Crystal structure of tetra­wickmanite, Mn²⁺Sn⁴⁺(OH)₆

The crystal structure of tetra­wickmanite, ideally Mn²⁺Sn⁴⁺(OH)₆ [mangan­ese(II) tin(IV) hexa­hydroxide], has been determined based on single-crystal X-ray diffraction data collected from a natural sample from Långban, Sweden. Tetra­wickmanite belongs to the octa­hedral-framework group of hydroxide-perovskite minerals, described by the general formula BB'(OH)₆ with a perovskite derivative structure. The structure differs from that of an ABO₃ perovskite in that the A site is empty while each O atom is bonded to an H atom. The perovskite B-type cations split into ordered B and B′ sites, which are occupied by Mn²⁺ and Sn⁴⁺, respectively. Tetra­wickmanite exhibits tetra­gonal symmetry and is topologically similar to its cubic polymorph, wickmanite. The tetra­wickmanite structure is characterized by a framework of alternating corner-linked [Mn²⁺(OH)₆] and [Sn⁴⁺(OH)₆] octa­hedra, both with point-group symmetry -1. Four of the five distinct H atoms in the structure are statistically disordered. The vacant A site is in a cavity in the centre of a distorted cube formed by eight octa­hedra at the corners. However, the hydrogen-atom positions and their hydrogen bonds are not equivalent in every cavity, resulting in two distinct environments. One of the cavities contains a ring of four hydrogen bonds, similar to that found in wickmanite, while the other cavity is more distorted and forms crankshaft-type chains of hydrogen bonds, as previously proposed for tetra­gonal stottite, Fe²⁺Ge⁴⁺(OH)₆.

1. Mineralogical and crystal-chemical context

Tetra­wickmanite, ideally Mn²⁺Sn⁴⁺(OH)₆, belongs to the octa­hedral-framework group of hydroxide-perovskites, described by the general formula BB'(OH)₆ with a perovskite derivative structure. The structure of hydroxide-perovskites differs from that of an ABO₃ perovskite in that the A site is empty while each O atom is bonded to a hydrogen atom. The lack of A-site cations makes them more compressible than perovskite structures (Kleppe et al., 2012) and elicits an industrial inter­est for their potential use in hydrogen storage at high pressures (Welch & Wunder, 2012).

The hydroxide-perovskite species with B = B′ include dzhalindite [In(OH)₃] (Genkin & Murav'eva, 1963), bernalite [Fe³⁺(OH)₃] (Birch et al., 1993) and söhngeite [Ga(OH)₃] (Strunz, 1965). The species with B ≠ B′ have the two cations fully ordered into B and B′ sites according to bond-valence constraints on the bridging O atoms. Valence states can range from +I to +III for B-site cations and from +III to +V for B′-site cations.

Tetra­wickmanite belongs to the group of hydrox­idostannate(IV) perovskites [BSn⁴⁺(OH)₆] which may exhibit cubic (Pn3, Pn3m) or tetra­gonal (P4₂/n, P4₂/nnm) symmetries. Burtite (B = Ca) (Sonnet, 1981), natanite (B = Fe²⁺) (Marshukova et al., 1981), schoenfliesite (B = Mg) (Faust & Schaller, 1971), vismirnovite (B = Zn) (Marshukova et al., 1981) and wickmanite (B = Mn²⁺) (Moore & Smith, 1967; Christensen & Hazell, 1969) display cubic symmetry while tetra­wickmanite (B = Mn²⁺), jeanbandyite (B = Fe³⁺) (Kampf, 1982) and mushistonite (B = Cu²⁺) (Marshukova et al., 1984) are tetra­gonal. The two hydroxide-perovskites stottite (B = Fe²⁺, B′ = Ge⁴⁺) (Strunz et al., 1958) and mopungite (B = Na, B′ = Sb⁵⁺) (Williams, 1985) are also tetra­gonal.

Tetra­wickmanite was initially described by White & Nelen (1973) from a pegmatite at the Foote Mineral Company's spodumene mine, Kings Mountain, North Carolina. From the X-ray diffraction pattern and the crystal morphology, they determined that tetra­wickmanite exhibits tetra­gonal symmetry and is topologically similar to its polymorph, the cubic wickmanite. A second occurrence of tetra­wickmanite at Långban, Sweden, was reported by Dunn (1978) and described as tungsten-rich tetra­wickmanite with tungsten substituting for tin in the structure.

