Mg_6.75(OH)₃(H_0.166AsO₄)₃(HAsO₄), a member of the M _{1- x}M′₆(OH)₃(H_2x/3AsO₄)₃(HAsO₄) family (M,M′ = Co; Ni)

Abstract

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.

Related literature  

For the isotypic Co and Ni members of the M _{1- x}M′₆(OH)₃(H_2x/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  

Crystal data  

• Mg_6.75(OH)₃(H_0.166AsO₄)₃(HAsO₄)

• M _r = 772.31

• Hexagonal,

• a = 12.7651 (3) Å

• c = 5.0844 (1) Å

• V = 717.49 (3) ų

• Z = 2

• Mo Kα radiation

• μ = 9.65 mm⁻¹

• T = 296 K

• 0.30 × 0.02 × 0.02 mm

Data collection  

• Bruker APEXII CCD diffractometer

• Absorption correction: multi-scan (SADABS; Bruker, 2008 ▶) T _min = 0.160, T _max = 0.830

• 21473 measured reflections

• 1144 independent reflections

• 1066 reflections with I > 2σ(I)

• R _int = 0.051

Refinement  

• R[F ² > 2σ(F ²)] = 0.029

• wR(F ²) = 0.058

• S = 1.21

• 1144 reflections

• 60 parameters

• 1 restraint

• H-atom parameters not refined

• Δρ_max = 1.31 e Å⁻³

• Δρ_min = −2.16 e Å⁻³

• Absolute structure: Flack (1983 ▶), 446 Friedel pairs

• Flack parameter: 0.022 (16)

Data collection: APEX2 (Bruker, 2008 ▶); cell refinement: SAINT (Bruker, 2008 ▶); data reduction: SAINT; 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 ▶).

Supplementary Material

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Table 1. Selected bond lengths (Å).

Mg1—O1i	2.100 (5)
Mg1—O1ii	2.102 (5)
Mg1—Mg1iii	2.5422 (1)
Mg2—O2	2.018 (3)
Mg2—O2iv	2.029 (3)
Mg2—O5	2.036 (3)
Mg2—O3v	2.053 (2)
Mg2—O4	2.139 (3)
Mg2—O4ii	2.225 (3)
As1—O2	1.678 (2)
As1—O1	1.687 (4)
As1—O3	1.696 (3)
As2—O5	1.644 (3)
As2—O6ii	1.710 (8)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) .

supplementary crystallographic information

Comment

Crystals of the title compound, Mg_6.75(OH)₃(H_0.166AsO₄)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).

Mg_6.75(OH)₃(H_0.166AsO₄)₃(HAsO₄) belongs to the isotypic series M_{1 -x}M'₆(OH)₃(H_2x/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).

Experimental

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

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.

Figures

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.

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.

Crystal data

Mg6.75(OH)3(H0.166AsO4)3(HAsO4)	Dx = 3.575 Mg m−3
Mr = 772.31	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63mc	Cell parameters from 5443 reflections
Hall symbol: P 6c -2c	θ = 6.2–34.4°
a = 12.7651 (3) Å	µ = 9.65 mm−1
c = 5.0844 (1) Å	T = 296 K
V = 717.49 (3) Å3	Needle, colourless
Z = 2	0.30 × 0.02 × 0.02 mm
F(000) = 739

Data collection

Bruker APEXII CCD diffractometer	1144 independent reflections
Radiation source: fine-focus sealed tube	1066 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.051
ω and φ scans	θmax = 36.2°, θmin = 1.8°
Absorption correction: multi-scan (SADABS; Bruker, 2008)	h = −21→21
Tmin = 0.160, Tmax = 0.830	k = −21→21
21473 measured reflections	l = −5→8

Refinement

Refinement on F2	H-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) = 0.058	Δρmax = 1.31 e Å−3
S = 1.21	Δρmin = −2.16 e Å−3
1144 reflections	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
60 parameters	Extinction coefficient: 0.0015 (4)
1 restraint	Absolute structure: Flack (1983), 446 Friedel pairs
Primary atom site location: isomorphous structure methods	Flack parameter: 0.022 (16)

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Mg1	0.0000	0.0000	0.8698 (12)	0.0227 (15)	0.750 (14)
Mg2	0.65062 (9)	0.57400 (10)	0.5491 (3)	0.0085 (2)
As1	0.69764 (4)	0.848821 (19)	0.57265 (9)	0.00778 (9)
As2	0.6667	0.3333	0.78398 (17)	0.00550 (14)
O1	0.8487 (3)	0.92434 (17)	0.6201 (9)	0.0257 (10)
O2	0.6564 (2)	0.7229 (2)	0.3969 (5)	0.0100 (4)
O3	0.6168 (3)	0.80839 (17)	0.8561 (7)	0.0115 (6)
O4	0.52447 (15)	0.47553 (15)	0.2427 (7)	0.0091 (6)
O5	0.59609 (16)	0.40391 (16)	0.6823 (8)	0.0173 (8)
O6	0.3333	0.6667	0.6202 (15)	0.0319 (19)

Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
Mg1	0.0120 (13)	0.0120 (13)	0.044 (4)	0.0060 (6)	0.000	0.000
Mg2	0.0103 (4)	0.0092 (4)	0.0073 (5)	0.0059 (3)	−0.0005 (4)	0.0001 (4)
As1	0.01061 (18)	0.00811 (12)	0.00544 (16)	0.00530 (9)	−0.0004 (2)	−0.00021 (10)
As2	0.00468 (17)	0.00468 (17)	0.0071 (3)	0.00234 (9)	0.000	0.000
O1	0.0089 (14)	0.0320 (17)	0.029 (3)	0.0044 (7)	−0.0038 (16)	−0.0019 (8)
O2	0.0141 (10)	0.0099 (9)	0.0072 (10)	0.0070 (8)	−0.0016 (8)	−0.0015 (8)
O3	0.0188 (16)	0.0110 (10)	0.0074 (14)	0.0094 (8)	0.0073 (12)	0.0036 (6)
O4	0.0099 (10)	0.0099 (10)	0.0082 (14)	0.0053 (11)	0.0009 (5)	−0.0009 (5)
O5	0.0099 (10)	0.0099 (10)	0.034 (2)	0.0065 (12)	−0.0037 (7)	0.0037 (7)
O6	0.042 (3)	0.042 (3)	0.011 (4)	0.0212 (15)	0.000	0.000

Geometric parameters (Å, º)

Mg1—O1i	2.100 (5)	Mg2—O3x	2.053 (2)
Mg1—O1ii	2.100 (5)	Mg2—O4	2.139 (3)
Mg1—O1iii	2.100 (5)	Mg2—O4iv	2.225 (3)
Mg1—O1iv	2.102 (5)	As1—O2	1.678 (2)
Mg1—O1v	2.102 (5)	As1—O2xi	1.678 (2)
Mg1—O1vi	2.102 (5)	As1—O1	1.687 (4)
Mg1—Mg1vii	2.5422 (1)	As1—O3	1.696 (3)
Mg1—Mg1viii	2.5422 (1)	As2—O5ii	1.644 (3)
Mg2—O2	2.018 (3)	As2—O5xii	1.644 (3)
Mg2—O2ix	2.029 (3)	As2—O5	1.644 (3)
Mg2—O5	2.036 (3)	As2—O6iv	1.710 (8)
O1i—Mg1—O1ii	87.2 (2)	O2—As1—O2xi	106.56 (16)
O1i—Mg1—O1iii	87.2 (2)	O2—As1—O1	110.19 (12)
O1ii—Mg1—O1iii	87.2 (2)	O2xi—As1—O1	110.19 (12)
O1i—Mg1—O1iv	179.9 (3)	O2—As1—O3	108.02 (11)
O1ii—Mg1—O1iv	92.81 (9)	O2xi—As1—O3	108.02 (11)
O1iii—Mg1—O1iv	92.81 (9)	O1—As1—O3	113.6 (2)
O1i—Mg1—O1v	92.81 (9)	O5ii—As2—O5xii	110.58 (15)
O1ii—Mg1—O1v	179.9 (3)	O5ii—As2—O5	110.58 (15)
O1iii—Mg1—O1v	92.81 (9)	O5xii—As2—O5	110.58 (15)
O1iv—Mg1—O1v	87.1 (2)	O5ii—As2—O6iv	108.33 (15)
O1i—Mg1—O1vi	92.81 (9)	O5xii—As2—O6iv	108.33 (15)
O1ii—Mg1—O1vi	92.81 (9)	O5—As2—O6iv	108.33 (15)
O1iii—Mg1—O1vi	179.9 (3)	As1—O1—Mg1xiii	151.0 (3)
O1iv—Mg1—O1vi	87.1 (2)	As1—O1—Mg1xiv	134.5 (3)
O1v—Mg1—O1vi	87.1 (2)	Mg1xiii—O1—Mg1xiv	74.46 (12)
O2—Mg2—O2ix	93.03 (10)	As1—O2—Mg2	122.99 (14)
O2—Mg2—O5	164.45 (12)	As1—O2—Mg2xv	137.98 (14)
O2ix—Mg2—O5	95.00 (14)	Mg2—O2—Mg2xv	98.03 (11)
O2—Mg2—O3x	89.84 (12)	As1—O3—Mg2ix	121.83 (9)
O2ix—Mg2—O3x	98.49 (13)	As1—O3—Mg2xvi	121.83 (9)
O5—Mg2—O3x	102.09 (14)	Mg2ix—O3—Mg2xvi	115.94 (18)
O2—Mg2—O4	86.36 (11)	Mg2—O4—Mg2xvii	84.17 (14)
O2ix—Mg2—O4	162.38 (12)	Mg2—O4—Mg2xiv	147.49 (17)
O5—Mg2—O4	81.96 (12)	Mg2xvii—O4—Mg2xiv	88.83 (7)
O3x—Mg2—O4	99.12 (14)	Mg2—O4—Mg2xv	88.83 (7)
O2—Mg2—O4iv	89.76 (11)	Mg2xvii—O4—Mg2xv	147.49 (17)
O2ix—Mg2—O4iv	83.86 (11)	Mg2xiv—O4—Mg2xv	80.23 (13)
O5—Mg2—O4iv	77.91 (13)	As2—O5—Mg2xvii	134.44 (9)
O3x—Mg2—O4iv	177.63 (15)	As2—O5—Mg2	134.44 (9)
O4—Mg2—O4iv	78.53 (7)	Mg2xvii—O5—Mg2	89.51 (15)

Symmetry codes: (i) x−1, y−1, z; (ii) −y+1, x−y, z; (iii) −x+y, −x+1, z; (iv) −x+1, −y+1, z+1/2; (v) y−1, −x+y, z+1/2; (vi) x−y, x−1, z+1/2; (vii) −x, −y, z+1/2; (viii) −x, −y, z−1/2; (ix) y, x, z+1/2; (x) x−y+1, x, z−1/2; (xi) x, x−y+1, z; (xii) −x+y+1, −x+1, z; (xiii) x+1, y+1, z; (xiv) −x+1, −y+1, z−1/2; (xv) y, x, z−1/2; (xvi) y, −x+y+1, z+1/2; (xvii) −y+1, −x+1, z.