Redetermination of brackebuschite, Pb₂Mn³⁺(VO₄)₂(OH)

The crystal structure of brackebuschite, ideally Pb₂Mn³⁺(VO₄)₂(OH) [dilead(II) manganese(III) vanadate(V) hydroxide], was redetermined based on single-crystal X-ray diffraction data of a natural sample from the type locality Sierra de Cordoba, Argentina. Improving on previous results, anisotropic displacement parameters for all non-H atoms were refined and the H atom located, obtaining a significant improvement of accuracy and an unambiguous hydrogen-bonding scheme. Brackebuschite belongs to the brackebuschite group of minerals with general formula A₂M(T1O₄)(T2O₄)(OH, H₂O), with A = Pb²⁺, Ba, Ca, Sr; M = Cu²⁺, Zn, Fe²⁺, Fe³⁺, Mn³⁺, Al; T1 = As⁵⁺, P, V⁵⁺; and T2 = As⁵⁺, P, V⁵⁺, S⁶⁺. The crystal structure of brackebuschite is based on a cubic closest-packed array of O and Pb atoms with infinite chains of edge-sharing [Mn³⁺O₆] octa­hedra located about inversion centres and decorated by two unique VO₄ tetra­hedra (each located on a special position 2e, site symmetry m). One type of VO₄ tetra­hedra is linked with the ¹_∞[MnO_4/2O_2/1] chain by one common vertex, alternating with H atoms along the chain, while the other type of VO₄ tetra­hedra link two adjacent octa­hedra by sharing two vertices with them and thereby participating in the formation of a three-membered Mn₂V ring between the central atoms. The ¹_∞[Mn³⁺(VO₄)₂OH] chains run parallel to [010] and are held together by two types of irregular [PbO_x] polyhedra (x = 8, 11), both located on special position 2e (site symmetry m). The magnitude of the libration component of the O atoms of the ¹_∞[Mn³⁺(VO₄)₂OH] chain increases linearly with the distance from the centerline of the chain, indicating a significant twisting to and fro of the chain along [010]. The hy­droxy group bridges one Pb²⁺ cation with two Mn³⁺ cations and forms an almost linear hydrogen bond with a vanadate group of a neighbouring chain. The O⋯O distance of this inter­action determined from the structure refinement agrees well with Raman spectroscopic data.

1. Mineralogical and crystal-chemical context

Brackebuschite, ideally Pb₂Mn³⁺(VO₄)₂(OH), belongs to the brackebuschite group of minerals with monoclinic symmetry and space group type P2₁/m. The brackebuschite group is defined by the general formula A₂M(T1O₄)(T2O₄)(OH, H₂O), with A = Pb²⁺, Ba, Ca, Sr; M = Cu²⁺, Zn, Fe²⁺, Fe³⁺, Mn³⁺, Al; T1 = As⁵⁺, P, V⁵⁺; and T2 = As⁵⁺, P, V⁵⁺, S⁶⁺. Together with brackebuschite, other secondary lead minerals within this group include arsenbrackebuschite [Pb₂(Fe³⁺,Zn)(AsO₄)₂(OH,H₂O)] (Abraham et al., 1978), calderónite [Pb₂Fe³⁺(VO₄)₂(OH)] (González del Tánago et al., 2003), tsumebite [Pb₂Cu(PO₄)(SO₄)(OH)] (Nichols, 1966), arsen­tsumebite [Pb₂Cu(AsO₄)(SO₄)(OH)] (Bideaux et al., 1966; Zubkova et al., 2002), bushmakinite [Pb₂Al(PO₄)(VO₄)(OH)] (Pekov et al., 2002), ferribushmakinite [Pb₂Fe³⁺(PO₄)(VO₄)(OH)] (Kampf et al., 2015), feinglosite [Pb₂Zn(AsO₄)₂·H₂O] (Clark et al., 1997), and possibly heyite [Pb₅Fe²⁺₂O₄(VO₄)₂], in which a cursory X-ray diffraction investigation suggest a similarity with brackebuschite (Williams, 1973).

Other lead minerals related to the brackebuschite group include fornacite [CuPb₂(CrO₄)(AsO₄)(OH)] (Cocco et al., 1967; Fanfani & Zanazzi, 1967), molybdofornacite [CuPb₂(MoO₄)(AsO₄)(OH)] (Medenbach et al., 1983), and vauque­len­ite [CuPb₂(CrO₄)(PO₄)(OH)] (Fanfani & Zanazzi, 1968). These minerals demonstrate a richness to the crystallography of the group because the first two are described in space group P2₁/c with doubled c-cell edge, while the last one is described in P2₁/n and exhibits doubling of both the a- and c-cell edges (Fanfani & Zanazzi, 1967).

In addition to the lead minerals, the brackebuschite group of minerals also includes bearthite [Ca₂Al(PO₄)₂OH] (Chopin et al., 1993), canosioite [Ba₂Fe³⁺(AsO₄)₂(OH)] (Hålenius et al., 2015), gamagarite [Ba₂Fe³⁺(VO₄)₂(OH)] (de Villiers et al., 1943; Basso et al., 1987), tokyoite [Ba₂Mn³⁺(VO₄)₂(OH)] (Matsubara et al., 2004), goedkenite [Sr₂Al(PO₄)₂(OH)] (Moore et al., 1975), and grandaite [Sr₂Al(AsO₄)₂(OH)] (Cámara et al., 2014).