In the course of identifying minerals for the RRUFF Project (http://rruff.info), we were able to isolate a single crystal of tetra­wickmanite from Långban with composition (Mn²⁺_0.94Mg_0.05Fe²⁺_0.01)_Σ=1(Sn⁴⁺_0.92W⁶⁺_0.05)_Σ=0.97(OH)₆. Thereby, this study presents the first crystal structure determination of tetra­wickmanite by means of single-crystal X-ray diffraction.

2. Structural commentary

The structure of tetra­wickmanite is characterized by a framework of alternating corner-linked [Mn²⁺(OH)₆] and [Sn⁴⁺(OH)₆] octa­hedra, centred at special positions 4d and 4c, respectively (site symmetry ) (Fig. 1b). The Mn—O distances are 2.2007 (13), 2.1933 (12) and 2.2009 (14) Å (average 2.198 Å) and the Sn—O distances are 2.0654 (13), 2.0523 (12) and 2.0446 (13) Å (average 2.054 Å), both similar to the inter­atomic distances determined from neutron powder diffraction data for synthetic wickmanite (Mn—O average 2.181 Å and Sn—O average 2.055 Å; Basciano et al., 1998). The tetra­wickmanite structure contains three non-equivalent O atoms, all protonated as OH groups and located at general positions. H1, H2, H3 and H4 are statistically disordered within the structure while H5 is ordered (Fig. 2).

Figure 1 Framework of alternating corner-linked [Mn^2+(OH)6] and [Sn^4+(OH)6] octa­hedra in (a) wickmanite (Basciano et al., 1998) and (b) tetra­wickmanite, with change in senses of rotation in alternate layers along the c-axis direction. Yellow and grey octa­hedra represent Mn and Sn sites, respectively. Blue spheres represent H atoms.

Figure 2 The crystal structure of tetra­wickmanite showing atoms with anisotropic displacement ellipsoids at the 99% probability level. Yellow, grey and red ellipsoids represent Mn, Sn and O atoms, respectively. Blue spheres of arbitrary radius represent H atoms.

Hydroxide-perovskites have the vacant A site in a cavity in the centre of a distorted cube formed by eight octa­hedra at the corners. According to the Glazer notation for octa­hedral-tilt systems in perovskites (Glazer, 1972), wickmanite, the cubic polymorph of tetra­wickmanite, is an a⁺a⁺a⁺-type perovskite, with three equal rotations (Fig. 1a) while tetra­wickmanite is of a⁺a⁺c⁻ type and it changes the senses of rotation in alternate layers along the c-axis direction (Fig. 1b). This difference in octa­hedral-tilt systems is similar to that observed during compressibility studies of cubic burtite [CaSn⁴⁺(OH)₆; Welch & Crichton, 2002] and tetra­gonal stottite [Fe²⁺Ge⁴⁺(OH)₆; Ross et al., 2002]. As the authors pointed out, the variance in the octa­hedral-tilt systems leads to distinct hydrogen-bonding topologies between burtite and stottite, similar to those observed between wickmanite and tetra­wickmanite.

Wickmanite has a single type of cavity with the H atom disordered over two positions, forming a ring of four hydrogen-bonds with two other hydrogen-bonds at the top and the bottom of the cavity (Basciano et al., 1998). However, in tetra­wickmanite, the hydrogen positions and their hydrogen bonds (Table 1) are not equivalent in every cavity, and exhibit two distinct environments. One of the cavities is similar to that of wickmanite, with isolated four-membered hydrogen-bonding ring motifs defined by O3—H5⋯O3 [2.752 (2) Å] and linkages O1—H1⋯O1 [3.047 (3) Å] at the top and bottom of the cavity (Fig. 3a). In tetra­wickmanite, the four-membered ring has equal O3⋯O3 distances [2.752 (2) Å] while in wickmanite, the O⋯O distances alternate between 2.928 and 2.752 Å. Presumably, the shorter O⋯O distances within the ring motif in tetra­wickmanite is correlated with the ordering of the H5 atom.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1⋯O1^i	1.10 (6)	2.22 (7)	3.047 (3)	131 (4)
O1—H1⋯O2^i	1.10 (6)	2.51 (6)	3.0846 (19)	111 (4)
O1—H2⋯O2^ii	0.89 (7)	1.98 (7)	2.859 (2)	171 (5)
O2—H3⋯O2^iii	1.15 (7)	1.80 (7)	2.760 (3)	138 (3)
O2—H3⋯O1^iv	1.15 (7)	2.30 (5)	3.140 (2)	128 (4)
O2—H4⋯O1^v	1.11 (5)	1.77 (5)	2.859 (2)	165 (5)
O3—H5⋯O3^vi	1.09 (3)	1.74 (3)	2.752 (2)	153 (3)
Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) ; (vi) .

