg m3
Mr = 772.31Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63mcCell parameters from 5443 reflections
Hall symbol: P 6c -2cθ = 6.2–34.4°
a = 12.7651 (3) ŵ = 9.65 mm1
c = 5.0844 (1) ÅT = 296 K
V = 717.49 (3) Å3Needle, colourless
Z = 20.30 × 0.02 × 0.02 mm
F(000) = 739
Data collection top
Bruker APEXII CCD
diffractometer
1144 independent reflections
Radiation source: fine-focus sealed tube1066 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.051
ω and φ scansθmax = 36.2°, θmin = 1.8°
Absorption correction: multi-scan
(SADABS; Bruker, 2008)
h = 2121
Tmin = 0.160, Tmax = 0.830k = 2121
21473 measured reflectionsl = 58
Refinement top
Refinement on F2H-atom parameters not refined
Least-squares matrix: full w = 1/[σ2(Fo2) + (0.0061P)2 + 2.8884P]
where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.029(Δ/σ)max < 0.001
wR(F2) = 0.058Δρmax = 1.31 e Å3
S = 1.21Δρmin = 2.16 e Å3
1144 reflectionsExtinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
60 parametersExtinction coefficient: 0.0015 (4)
1 restraintAbsolute structure: Flack (1983), 446 Friedel pairs
Primary atom site location: isomorphous structure methodsFlack parameter: 0.022 (16)
Crystal data top
Mg6.75(OH)3(H0.166AsO4)3(HAsO4)Z = 2
Mr = 772.31Mo Kα radiation
Hexagonal, P63mcµ = 9.65 mm1
a = 12.7651 (3) ÅT = 296 K
c = 5.0844 (1) Å0.30 × 0.02 × 0.02 mm
V = 717.49 (3) Å3
Data collection top
Bruker APEXII CCD
diffractometer
1144 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 2008)
1066 reflections with I > 2σ(I)
Tmin = 0.160, Tmax = 0.830Rint = 0.051
21473 measured reflectionsθmax = 36.2°
Refinement top
R[F2 > 2σ(F2)] = 0.029H-atom parameters not refined
wR(F2) = 0.058Δρmax = 1.31 e Å3
S = 1.21Δρmin = 2.16 e Å3
1144 reflectionsAbsolute structure: Flack (1983), 446 Friedel pairs
60 parametersFlack parameter: 0.022 (16)
1 restraint
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
xyzUiso*/UeqOcc. (<1)
Mg10.00000.00000.8698 (12)0.0227 (15)0.750 (14)
Mg20.65062 (9)0.57400 (10)0.5491 (3)0.0085 (2)
As10.69764 (4)0.848821 (19)0.57265 (9)0.00778 (9)
As20.66670.33330.78398 (17)0.00550 (14)
O10.8487 (3)0.92434 (17)0.6201 (9)0.0257 (10)
O20.6564 (2)0.7229 (2)0.3969 (5)0.0100 (4)
O30.6168 (3)0.80839 (17)0.8561 (7)0.0115 (6)
O40.52447 (15)0.47553 (15)0.2427 (7)0.0091 (6)
O50.59609 (16)0.40391 (16)0.6823 (8)0.0173 (8)
O60.33330.66670.6202 (15)0.0319 (19)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Mg10.0120 (13)0.0120 (13)0.044 (4)0.0060 (6)0.0000.000
Mg20.0103 (4)0.0092 (4)0.0073 (5)0.0059 (3)0.0005 (4)0.0001 (4)
As10.01061 (18)0.00811 (12)0.00544 (16)0.00530 (9)0.0004 (2)0.00021 (10)
As20.00468 (17)0.00468 (17)0.0071 (3)0.00234 (9)0.0000.000
O10.0089 (14)0.0320 (17)0.029 (3)0.0044 (7)0.0038 (16)0.0019 (8)
O20.0141 (10)0.0099 (9)0.0072 (10)0.0070 (8)0.0016 (8)0.0015 (8)
O30.0188 (16)0.0110 (10)0.0074 (14)0.0094 (8)0.0073 (12)0.0036 (6)