In the course of characterizing minerals for the RRUFF Project (https://rruff.info), we were able to isolate a single crystal of brackebuschite from the type locality Sierra de Cordoba (Argentina), with composition Pb_1.91(Mn³⁺_0.81Fe³⁺_0.07Zn_0.07)_Σ=0.95(V_1.98As_0.02)_Σ=2.00O_8.00(OH)_1.00. The ratio Mn:Fe varies from grain to grain as shown in Fig. 1, where the colour of the crystals range from light-brown (Mn-rich, this crystal) to dark-brown [Fe-rich, (Mn³⁺_0.43Fe³⁺_0.42Zn_0.10)_Σ=0.95]. González del Tánago et al. (2003) suggested that brackebuschite [Pb₂Mn³⁺(VO₄)₂(OH)] and calderónite [Pb₂Fe³⁺(VO₄)₂(OH)] probably form a complete solid solution, as both phases have been found to coexist on a single specimen.

Figure 1 Photograph of the brackebuschite specimen analysed in this study.

The crystal structure of brackebuschite was first determined by Donaldson & Barnes (1955) in space group B2₁/m assuming a chemical formula Pb₂Mn²⁺(VO₄)₂·H₂O. Foley et al. (1997) redefined its structure in space group P2₁/m and revised its composition to the currently accepted Pb₂Mn³⁺(VO₄)₂(OH). Structure refinement of the latter converged at a reliability factor R1 of 0.056 and was based on anisotropic displacement parameters for all non-O atoms [note that the deposited data in the Inorganic Crystal Structure Database (ICSD, 2016), entry #89256, report only isotropic displacement parameters], and the H atom undetermined. In the current work, all non-hydrogen atoms were refined with anisotropic displacement parameters, and the H atom was located, leading to a significant improvement of accuracy and precision, and to an unambiguous hydrogen bonding scheme.

2. Structural commentary

The structure of brackebuschite is characterized by a distorted cubic closest-packed array of O and Pb atoms along [10] as stacking direction. Infinite chains of edge-sharing [Mn³⁺O₆] octa­hedra decorated by V1O₄ and V2O₄ tetra­hedra are aligned parallel to [010], perpendicular to the stacking direction. Mn³⁺, located on an inversion centre, is coordinated by the oxygen anions belonging to VO₄ tetra­hedra (O3×2 and O5×2) and the hydroxyl group (O7×2) in an overall distorted octa­hedral arrangement. Each V1O₄ tetra­hedron is linked to the ¹_∞[MnO_4/2O_2/1] chain by one common vertex (O3) while each V2O₄ links two adjacent octa­hedra by sharing two vertices (O5) with them. The V1O₄ groups and the H atoms alternate, belong to the edge-sharing atoms and are arranged along one side of the ¹_∞[MnO_4/2O_2/1] chain. The thus resulting ¹_∞[Mn³⁺(VO₄)₂OH] chains are held together by irregular [Pb1O₁₁] and [Pb2O₈] polyhedra (Fig. 2).

Figure 2 Crystal structure of brackebuschite. The edge-sharing [MnO6] octa­hedra (green) form chains parallel to [010] with V1O4 and V2O4 tetra­hedra (purple and violet, respectively) linked to them. These chains are held together by Pb1 and Pb2 (orange and yellow ellipsoids, respectively). Blue spheres of arbitrary radius represent H atoms.

Looking down the axis of the [Mn³⁺(VO₄)₂OH] chain, there is a radial increase in the amplitude of the displacement parameters. We inter­pret this to indicate that the entire chain is undergoing rigid-body libration, oscillating to and fro along its axis. The radial change in amplitude is indicated by three concentric rings in Fig. 3a. The average amplitudes of the inner, middle, and outer rings (1.34, 2.00, and 4.06 Å, respectively) increase roughly linearly with the radial distance from the chain axis.

Figure 3 (a) View of the ^1∞[Mn^3+(VO4)2OH] chain looking down [010]. The black rings represent different radii that correlate with the progressive increase of the libration component of oxygen atoms further away from the centre of the chain. Purple and violet tetra­hedra and green octa­hedra represent V1O4, V2O4 and [MnO6], respectively. Red ellipsoids represent O atoms; (b) large O2 disk-shaped ellipsoid oriented perpendicular to the V1—O bond (anisotropic displacement ellipsoids at the 99% probability level). Note the hydrogen bond O2⋯H—O7 (dashed lines). Purple, yellow and red ellipsoids represent V1O4, [Pb2O8] and O atoms, respectively. The blue sphere represents the H atom.