Figure 3 Cavity (left) and hydrogen-bonding linkages (right) in tetra­wickmanite. (a) Wickmanite-like cavity with isolated four-membered ring motif O3—H5⋯O3 and linkages O1—H1⋯O1 at the top and bottom of the cavity. (b) Sets of <100> crankshaft-type motifs with the isolated four-membered rings lining in the plane perpendicular to the c axis. Yellow, grey and red spheres represent Mn, Sn and O atoms. Blue, purple, pink, aqua­marine and orange spheres represent H1, H2, H3, H4 and H5 hydrogen atoms, respectively.

The other cavity in tetra­wickmanite is more distorted, with the four-membered rings converted into <100> crankshaft-type motifs defined by three hydrogen bonds: O2—H3⋯O2 [2.760 (3) Å], O1—H2⋯O2/ O2—H4⋯O1 [2.859 (2) Å] and O1—H1⋯O1 [3.047 (3) Å] and the isolated four-membered rings lying in the plane perpendicular to the c axes. The hydrogen bonds O2—H3⋯O1 [3.140 (2) Å] and O1—H1⋯O2 [3.085 (2) Å] are located between the crankshafts, at the top and the bottom, respectively (Fig. 3b). There are no hydrogen bonds parallel to [001].

As stated earlier, the compressibilities of cubic burtite and tetra­gonal stottite, with unit-cell volumes 535.8 and 426 ų, respectively, have been studied and their hydrogen bonding has been compared (Welch & Crichton, 2002; Ross et al., 2002). By analogy, a study of the compressibility of the polymorphs wickmanite and tetra­wickmanite, with much closer unit-cell volume values (488.26 and 482.17 ų, respectively), might also help in understanding the connection between hydrogen-bonding topologies and compression mechanisms in hydroxide-perovskites.

Kleppe et al. (2012) studied pressure-induced phase trans­itions in hydroxide-perovskites based on Raman spectroscopy measurements of stottite [Fe²⁺Ge⁴⁺(OH)₆] up to 21 GPa. In their work, they proposed the monoclinic space group P2/n for stottite at ambient conditions derived from the presence of six OH-stretching bands in the Raman spectra in the range 3064–3352 cm⁻¹. We refined the structure of tetra­wickmanite in space group P2/n (R₁ = 0.0215) and performed the Hamilton reliability test (Hamilton, 1965). The test indicated that the better structural model for tetra­wickmanite is based on the tetra­gonal space group P4₂/n at the 92% confidence level. Moreover, analysis of the anisotropic displacement parameters showed that the tetra­gonal model displays ideal rigid-body motion of the strong polyhedral groups (Downs, 2000), thus corroborating a tetra­gonal structure for tetra­wickmanite.

The Raman spectrum of tetra­wickmanite in the OH-stretching region (2800–3900 cm⁻¹) is displayed in Fig. 4. The minimum number of peaks needed to fit the spectrum in this region (using pseudo-Voigt line profiles) is seven, which is in agreement with the number of hydrogen bonds derived from the structure (Table 1). According to the correlation of O—H stretching frequencies and O—H⋯O hydrogen-bond lengths in minerals by Libowitzky (1999), the most intense peaks (3062, 3145, 3253 and 3374 cm⁻¹) are within the range of calculated wavenumbers for the H⋯O distances between 2.75 and 2.86 Å and they correspond to the strongest hydrogen bonds in the structure.

Figure 4 Raman spectrum of tetra­wickmanite in the OH-stretching region (2800–3900 cm^−1). At the top right, the spectral deconvolution obtained with seven fitting peaks using pseudo-Voigt line profiles.

3. Experimental

The tetra­wickmanite specimen used in this study was from Långban, Sweden, and is in the collection of the RRUFF project (deposition R100003: http://rruff.info/R100003). Its chemical composition was determined with a CAMECA SX100 electron microprobe at the conditions of 20 kV, 20 nA and a beam size of 5 mm.