O40.0099 (10)0.0099 (10)0.0082 (14)0.0053 (11)0.0009 (5)0.0009 (5)
O50.0099 (10)0.0099 (10)0.034 (2)0.0065 (12)0.0037 (7)0.0037 (7)
O60.042 (3)0.042 (3)0.011 (4)0.0212 (15)0.0000.000
Geometric parameters (Å, º) top
Mg1—O1i2.100 (5)Mg2—O3x2.053 (2)
Mg1—O1ii2.100 (5)Mg2—O42.139 (3)
Mg1—O1iii2.100 (5)Mg2—O4iv2.225 (3)
Mg1—O1iv2.102 (5)As1—O21.678 (2)
Mg1—O1v2.102 (5)As1—O2xi1.678 (2)
Mg1—O1vi2.102 (5)As1—O11.687 (4)
Mg1—Mg1vii2.5422 (1)As1—O31.696 (3)
Mg1—Mg1viii2.5422 (1)As2—O5ii1.644 (3)
Mg2—O22.018 (3)As2—O5xii1.644 (3)
Mg2—O2ix2.029 (3)As2—O51.644 (3)
Mg2—O52.036 (3)As2—O6iv1.710 (8)
O1i—Mg1—O1ii87.2 (2)O2—As1—O2xi106.56 (16)
O1i—Mg1—O1iii87.2 (2)O2—As1—O1110.19 (12)
O1ii—Mg1—O1iii87.2 (2)O2xi—As1—O1110.19 (12)
O1i—Mg1—O1iv179.9 (3)O2—As1—O3108.02 (11)
O1ii—Mg1—O1iv92.81 (9)O2xi—As1—O3108.02 (11)
O1iii—Mg1—O1iv92.81 (9)O1—As1—O3113.6 (2)
O1i—Mg1—O1v92.81 (9)O5ii—As2—O5xii110.58 (15)
O1ii—Mg1—O1v179.9 (3)O5ii—As2—O5110.58 (15)
O1iii—Mg1—O1v92.81 (9)O5xii—As2—O5110.58 (15)
O1iv—Mg1—O1v87.1 (2)O5ii—As2—O6iv108.33 (15)
O1i—Mg1—O1vi92.81 (9)O5xii—As2—O6iv108.33 (15)
O1ii—Mg1—O1vi92.81 (9)O5—As2—O6iv108.33 (15)
O1iii—Mg1—O1vi179.9 (3)As1—O1—Mg1xiii151.0 (3)
O1iv—Mg1—O1vi87.1 (2)As1—O1—Mg1xiv134.5 (3)
O1v—Mg1—O1vi87.1 (2)Mg1xiii—O1—Mg1xiv74.46 (12)
O2—Mg2—O2ix93.03 (10)As1—O2—Mg2122.99 (14)
O2—Mg2—O5164.45 (12)As1—O2—Mg2xv137.98 (14)
O2ix—Mg2—O595.00 (14)Mg2—O2—Mg2xv98.03 (11)
O2—Mg2—O3x89.84 (12)As1—O3—Mg2ix121.83 (9)
O2ix—Mg2—O3x98.49 (13)As1—O3—Mg2xvi121.83 (9)
O5—Mg2—O3x102.09 (14)Mg2ix—O3—Mg2xvi115.94 (18)
O2—Mg2—O486.36 (11)Mg2—O4—Mg2xvii84.17 (14)
O2ix—Mg2—O4162.38 (12)Mg2—O4—Mg2xiv147.49 (17)
O5—Mg2—O481.96 (12)Mg2xvii—O4—Mg2xiv88.83 (7)
O3x—Mg2—O499.12 (14)Mg2—O4—Mg2xv88.83 (7)
O2—Mg2—O4iv89.76 (11)Mg2xvii—O4—Mg2xv147.49 (17)
O2ix—Mg2—O4iv83.86 (11)Mg2xiv—O4—Mg2xv80.23 (13)
O5—Mg2—O4iv77.91 (13)As2—O5—Mg2xvii134.44 (9)
O3x—Mg2—O4iv177.63 (15)As2—O5—Mg2134.44 (9)
O4—Mg2—O4iv78.53 (7)Mg2xvii—O5—Mg289.51 (15)
Symmetry codes: (i) x1, y1, z; (ii) y+1, xy, z; (iii) x+y, x+1, z; (iv) x+1, y+1, z+1/2; (v) y1, x+y, z+1/2; (vi) xy, x1, z+1/2; (vii) x, y, z+1/2; (viii) x, y, z1/2; (ix) y, x, z+1/2; (x) xy+1, x, z1/2; (xi) x, xy+1, z; (xii) x+y+1, x+1, z; (xiii) x+1, y+1, z; (xiv) x+1, y+1, z1/2; (xv) y, x, z1/2; (xvi) y, x+y+1, z+1/2; (xvii) y+1, x+1, z.
Selected bond lengths (Å) top
Mg1—O1i2.100 (5)Mg2—O42.139 (3)
Mg1—O1ii2.102 (5)Mg2—O4ii2.225 (3)
Mg1—Mg1iii2.5422 (1)As1—O21.678 (2)
Mg2—O22.018 (3)As1—O11.687 (4)
Mg2—O2iv2.029 (3)As1—O31.696 (3)
Mg2—O52.036 (3)As2—O51.644 (3)
Mg2—O3v2.053 (2)As2—O6ii1.710 (8)
Symmetry codes: (i) x1, y1, z; (ii) x+1, y+1, z+1/2; (iii) x, y, z+1/2; (iv) y, x, z+1/2; (v) xy+1, x, z1/2.
Acknowledgements top

The X-ray centre of the Vienna University of Technology is acknowledged for financial support and for providing access to the single-crystal diffractometer.

references
References top

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