Bond-valence calculations (Brown, 2002) confirm that O7 corresponds to the hydroxyl group (bond-valence sum of 1.25 valence units), which is approximately tetra­hedrally coordinated by three cations and O2 (bond-valence sum of 1.61 v.u.) with which it forms an almost linear hydrogen bond (Table 1). The Raman spectrum of brackebuschite (Fig. 4) shows a broad band around 3145 cm⁻¹ that is assigned to the OH-stretching vibration (υ_OH). According to the correlation of O—H IR stretching frequencies and O—H⋯O hydrogen-bond lengths in minerals (Libowitzky, 1999), the stretching frequency inferred from this bond-length is 3143 cm⁻¹.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O7—H⋯O2^i	0.89 (2)	1.80 (2)	2.686 (7)	174 (10)
Symmetry code: (i) x, y, z-1.

Figure 4 Raman spectrum of brackebuschite. The weak Raman band around 3145 cm^−1 is assigned to the OH stretching vibrations associated with the OH group (νOH).

The O2 atom, the one associated as the acceptor atom of the hydrogen bond, displays quite large anisotropic displacement parameters relative to the other atoms (Fig. 3b). The disk-shaped ellipsoid is oriented parallel to the weaker Pb2—O bond and perpendicular to the stronger V1—O bond, which constrains the thermal vibration in that direction.

3. Synthesis and crystallization

The natural brackebuschite specimen used in this study is from the type locality Sierra de Cordoba, Argentina, and is in the collection of the RRUFF project (https://rruff.info/R050547). The chemical composition of the brackebuschite specimen was determined with a CAMECA SX100 electron microprobe operated at 20 kV and 20 nA. Seven analysis points yielded an average composition (wt. %): PbO 60.8 (4), V₂O₅ 25.6 (2), Mn₂O₃ 9.1 (5), Fe₂O₃ 0.8 (5), ZnO 0.8 (2), and As₂O₅ 0.33 (8), with H₂O 1.28 estimated to provide one H₂O mol­ecule per formula unit. The empirical chemical formula, based on nine oxygen atoms, is Pb_1.91(Mn³⁺_0.81Fe³⁺_0.07Zn_0.07)_Σ=0.95(V_1.98As_0.02)_Σ=2.00O_8.00(OH)_1.00.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. Electron microprobe analysis revealed that the brackebuschite sample studied here contains small amounts of Fe, Zn and As. However, the structure refinements, with and without a minor contribution of these elements in the octa­hedral and tetra­hedral sites, did not produce any significant differences in terms of reliability factors or displacement parameters. Hence, the ideal chemical formula Pb₂Mn³⁺(VO₄)₂(OH) was assumed during the refinement. The H atom was located from difference Fourier syntheses and its position refined with a fixed isotropic displacement parameter (U_iso = 0.03), and soft DFIX constraint of 0.9 Å from O7. The maximum residual electron density in the difference Fourier map, 2.66 e Å⁻³, was located at (0.6661, 0.1681, 0.5521), 0.66 Å from Pb1 and the minimum, −2.16 e Å⁻³, at (0.7036, 0.25, 0.5714), 0.41 Å from Pb1.

Table 2
Experimental details

Crystal data
Chemical formula	Pb2Mn(VO4)2(OH)
Mr	716.21
Crystal system, space group	Monoclinic, P21/m
Temperature (K)	293
a, b, c (Å)	7.6492 (1), 6.1262 (1), 8.9241 (2)
β (°)	112.195 (1)
V (Å^3)	387.20 (1)
Z	2
Radiation type	Mo Kα
μ (mm^−1)	47.27
Crystal size (mm)	0.05 × 0.05 × 0.05
Data collection
Diffractometer	Bruker APEXII CCD area detector
Absorption correction	Multi-scan (SADABS; Bruker, 2004)
Tmin, Tmax	0.201, 0.201
No. of measured, independent and observed [I > 2σ(I)] reflections	11674, 1521, 1356
Rint	0.037
(sin θ/λ)max (Å^−1)	0.759
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.025, 0.056, 1.05
No. of reflections	1521
No. of parameters	82
No. of restraints	1
H-atom treatment	Only H-atom coordinates refined
Δρmax, Δρmin (e Å^−3)	2.67, −2.16
Computer programs: APEX2 and SAINT (Bruker, 2004), SHELXL2014 (Sheldrick, 2015), XtalDraw (Downs & Hall-Wallace, 2003) and publCIF (Westrip, 2010). Coordinates taken from a previous refinement.

Supporting information

S1. Mineralogical and crystal-chemical context

Brackebuschite, ideally Pb₂Mn³⁺(VO₄)₂(OH), belongs to the brackebuschite group of minerals with monoclinic symmetry and space group type P2₁/m. The brackebuschite group is defined by the general formula A₂M(T1O₄)(T2O₄)(OH, H₂O), with A = Pb²⁺, Ba, Ca, Sr; M = Cu²⁺, Zn, Fe²⁺, Fe³⁺, Mn³⁺, Al; T1 = As⁵⁺, P, V⁵⁺; and T2 = As⁵⁺, P, V⁵⁺, S⁶⁺. Together with brackebuschite, other secondary lead minerals within this group include arsenbrackebuschite [Pb₂(Fe³⁺,Zn)(AsO₄)₂(OH,H₂O)] (Abraham et al., 1978), calderónite [Pb₂Fe³⁺(VO₄)₂(OH)] (González del Tánago et al., 2003), tsumebite [Pb₂Cu(PO₄)(SO₄)(OH)] (Nichols, 1966), arsentsumebite [Pb₂Cu(AsO₄)(SO₄)(OH)] (Bideaux et al., 1966; Zubkova et al., 2002), bushmakinite [Pb₂Al(PO₄)(VO₄)(OH)] (Pekov et al., 2002), ferribushmakinite [Pb₂Fe³⁺(PO₄)(VO₄)(OH)] (Kampf et al., 2015), feinglosite [Pb₂Zn(AsO₄)₂·H₂O] (Clark et al., 1997), and possibly heyite [Pb₅Fe²⁺₂O₄(VO₄)₂], in which a cursory X-ray diffraction investigation suggest a similarity with brackebuschite (Williams, 1973).