The analysis of thirteen points yielded an average composition (wt. %): MnO 24.47 (15), MgO 0.71 (11), FeO 0.34 (19), SnO₂ 50.57 (15) and WO₃ 4.49(1.21) with H₂O 19.76 added to obtain a total close to 100%. The empirical chemical formula, calculated based on six oxygen atoms, is (Mn²⁺_0.94Mg_0.05Fe²⁺_0.01)_Σ=1(Sn⁴⁺_0.92W⁶⁺_0.05)_Σ=0.97(OH)₆.

The Raman spectrum of tetra­wickmanite was collected from a randomly oriented crystal on a Thermo-Almega microRaman system, using a 532 nm solid-state laser with a thermoelectric cooled CCD detector. The laser was partially polarized with 4 cm⁻¹ resolution and a spot size of 1 mm.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. Electron microprobe analysis revealed that the tetra­wickmanite sample studied here contains small amounts of W, Mg and Fe. However, the structure refinements with and without a minor contribution of these elements in the octa­hedral sites did not produce any significant differences in terms of reliability factors or displacement parameters. Hence, the ideal chemical formula Mn²⁺Sn⁴⁺(OH)₆ was assumed during the refinement, and all non-hydrogen atoms were refined with anisotropic displacement parameters. All H atoms were located from difference Fourier syntheses. The hydrogen atoms H1–H4 were modelled as statistically disordered around the parent O atom. H atom positions were refined freely; a fixed isotropic displacement parameter (U_iso = 0.03 Å) was used for all H atoms.

Table 2
Experimental details

Crystal data
Chemical formula	MnSn(OH)6
Mr	275.68
Crystal system, space group	Tetragonal, P42/n
Temperature (K)	293
a, c (Å)	7.8655 (4), 7.7938 (6)
V (Å^3)	482.17 (5)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	7.74
Crystal size (mm)	0.05 × 0.05 × 0.04
Data collection
Diffractometer	Bruker APEXII CCD area detector
Absorption correction	Multi-scan (SADABS; Bruker, 2004)
Tmin, Tmax	0.698, 0.747
No. of measured, independent and observed [I > 2σ(I)] reflections	4394, 1272, 681
Rint	0.020
(sin θ/λ)max (Å^−1)	0.863
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.021, 0.056, 1.00
No. of reflections	1272
No. of parameters	56
H-atom treatment	All H-atom parameters refined
Δρmax, Δρmin (e Å^−3)	0.55, −0.54
Computer programs: APEX2 and SAINT (Bruker, 2004), SHELXS97 and SHELXL97 (Sheldrick, 2008), XtalDraw (Downs & Hall-Wallace, 2003) and publCIF (Westrip, 2010).

The maximum residual electron density in the difference Fourier map, 0.55 e Å⁻³, was located at (0.7590 0.5372 0.0856), 1.28 Å from H5 and the minimum, −0.54 e Å⁻³, at (0.7181 0.5102 0.2313), 0.22 Å from H5.

Supporting information

S1. Mineralogical and crystal-chemical context

Tetra­wickmanite, ideally Mn²⁺Sn⁴⁺(OH)₆, belongs to the o­cta­hedral-framework group of hydroxide-perovskites, described by the general formula BB'(OH)₆ with a perovskite derivative structure. The structure of hydroxide-perovskites differs from that of an ABO₃ perovskite in that the A-site is empty while each O atom is bonded to a hydrogen atom. The lack of A-site cations makes them more compressible than perovskite structures (Kleppe et al., 2012) and elicits an industrial inter­est for their potential use in hydrogen storage at high pressures (Welch & Wunder, 2012).

The hydroxide-perovskite species with B = B' include dzhalindite [In(OH)₃] (Genkin & Murav'eva, 1963), bernalite [Fe³⁺(OH)₃] (Birch et al., 1993) and söhngeite [Ga(OH)₃] (Strunz, 1965). The species with B ≠ B' have the two cations fully ordered into B and B' sites according to bond-valence constraints on the bridging O atoms. Valence states can range from +I to +III for B-site cations and from +III to +V for B'-site cations.

Tetra­wickmanite belongs to the group of hydroxidostannate(IV) perovskites [BSn⁴⁺(OH)₆] which may exhibit cubic (Pn3, Pn3m) or tetra­gonal (P4₂/n, P4₂/nnm) symmetries. Burtite (B = Ca) (Sonnet, 1981), natanite (B = Fe²⁺) (Marshukova et al., 1981), schoenfliesite (B = Mg) (Faust & Schaller, 1971), vismirnovite (B = Zn) (Marshukova et al., 1981) and wickmanite (B = Mn²⁺) (Moore & Smith, 1967; Christensen & Hazell, 1969) display cubic symmetry while tetra­wickmanite (B = Mn²⁺), jeanbandyite (B = Fe³⁺) (Kampf, 1982) and mushistonite (B = Cu²⁺) (Marshukova et al., 1984) are tetra­gonal. The two hydroxide-perovskites stottite (B = Fe²⁺, B' = Ge⁴⁺) (Strunz et al., 1958) and mopungite (B = Na, B' = Sb⁵⁺) (Williams, 1985) are also tetra­gonal.

