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

The crystal structure of brackebuschite, ideally Pb2Mn3+(VO4)2(OH), 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 lowering of the reliability factors.

1 1 [Mn 3+ (VO 4 ) 2 OH] 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 1 [Mn 3+ (VO 4 ) 2 OH] 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 Pb 2+ cation with two Mn 3+ cations and forms an almost linear hydrogen bond with a vanadate group of a neighbouring chain. The OÁ Á ÁO distance of this interaction determined from the structure refinement agrees well with Raman spectroscopic data.
In the course of characterizing minerals for the RRUFF Project (http://rruff.info), we were able to isolate a single crystal of brackebuschite from the type locality Sierra de  The crystal structure of brackebuschite was first determined by Donaldson & Barnes (1955) in space group B2 1 /m assuming a chemical formula Pb 2 Mn 2+ (VO 4 ) 2 ÁH 2 O. Foley et al. (1997) redefined its structure in space group P2 1 /m and revised its composition to the currently accepted Pb 2 Mn 3+ (VO 4 ) 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 nonhydrogen 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. Photograph of the brackebuschite specimen analysed in this study.

Figure 2
Crystal structure of brackebuschite. The edge-sharing [MnO 6 ] octahedra 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 À1 that is assigned to the OH-stretching vibration ( OH ). According to the correlation of O-H IR stretching frequencies and O-HÁ Á ÁO hydrogen-bond lengths in minerals (Libowitzky, 1999), the stretching frequency inferred from this bond-length is 3143 cm À1 .
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) (7) 174 (10) Symmetry code: (i) x; y; z À 1.

Figure 4
Raman spectrum of brackebuschite. The weak Raman band around 3145 cm À1 is assigned to the OH stretching vibrations associated with the OH group ( OH ).

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 Pb 2 Mn 3+ (VO 4 ) 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.