Other lead minerals related to the brackebuschite group include fornacite [CuPb₂(CrO₄)(AsO₄)(OH)] (Cocco et al., 1967; Fanfani & Zanazzi, 1967), molybdofornacite [CuPb₂(MoO₄)(AsO₄)(OH)] (Medenbach et al., 1983), and vauquelenite [CuPb₂(CrO₄)(PO₄)(OH)] (Fanfani & Zanazzi, 1968). These minerals demonstrate a richness to the crystallography of the group because the first two are described in space group P2₁/c with doubled c-cell edge, while the last one is described in P2₁/n and exhibits doubling of both the a- and c-cell edges (Fanfani & Zanazzi, 1967).

In addition to the lead minerals, the brackebuschite group of minerals also includes bearthite [Ca₂Al(PO₄)₂OH] (Chopin et al., 1993), canosioite [Ba₂Fe³⁺(AsO₄)₂(OH)] (Hålenius et al., 2015), gamagarite [Ba₂Fe³⁺(VO₄)₂(OH)] (de Villiers et al., 1943; Basso et al., 1987), tokyoite [Ba₂Mn³⁺(VO₄)₂(OH)] (Matsubara et al., 2004), goedkenite [Sr₂Al(PO₄)₂(OH)] (Moore et al., 1975), and grandaite [Sr₂Al(AsO₄)₂(OH)] (Cámara et al., 2014).

In the course of characterizing minerals for the RRUFF Project (https://rruff.info), we were able to isolate a single-crystal of brackebuschite from the type locality Sierra de Cordoba (Argentina), with composition Pb_1.91(Mn³⁺_0.81Fe³⁺_0.07Zn_0.07)_Σ=0.95(V_1.98As_0.02) _Σ=2.00O_8.00(OH)_1.00. The ratio Mn:Fe varies from grain to grain as shown in Fig. 1, where the colour of the crystals range from light-brown (Mn-rich, this crystal) to dark-brown [Fe-rich, (Mn³⁺_0.43Fe³⁺_0.42Zn_0.10)_Σ=0.95]. González del Tánago et al. (2003) suggested that brackebuschite [Pb₂Mn³⁺(VO₄)₂(OH)] and calderónite [Pb₂Fe³⁺(VO₄)₂(OH)] probably form a complete solid solution, as both phases have been found to coexist on a single specimen.

The crystal structure of brackebuschite was first determined by Donaldson & Barnes (1955) in space group B2₁/m assuming a chemical formula Pb₂Mn²⁺(VO₄)₂·H₂O. Foley et al. (1997) redefined its structure in space group P2₁/m and revised its composition to the currently accepted Pb₂Mn³⁺(VO₄)₂(OH). Structure refinement of the latter converged at a reliability factor R1 of 0.056 and was based on anisotropic displacement parameters for all non-O atoms [note that the deposited data in the Inorganic Crystal Structure Database (ICSD, 2016), entry #89256, report only isotropic displacement parameters], and the H atom undetermined. In the current work, all non-hydrogen atoms were refined with anisotropic displacement parameters, and the H atom was located, leading to a significant improvement of accuracy and precision, and to an unambiguous hydrogen bonding scheme.

S2. Structural commentary

The structure of brackebuschite is characterized by a distorted cubic closest-packed array of O and Pb atoms along [101] as stacking direction. Infinite chains of edge-sharing [Mn³⁺O₆] o­cta­hedra decorated by V1O₄ and V2O₄ tetra­hedra are aligned parallel to [010], perpendicular to the stacking direction. Mn³⁺, located on an inversion centre, is coordinated by the oxygen anions belonging to VO₄ tetra­hedra (O3×2 and O5×2) and the hydroxyl group (O7×2) in an overall distorted o­cta­hedral arrangement. Each V1O₄ tetra­hedron is linked to the ¹_∞[MnO_4/2O_2/1] chain by one common vertex (O3) while each V2O₄ links two adjacent o­cta­hedra by sharing two vertices (O5) with them. The V1O₄ groups and the H-atoms alternate belong to the edge-sharing atoms and are arranged along one side of the ¹_∞[MnO_4/2O_2/1] chain. The thus resulting ¹_∞[Mn³⁺(VO₄)₂OH] chains are held together by irregular [Pb1O₁₁] and [Pb2O₈] polyhedra (Fig. 2).

Looking down the axis of the [Mn³⁺(VO₄)₂OH] chain, there is a radial increase in the amplitude of the displacement parameters. We inter­pret this to indicate that the entire chain is undergoing rigid-body libration, oscillating to and fro along its axis. The radial change in amplitude is indicated by three concentric rings in Fig. 3a. The average amplitudes of the inner, middle, and outer rings (1.34, 2.00, and 4.06 Å, respectively) increase roughly linearly with the radial distance from the chain axis.