Tetra­wickmanite was initially described by White & Nelen (1973) from a pegmatite at the Foote Mineral Company's spodumene mine, Kings Mountain, North Carolina. From the X-ray diffraction pattern and the crystal morphology, they determined that tetra­wickmanite exhibits tetra­gonal symmetry and is topologically similar to its polymorph, the cubic wickmanite. A second occurrence of tetra­wickmanite at Långban, Sweden, was reported by Dunn (1978) and described as tungsten-rich tetra­wickmanite with tungsten substituting for tin in the structure.

In the course of identifying minerals for the RRUFF Project (http://rruff.info), we were able to isolate a single crystal of tetra­wickmanite from Långban with composition (Mn²⁺_0.94Mg_0.05Fe²⁺_0.01)_Σ=1(Sn⁴⁺_0.92W⁶⁺_0.05)_Σ=0.97(OH)₆. Thereby, this study presents the first crystal structure determination of tetra­wickmanite by means of single-crystal X-ray diffraction.

S2. Structural commentary

The structure of tetra­wickmanite is characterized by a framework of alternating corner-linked [Mn²⁺(OH)₆] and [Sn⁴⁺(OH)₆] o­cta­hedra, centred at special positions 4d and 4c, respectively [site symmetry (1)] (Fig. 1b). The Mn—O distances are 2.2007 (13), 2.1933 (12) and 2.2009 (14) Å (average 2.198 Å) and the Sn—O distances are 2.0654 (13), 2.0523 (12) and 2.0446 (13) Å (average 2.054 Å), both similar to the inter­atomic distances determined from neutron powder diffraction data for synthetic wickmanite (Mn—O average 2.181 Å and Sn—O average 2.055 Å; Basciano et al., 1998). The tetra­wickmanite structure contains three non-equivalent O atoms, all protonated as OH groups and located at general positions. H1, H2, H3 and H4 are statistically disordered within the structure while H5 is ordered (Fig. 2).

Hydroxide-perovskites have the vacant A-site in a cavity in the centre of a distorted cube formed by eight o­cta­hedra at the corners. According to the Glazer notation for o­cta­hedral-tilt systems in perovskites (Glazer, 1972), wickmanite, the cubic polymorph of tetra­wickmanite, is an a⁺a⁺a⁺-type perovskite, with three equal rotations (Fig. 1a) while tetra­wickmanite is of a⁺a⁺c^--type and it changes the senses of rotation in alternate layers along the c-axis direction (Fig. 1b). This difference in o­cta­hedral-tilt systems is similar to that observed during compressibility studies of cubic burtite [CaSn⁴⁺(OH)₆; Welch & Crichton, 2002] and tetra­gonal stottite [Fe²⁺Ge⁴⁺(OH)₆; Ross et al., 2002]. As the authors pointed out, the variance in the o­cta­hedral-tilt systems leads to distinct hydrogen-bonding topologies between burtite and stottite, similar to those observed between wickmanite and tetra­wickmanite.

Wickmanite has a single type of cavity with the H atom disordered over two positions, forming a ring of four hydrogen-bonds with two other hydrogen-bonds at the top and the bottom of the cavity (Basciano et al., 1998). However, in tetra­wickmanite, the hydrogen positions and their hydrogen bonds (Table 1) are not equivalent in every cavity, and exhibit two distinct environments. One of the cavities is similar to that of wickmanite, with isolated four-membered hydrogen-bonding ring motifs defined by O3—H5···O3 [2.752 (2) Å] and linkages O1—H1···O1 [3.047 (3) Å] at the top and bottom of the cavity (Fig. 3a). In tetra­wickmanite, the four-membered ring has equal O3···O3 distances [2.752 (2) Å] while in wickmanite, the O···O distances alternate between 2.928 and 2.752 Å. Presumably, the shorter O···O distances within the ring motif in tetra­wickmanite is correlated with the ordering of the H5 atom.

The other cavity in tetra­wickmanite is more distorted, with the four-membered rings converted into <100> crankshafts-type motifs defined by three hydrogen bonds: O2—H3···O2 [2.760 (3) Å], O1—H2···O2/ O2—H4···O1 [2.859 (2) Å] and O1—H1···O1 [3.047 (3) Å] and the isolated four-membered rings lying in the plane perpendicular to the c axes. The hydrogen bonds O2—H3···O1 [3.140 (2) Å] and O1—H1···O2 [3.085 (2) Å] are located between the crankshafts, at the top and the bottom, respectively (Fig. 3b). There are no hydrogen bonds parallel to [001].

As stated earlier, the compressibilities of cubic burtite and tetra­gonal stottite, with unit-cell volumes 535.8 and 426 ų, respectively, have been studied and their hydrogen bonding has been compared (Welch & Crichton, 2002; Ross et al., 2002). By analogy, a study of the compressibility of the polymorphs wickmanite and tetra­wickmanite, with much closer unit-cell volume values (488.26 and 482.17 ų, respectively), might also help in understanding the connection between hydrogen-bonding topologies and compression mechanisms in hydroxide-perovskites.

Kleppe et al. (2012) studied pressure-induced phase transitions in hydroxide-perovskites based on Raman spectroscopy measurements of stottite [Fe²⁺Ge⁴⁺(OH)₆] up to 21 GPa. In their work, they proposed the monoclinic space group P2/n for stottite at ambient conditions derived from the presence of six OH-stretching bands in the Raman spectra in the range of 3064–3352 cm^-1. We refined the structure of tetra­wickmanite in space group P2/n (R₁ = 0.0215) and performed the Hamilton reliability test (Hamilton, 1965). The test indicated that the better structural model for tetra­wickmanite is based on the tetra­gonal space group P4₂/n at the 92% confidence level. Moreover, analysis of the anisotropic displacement parameters showed that the tetra­gonal model displays ideal rigid-body motion of the strong polyhedral groups (Downs, 2000), thus corroborating a tetra­gonal structure for tetra­wickmanite.

The Raman spectrum of tetra­wickmanite in the OH-stretching region (2800–3900 cm^-1) is displayed in Fig. 4. The minimum number of peaks needed to fit the spectrum in this region (using pseudo-Voigt line profiles) is seven, which is in agreement with the number of hydrogen bonds derived from the structure (Table 1). According to the correlation of O—H stretching frequencies and O—H···O hydrogen-bond lengths in minerals by Libowitzky (1999), the most intense peaks (3062, 3145, 3253 and 3374 cm^-1) are within the range of calculated wavenumbers for the H···O distances between 2.75 and 2.86 Å and they correspond to the strongest hydrogen bonds in the structure.

S3. Experimental

The tetra­wickmanite specimen used in this study was from Långban, Sweden, and is in the collection of the RRUFF project (deposition R100003: http://rruff.info/R100003). Its chemical composition was determined with a CAMECA SX100 electron microprobe at the conditions of 20 kV, 20 nA and a beam size of 5 mm.

The analysis of thirteen points yielded an average composition (wt. %): MnO 24.47 (15), MgO 0.71 (11), FeO 0.34 (19), SnO₂ 50.57 (15) and WO₃ 4.49(1.21) with H₂O 19.76 added to obtain a total close to 100%. The empirical chemical formula, calculated based on 6 O atoms, is (Mn²⁺_0.94Mg_0.05Fe²⁺_0.01)_Σ=1(Sn⁴⁺_0.92W⁶⁺_0.05)_Σ=0.97(OH)₆.

The Raman spectrum of tetra­wickmanite was collected from a randomly oriented crystal on a Thermo-Almega microRaman system, using a 532 nm solid-state laser with a thermoelectric cooled CCD detector. The laser was partially polarized with 4 cm^–1 resolution and a spot size of 1 mm.

S4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. Electron microprobe analysis revealed that the tetra­wickmanite sample studied here contains small amounts of W, Mg and Fe. However, the structure refinements with and without a minor contribution of these elements in the o­cta­hedral sites did not produce any significant differences in terms of reliability factors or displacement parameters. Hence, the ideal chemical formula Mn²⁺Sn⁴⁺(OH)₆ was assumed during the refinement, and all non-hydrogen atoms were refined with anisotropic displacement parameters. All H atoms were located from difference Fourier syntheses. The hydrogen atoms H1–H4 were modelled as statistically disordered around the parent O atom. H atom positions were refined freely; a fixed isotropic displacement parameter (U_iso = 0.03 Å) was used for all H atoms.