Bond-valence calculations (Brown, 2002) confirm that O7 corresponds to the hydroxyl group (bond-valence sum of 1.25 valence units), which is approximately tetra­hedrally coordinated by three cations and O2 (bond-valence sum of 1.61 v.u.) with which it forms an almost linear hydrogen bond (Table 1). The Raman spectrum of brackebuschite (Fig. 4) shows a broad band around 3145 cm⁻¹ that is assigned to the OH-stretching vibration (υ_OH). According to the correlation of O—H IR stretching frequencies and O—H···O hydrogen-bond lengths in minerals (Libowitzky, 1999), the stretching frequency inferred from this bond-length is 3143 cm⁻¹.

The O2 atom, the one associated as the acceptor atom of the hydrogen bond, displays quite large anisotropic displacement parameters relative to the other atoms (Fig. 3b). The disk-shaped ellipsoid is oriented parallel to the weaker Pb2—O bond and perpendicular to the stronger V1—O bond, which constrains the thermal vibration in that direction.

S3. Synthesis and crystallization

\ The natural brackebuschite specimen used in this study is from the type locality Sierra de Cordoba, Argentina, and is in the collection of the RRUFF project (https://rruff.info/R050547). The chemical composition of the brackebuschite specimen was determined with a CAMECA SX100 electron microprobe operated at 20 kV and 20 nA. Seven analysis points yielded an average composition (wt. %): PbO 60.8 (4), V₂O₅ 25.6 (2), Mn₂O₃ 9.1 (5), Fe₂O₃ 0.8 (5), ZnO 0.8 (2), and As₂O₅ 0.33 (8), with H₂O 1.28 estimated to provide one H₂O molecule per formula unit. The empirical chemical formula, based on nine oxygen atoms, is Pb_1.91(Mn³⁺_0.81Fe³⁺_0.07Zn_0.07)_Σ=0.95(V_1.98As_0.02)\ _Σ=2.00O_8.00(OH)_1.00.

S4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. Electron microprobe analysis revealed that the brackebuschite sample studied here contains small amounts of Fe, Zn and As. However, the structure refinements, with and without a minor contribution of these elements in the o­cta­hedral and tetra­hedral sites, did not produce any significant differences in terms of reliability factors or displacement parameters. Hence, the ideal chemical formula Pb₂Mn³⁺(VO₄)₂(OH) was assumed during the refinement. The H atom was located from difference Fourier syntheses and its position refined with a fixed isotropic displacement parameter (U_iso = 0.03), and soft DFIX constraint of 0.9 Å from O7. The maximum residual electron density in the difference Fourier map, 2.66 e Å⁻³, was located at (0.6661, 0.1681, 0.5521), 0.66 Å from Pb1 and the minimum, −2.16 e Å⁻³, at (0.7036, 1/4, 0.5714), 0.41 Å from Pb1.

Structure description

Brackebuschite, ideally Pb₂Mn³⁺(VO₄)₂(OH), belongs to the brackebuschite group of minerals with monoclinic symmetry and space group type P2₁/m. The brackebuschite group is defined by the general formula A₂M(T1O₄)(T2O₄)(OH, H₂O), with A = Pb²⁺, Ba, Ca, Sr; M = Cu²⁺, Zn, Fe²⁺, Fe³⁺, Mn³⁺, Al; T1 = As⁵⁺, P, V⁵⁺; and T2 = As⁵⁺, P, V⁵⁺, S⁶⁺. Together with brackebuschite, other secondary lead minerals within this group include arsenbrackebuschite [Pb₂(Fe³⁺,Zn)(AsO₄)₂(OH,H₂O)] (Abraham et al., 1978), calderónite [Pb₂Fe³⁺(VO₄)₂(OH)] (González del Tánago et al., 2003), tsumebite [Pb₂Cu(PO₄)(SO₄)(OH)] (Nichols, 1966), arsentsumebite [Pb₂Cu(AsO₄)(SO₄)(OH)] (Bideaux et al., 1966; Zubkova et al., 2002), bushmakinite [Pb₂Al(PO₄)(VO₄)(OH)] (Pekov et al., 2002), ferribushmakinite [Pb₂Fe³⁺(PO₄)(VO₄)(OH)] (Kampf et al., 2015), feinglosite [Pb₂Zn(AsO₄)₂·H₂O] (Clark et al., 1997), and possibly heyite [Pb₅Fe²⁺₂O₄(VO₄)₂], in which a cursory X-ray diffraction investigation suggest a similarity with brackebuschite (Williams, 1973).

Other lead minerals related to the brackebuschite group include fornacite [CuPb₂(CrO₄)(AsO₄)(OH)] (Cocco et al., 1967; Fanfani & Zanazzi, 1967), molybdofornacite [CuPb₂(MoO₄)(AsO₄)(OH)] (Medenbach et al., 1983), and vauquelenite [CuPb₂(CrO₄)(PO₄)(OH)] (Fanfani & Zanazzi, 1968). These minerals demonstrate a richness to the crystallography of the group because the first two are described in space group P2₁/c with doubled c-cell edge, while the last one is described in P2₁/n and exhibits doubling of both the a- and c-cell edges (Fanfani & Zanazzi, 1967).

In addition to the lead minerals, the brackebuschite group of minerals also includes bearthite [Ca₂Al(PO₄)₂OH] (Chopin et al., 1993), canosioite [Ba₂Fe³⁺(AsO₄)₂(OH)] (Hålenius et al., 2015), gamagarite [Ba₂Fe³⁺(VO₄)₂(OH)] (de Villiers et al., 1943; Basso et al., 1987), tokyoite [Ba₂Mn³⁺(VO₄)₂(OH)] (Matsubara et al., 2004), goedkenite [Sr₂Al(PO₄)₂(OH)] (Moore et al., 1975), and grandaite [Sr₂Al(AsO₄)₂(OH)] (Cámara et al., 2014).

In the course of characterizing minerals for the RRUFF Project (https://rruff.info), we were able to isolate a single-crystal of brackebuschite from the type locality Sierra de Cordoba (Argentina), with composition Pb_1.91(Mn³⁺_0.81Fe³⁺_0.07Zn_0.07)_Σ=0.95(V_1.98As_0.02) _Σ=2.00O_8.00(OH)_1.00. The ratio Mn:Fe varies from grain to grain as shown in Fig. 1, where the colour of the crystals range from light-brown (Mn-rich, this crystal) to dark-brown [Fe-rich, (Mn³⁺_0.43Fe³⁺_0.42Zn_0.10)_Σ=0.95]. González del Tánago et al. (2003) suggested that brackebuschite [Pb₂Mn³⁺(VO₄)₂(OH)] and calderónite [Pb₂Fe³⁺(VO₄)₂(OH)] probably form a complete solid solution, as both phases have been found to coexist on a single specimen.

The crystal structure of brackebuschite was first determined by Donaldson & Barnes (1955) in space group B2₁/m assuming a chemical formula Pb₂Mn²⁺(VO₄)₂·H₂O. Foley et al. (1997) redefined its structure in space group P2₁/m and revised its composition to the currently accepted Pb₂Mn³⁺(VO₄)₂(OH). Structure refinement of the latter converged at a reliability factor R1 of 0.056 and was based on anisotropic displacement parameters for all non-O atoms [note that the deposited data in the Inorganic Crystal Structure Database (ICSD, 2016), entry #89256, report only isotropic displacement parameters], and the H atom undetermined. In the current work, all non-hydrogen atoms were refined with anisotropic displacement parameters, and the H atom was located, leading to a significant improvement of accuracy and precision, and to an unambiguous hydrogen bonding scheme.

The structure of brackebuschite is characterized by a distorted cubic closest-packed array of O and Pb atoms along [101] as stacking direction. Infinite chains of edge-sharing [Mn³⁺O₆] o­cta­hedra decorated by V1O₄ and V2O₄ tetra­hedra are aligned parallel to [010], perpendicular to the stacking direction. Mn³⁺, located on an inversion centre, is coordinated by the oxygen anions belonging to VO₄ tetra­hedra (O3×2 and O5×2) and the hydroxyl group (O7×2) in an overall distorted o­cta­hedral arrangement. Each V1O₄ tetra­hedron is linked to the ¹_∞[MnO_4/2O_2/1] chain by one common vertex (O3) while each V2O₄ links two adjacent o­cta­hedra by sharing two vertices (O5) with them. The V1O₄ groups and the H-atoms alternate belong to the edge-sharing atoms and are arranged along one side of the ¹_∞[MnO_4/2O_2/1] chain. The thus resulting ¹_∞[Mn³⁺(VO₄)₂OH] chains are held together by irregular [Pb1O₁₁] and [Pb2O₈] polyhedra (Fig. 2).

Looking down the axis of the [Mn³⁺(VO₄)₂OH] chain, there is a radial increase in the amplitude of the displacement parameters. We inter­pret this to indicate that the entire chain is undergoing rigid-body libration, oscillating to and fro along its axis. The radial change in amplitude is indicated by three concentric rings in Fig. 3a. The average amplitudes of the inner, middle, and outer rings (1.34, 2.00, and 4.06 Å, respectively) increase roughly linearly with the radial distance from the chain axis.

Bond-valence calculations (Brown, 2002) confirm that O7 corresponds to the hydroxyl group (bond-valence sum of 1.25 valence units), which is approximately tetra­hedrally coordinated by three cations and O2 (bond-valence sum of 1.61 v.u.) with which it forms an almost linear hydrogen bond (Table 1). The Raman spectrum of brackebuschite (Fig. 4) shows a broad band around 3145 cm⁻¹ that is assigned to the OH-stretching vibration (υ_OH). According to the correlation of O—H IR stretching frequencies and O—H···O hydrogen-bond lengths in minerals (Libowitzky, 1999), the stretching frequency inferred from this bond-length is 3143 cm⁻¹.

The O2 atom, the one associated as the acceptor atom of the hydrogen bond, displays quite large anisotropic displacement parameters relative to the other atoms (Fig. 3b). The disk-shaped ellipsoid is oriented parallel to the weaker Pb2—O bond and perpendicular to the stronger V1—O bond, which constrains the thermal vibration in that direction.