The maximum residual electron density in the difference Fourier map, 0.55 e Å^-3, was located at (0.7590 0.5372 0.0856), 1.28 Å from H5 and the minimum, -0.54 e Å^-3, at (0.7181 0.5102 0.2313), 0.22 Å from H5.

Related literature

For related literature, see: Basciano et al. (1998); Birch et al. (1993); Christensen & Hazell (1969); Downs (2000); Dunn (1978); Faust & Schaller (1971); Genkin & Murav'eva (1963); Glazer (1972); Hamilton (1965); Kampf (1982); Kleppe et al. (2012); Libowitzky (1999); Marshukova et al. (1981, 1984); Moore & Smith (1967); Ross et al. (2002); Sonnet (1981); Strunz (1965); Strunz et al. (1958); Welch & Crichton (2002); Welch & Wunder (2012); White & Nelen (1973); Williams (1985).

Computing details

Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XtalDraw (Downs & Hall-Wallace, 2003); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

	Fig. 1. Framework of alternating corner-linked [Mn^2+(OH)6] and [Sn^4+(OH)6] octahedra in (a) wickmanite (Basciano et al., 1998) and (b) tetrawickmanite, with change in senses of rotation in alternate layers along the c-axis direction. Yellow and grey octahedra represent Mn and Sn sites, respectively. Blue spheres represent H atoms.
	Fig. 2. The crystal structure of tetrawickmanite showing atoms with anisotropic displacement ellipsoids at the 99% probability level. Yellow, grey and red ellipsoids represent Mn, Sn and O atoms, respectively. Blue spheres of arbitrary radius represent H atoms.
	Fig. 3. Cavity (left) and hydrogen-bonding linkages (right) in tetrawickmanite. (a) Wickmanite-like cavity with isolated four-membered ring motif O3—H5···O3 and linkages O1—H1···O1 at the top and bottom of the cavity. (b) Sets of <100> crankshaft-type motifs with the isolated four-membered rings lining in the plane perpendicular to the c axes. Yellow, grey and red spheres represent Mn, Sn and O atoms. Blue, purple, pink, aquamarine and orange spheres represent H1, H2, H3, H4 and H5 hydrogen atoms, respectively.
	Fig. 4. Raman spectra of tetrawickmanite in the OH-stretching region (2800–3900 cm^-1). At the top right, the spectral deconvolution obtained with seven fitting peaks using pseudo-Voigt line profiles.

Manganese(II) tin(IV) hexahydroxide]

Crystal data

MnSn(OH)6	Dx = 3.798 Mg m^−3
Mr = 275.68	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P42/n	Cell parameters from 1249 reflections
Hall symbol: -P 4bc	θ = 4.5–37.8°
a = 7.8655 (4) Å	µ = 7.74 mm^−1
c = 7.7938 (6) Å	T = 293 K
V = 482.17 (5) Å^3	Pseudocubic, yellow–orange
Z = 4	0.05 × 0.05 × 0.04 mm
F(000) = 516

Data collection

Bruker APEXII CCD area-detector diffractometer	1272 independent reflections
Radiation source: fine-focus sealed tube	681 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.020
ϕ and ω scan	θmax = 37.8°, θmin = 3.7°
Absorption correction: multi-scan (SADABS; Bruker, 2004)	h = −11→7
Tmin = 0.698, Tmax = 0.747	k = −12→13
4394 measured reflections	l = −13→5

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.021	All H-atom parameters refined
wR(F^2) = 0.056	w = 1/[σ^2(Fo^2) + (0.022P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.00	(Δ/σ)max < 0.001
1272 reflections	Δρmax = 0.55 e Å^−3
56 parameters	Δρmin = −0.54 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0044 (3)