Synthesis and crystallization

\ The natural brackebuschite specimen used in this study is from the type locality Sierra de Cordoba, Argentina, and is in the collection of the RRUFF project (https://rruff.info/R050547). The chemical composition of the brackebuschite specimen was determined with a CAMECA SX100 electron microprobe operated at 20 kV and 20 nA. Seven analysis points yielded an average composition (wt. %): PbO 60.8 (4), V₂O₅ 25.6 (2), Mn₂O₃ 9.1 (5), Fe₂O₃ 0.8 (5), ZnO 0.8 (2), and As₂O₅ 0.33 (8), with H₂O 1.28 estimated to provide one H₂O molecule per formula unit. The empirical chemical formula, based on nine oxygen atoms, is Pb_1.91(Mn³⁺_0.81Fe³⁺_0.07Zn_0.07)_Σ=0.95(V_1.98As_0.02)\ _Σ=2.00O_8.00(OH)_1.00.

Refinement details

Crystal data, data collection and structure refinement details are summarized in Table 2. Electron microprobe analysis revealed that the brackebuschite sample studied here contains small amounts of Fe, Zn and As. However, the structure refinements, with and without a minor contribution of these elements in the o­cta­hedral and tetra­hedral sites, did not produce any significant differences in terms of reliability factors or displacement parameters. Hence, the ideal chemical formula Pb₂Mn³⁺(VO₄)₂(OH) was assumed during the refinement. The H atom was located from difference Fourier syntheses and its position refined with a fixed isotropic displacement parameter (U_iso = 0.03), and soft DFIX constraint of 0.9 Å from O7. The maximum residual electron density in the difference Fourier map, 2.66 e Å⁻³, was located at (0.6661, 0.1681, 0.5521), 0.66 Å from Pb1 and the minimum, −2.16 e Å⁻³, at (0.7036, 1/4, 0.5714), 0.41 Å from Pb1.

Computing details

Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT (Bruker, 2004); program(s) used to solve structure: coordinates taken from a previous refinement; program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: XtalDraw (Downs & Hall-Wallace, 2003); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

	Fig. 1. Photograph of the brackebuschite specimen analysed in this study.
	Fig. 2. Crystal structure of brackebuschite. The edge-sharing [MnO6] octahedra (green) form chains parallel to [010] with V1O4 and V2O4 tetrahedra (purple and violet, respectively) linked to them. These chains are held together by Pb1 and Pb2 (orange and yellow ellipsoids, respectively). Blue spheres of arbitrary radius represent H atoms.
	Fig. 3. (a) View of the ^1∞[Mn^3+(VO4)2OH] chain looking down [010]. The black rings represent different radii that correlate with the progressive increase of the libration component of oxygen atoms further away from the centre of the chain. Purple and violet tetrahedra and green octahedra represent V1O4, V2O4 and [MnO6], respectively. Red ellipsoids represent O atoms; (b) large O2 disk-shaped ellipsoid oriented perpendicular to the V1—O bond (anisotropic displacement ellipsoids at the 99% probability level). Note the hydrogen bond O2···H—O7 (dashed lines). Purple, yellow and red ellipsoids represent V1O4, [Pb2O8] and O atoms, respectively. The blue sphere represents the H atom.
	Fig. 4. Raman spectrum of brackebuschite. The weak Raman band around 3145 cm^−1 is assigned to the OH stretching vibrations associated with the OH group (νOH).

Dilead(II) manganese(III) vanadate(V) hydroxide

Crystal data

Pb2Mn(VO4)2(OH)	F(000) = 616
Mr = 716.21	Dx = 6.143 Mg m^−3
Monoclinic, P21/m	Mo Kα radiation, λ = 0.71073 Å
a = 7.6492 (1) Å	Cell parameters from 4222 reflections
b = 6.1262 (1) Å	θ = 2.5–32.3°
c = 8.9241 (2) Å	µ = 47.27 mm^−1
β = 112.195 (1)°	T = 293 K
V = 387.20 (1) Å^3	Tabular, light-brown
Z = 2	0.05 × 0.05 × 0.05 mm

Data collection

Bruker APEXII CCD area-detector diffractometer	1356 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.037
φ and ω scan	θmax = 32.6°, θmin = 2.9°
Absorption correction: multi-scan (SADABS; Bruker, 2004)	h = −11→11
Tmin = 0.201, Tmax = 0.201	k = −9→9
11674 measured reflections	l = −13→13
1521 independent reflections

Refinement

Refinement on F^2	Hydrogen site location: difference Fourier map
Least-squares matrix: full	Only H-atom coordinates refined
R[F^2 > 2σ(F^2)] = 0.025	w = 1/[σ^2(Fo^2) + (0.0195P)^2 + 3.6132P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.056	(Δ/σ)max < 0.001
S = 1.05	Δρmax = 2.67 e Å^−3
1521 reflections	Δρmin = −2.16 e Å^−3
82 parameters	Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
1 restraint	Extinction coefficient: 0.00067 (18)

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.

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

	x	y	z	Uiso*/Ueq
Pb1	0.32423 (5)	−0.2500	0.39735 (4)	0.02766 (9)
Pb2	−0.25814 (4)	−0.2500	0.25617 (4)	0.02176 (8)
Mn	0.0000	0.0000	0.0000	0.00476 (15)
V1	0.55901 (14)	0.7500	0.82401 (12)	0.00821 (18)
V2	0.96053 (14)	0.7500	0.66182 (12)	0.00864 (17)
O1	0.4927 (5)	0.9756 (6)	0.7041 (4)	0.0167 (6)
O2	0.4546 (8)	0.7500	0.9583 (8)	0.0304 (13)
O3	0.8084 (6)	0.7500	0.9407 (6)	0.0124 (8)
O4	0.7301 (8)	0.7500	0.5441 (7)	0.0256 (12)
O5	0.0115 (5)	0.9882 (5)	0.7801 (4)	0.0147 (6)
O6	0.0767 (8)	0.7500	0.5377 (7)	0.0249 (12)
O7	0.1837 (6)	0.7500	0.0814 (6)	0.0102 (8)
H	0.268 (11)	0.7500	0.034 (11)	0.030*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Pb1	0.03242 (17)	0.02685 (15)	0.03049 (18)	0.000	0.01957 (14)	0.000
Pb2	0.02570 (15)	0.01980 (13)	0.02074 (14)	0.000	0.00988 (11)	0.000
Mn	0.0067 (3)	0.0032 (3)	0.0038 (3)	0.0001 (3)	0.0013 (3)	−0.0001 (3)
V1	0.0071 (4)	0.0080 (4)	0.0093 (4)	0.000	0.0029 (3)	0.000
V2	0.0111 (4)	0.0079 (4)	0.0073 (4)	0.000	0.0039 (4)	0.000
O1	0.0161 (15)	0.0130 (14)	0.0175 (17)	0.0020 (12)	0.0023 (13)	0.0045 (12)
O2	0.023 (3)	0.048 (4)	0.029 (3)	0.000	0.019 (3)	0.000
O3	0.0056 (18)	0.0109 (18)	0.016 (2)	0.000	−0.0019 (16)	0.000
O4	0.016 (2)	0.039 (3)	0.015 (3)	0.000	−0.001 (2)	0.000
O5	0.0274 (17)	0.0111 (13)	0.0077 (14)	−0.0031 (12)	0.0089 (13)	−0.0026 (11)
O6	0.033 (3)	0.028 (3)	0.025 (3)	0.000	0.024 (2)	0.000
O7	0.0085 (18)	0.0111 (18)	0.012 (2)	0.000	0.0054 (16)	0.000

Geometric parameters (Å, º)

Pb1—O1^i	2.563 (3)	Pb2—O6^ii	2.830 (6)
Pb1—O7^ii	2.611 (5)	Pb2—O3^vi	3.045 (5)
Pb1—O6^ii	2.635 (5)	Mn—O5^iv	1.999 (3)
Pb1—O4^ii	2.878 (5)	Mn—O7^vii	2.019 (3)
Pb1—O1^ii	2.898 (4)	Mn—O3^i	2.046 (3)
Pb1—O5^iii	2.933 (4)	V1—O2	1.673 (6)
Pb1—O4^i	3.1610 (15)	V1—O1	1.703 (3)
Pb2—O1^iii	2.579 (3)	V1—O3	1.795 (4)
Pb2—O5^iv	2.588 (3)	V2—O6^viii	1.661 (5)
Pb2—O4^v	2.605 (6)	V2—O4	1.677 (5)
Pb2—O2^vi	2.735 (6)	V2—O5^viii	1.756 (3)
O5^iv—Mn—O5^ix	180.0	O3^i—Mn—O3^vi	180.0 (3)
O5^iv—Mn—O7^vii	92.45 (16)	O2—V1—O1	109.90 (17)
O5^ix—Mn—O7^vii	87.55 (16)	O1—V1—O1^x	108.4 (3)
O5^ix—Mn—O7^ii	92.45 (16)	O2—V1—O3	106.0 (3)
O7^vii—Mn—O7^ii	180.0 (2)	O1—V1—O3	111.31 (14)
O5^iv—Mn—O3^i	90.67 (17)	O6^viii—V2—O4	106.4 (3)
O5^ix—Mn—O3^i	89.33 (17)	O6^viii—V2—O5^viii	110.34 (16)
O7^vii—Mn—O3^i	81.87 (12)	O4—V2—O5^viii	108.58 (16)
O7^ii—Mn—O3^i	98.13 (12)	O5^viii—V2—O5^xi	112.4 (2)
O5^iv—Mn—O3^vi	89.33 (17)

Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) x, y−1, z; (iii) −x, y−3/2, −z+1; (iv) −x, −y+1, −z+1; (v) x−1, y−1, z; (vi) x−1, y−1, z−1; (vii) −x, −y+1, −z; (viii) x+1, y, z; (ix) x, y−1, z−1; (x) x, −y+3/2, z; (xi) x+1, −y+3/2, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O7—H···O2^xii	0.89 (2)	1.80 (2)	2.686 (7)	174 (10)

Symmetry code: (xii) x, y, z−1.