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)
Sn	0.5000	0.0000	0.5000	0.00790 (6)
Mn	0.5000	0.0000	0.0000	0.01017 (8)
O1	0.74065 (17)	−0.05652 (19)	0.5894 (2)	0.0133 (3)
O2	0.42512 (19)	−0.23929 (17)	0.56923 (18)	0.0138 (3)
O3	0.43079 (18)	0.08165 (18)	0.74014 (15)	0.0113 (3)
H1	0.772 (7)	−0.171 (8)	0.517 (7)	0.030*	0.50
H2	0.740 (7)	−0.025 (7)	0.699 (9)	0.030*	0.50
H3	0.297 (9)	−0.292 (7)	0.525 (4)	0.030*	0.50
H4	0.455 (8)	−0.252 (7)	0.707 (7)	0.030*	0.50
H5	0.465 (4)	0.215 (4)	0.743 (2)	0.030*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sn	0.00751 (11)	0.00801 (11)	0.00817 (8)	−0.00042 (8)	0.00044 (6)	0.00029 (6)
Mn	0.0102 (2)	0.0102 (2)	0.01012 (17)	0.0004 (2)	0.00047 (14)	0.00021 (15)
O1	0.0098 (6)	0.0174 (7)	0.0128 (6)	−0.0002 (5)	−0.0011 (6)	0.0014 (6)
O2	0.0147 (7)	0.0091 (7)	0.0176 (7)	0.0001 (5)	0.0009 (6)	0.0012 (6)
O3	0.0145 (7)	0.0107 (8)	0.0087 (5)	−0.0002 (6)	0.0005 (5)	−0.0006 (5)

Geometric parameters (Å, º)

Sn—O2^i	2.0446 (13)	Mn—O3^ii	2.1933 (12)
Sn—O2	2.0446 (13)	Mn—O3^i	2.1933 (12)
Sn—O3	2.0523 (12)	Mn—O2^iii	2.2007 (13)
Sn—O3^i	2.0523 (12)	Mn—O2^iv	2.2007 (13)
Sn—O1^i	2.0654 (13)	Mn—O1^v	2.2009 (14)
Sn—O1	2.0654 (13)	Mn—O1^vi	2.2009 (14)
O2^i—Sn—O2	180.0	O3^ii—Mn—O3^i	180.00 (7)
O2^i—Sn—O3	91.66 (5)	O3^ii—Mn—O2^iii	94.21 (5)
O2—Sn—O3	88.34 (5)	O3^i—Mn—O2^iii	85.79 (5)
O2^i—Sn—O3^i	88.34 (5)	O3^ii—Mn—O2^iv	85.79 (5)
O2—Sn—O3^i	91.66 (5)	O3^i—Mn—O2^iv	94.21 (5)
O3—Sn—O3^i	180.00 (3)	O2^iii—Mn—O2^iv	180.00 (7)
O2^i—Sn—O1^i	88.67 (5)	O3^ii—Mn—O1^v	88.32 (5)
O2—Sn—O1^i	91.33 (5)	O3^i—Mn—O1^v	91.68 (5)
O3—Sn—O1^i	89.84 (6)	O2^iii—Mn—O1^v	88.98 (5)
O3^i—Sn—O1^i	90.16 (6)	O2^iv—Mn—O1^v	91.02 (5)
O2^i—Sn—O1	91.33 (5)	O3^ii—Mn—O1^vi	91.68 (5)
O2—Sn—O1	88.67 (5)	O3^i—Mn—O1^vi	88.32 (5)
O3—Sn—O1	90.16 (6)	O2^iii—Mn—O1^vi	91.02 (5)
O3^i—Sn—O1	89.84 (6)	O2^iv—Mn—O1^vi	88.98 (5)
O1^i—Sn—O1	180.0	O1^v—Mn—O1^vi	180.0

Symmetry codes: (i) −x+1, −y, −z+1; (ii) x, y, z−1; (iii) −y, x−1/2, z−1/2; (iv) y+1, −x+1/2, −z+1/2; (v) y+1/2, −x+1, z−1/2; (vi) −y+1/2, x−1, −z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O1^vii	1.10 (6)	2.22 (7)	3.047 (3)	131 (4)
O1—H1···O2^vii	1.10 (6)	2.51 (6)	3.0846 (19)	111 (4)
O1—H2···O2^viii	0.89 (7)	1.98 (7)	2.859 (2)	171 (5)
O2—H3···O2^ix	1.15 (7)	1.80 (7)	2.760 (3)	138 (3)
O2—H3···O1^x	1.15 (7)	2.30 (5)	3.140 (2)	128 (4)
O2—H4···O1^xi	1.11 (5)	1.77 (5)	2.859 (2)	165 (5)
O3—H5···O3^xii	1.09 (3)	1.74 (3)	2.752 (2)	153 (3)

Symmetry codes: (vii) −x+3/2, −y−1/2, z; (viii) y+1, −x+1/2, −z+3/2; (ix) −x+1/2, −y−1/2, z; (x) x−1/2, y−1/2, −z+1; (xi) −y+1/2, x−1, −z+3/2; (xii) −y+1/2, x, −z+3/2.