Redetermination of metarossite, CaV5+ 2O6·2H2O

The redetermination of metarossite, CaV5+ 2O6·2H2O, based on modern single-crystal diffraction data confirms the previous study based on precession photographs, however, with the H atoms located and all atoms refined with anisotropic displacement parameters.

The crystal structure of metarossite, ideally CaV 2 O 6 Á2H 2 O [chemical name: calcium divanadium(V) hexaoxide dihydrate], was first determined using precession photographs, with fixed isotropic displacement parameters and without locating the positions of the H atoms, leading to a reliability factor R = 0.11 [Kelsey & Barnes (1960). Can. Mineral. 6,[448][449][450][451][452][453][454][455][456][457][458][459][460][461][462][463][464][465][466]. This communication reports a structure redetermination of this mineral on the basis of single-crystal X-ray diffraction data of a natural sample from the Blue Cap mine, San Juan County, Utah, USA (R1 = 0.036). Our study not only confirms the structural topology reported in the previous study, but also makes possible the refinement of all non-H atoms with anisotropic displacement parameters and all H atoms located. The metarossite structure is characterized by chains of edge-sharing [CaO 8 ] polyhedra parallel to [100] that are themselves connected by chains of alternating [VO 5 ] trigonal bipyramids parallel to [010]. The two H 2 O molecules are bonded to Ca. Analysis of the displacement parameters show that the [VO 5 ] chains librate around [010]. In addition, we measured the Raman spectrum of metarossite and compared it with IR and Raman data previously reported. Moreover, heating of metarossite led to a loss of water, which results in a transformation to the brannerite-type structure, CaV 2 O 6 , implying a possible dehydration pathway for the compounds M 2+ V 2 O 6 ÁxH 2 O, with M = Cu, Cd, Mg or Mn, and x = 2 or 4.
This study reports the refinement of the structure of a metarossite sample (Fig. 1) from the Blue Cap mine, San Juan County, Utah, USA, with anisotropic displacement para- ISSN 2056-9890 meters for all non-hydrogen atoms, positions of hydrogen atoms determined, and improvement of the reliability factor to 0.036. Raman spectra were also recorded and compared with that reported in the two studies by Frost et al. (2004Frost et al. ( , 2005, on a sample from the Burro mine, San Miguel County of Colorado, USA.
It is interesting to note that there is a radial orientation of the displacement ellipsoids associated with the [VO 5 ] chains when viewed along the chain direction (Fig. 3). The amplitude also slightly radially increases, as indicated by the black dashed circles in Fig. 3. We interpret this as the oscillation or libration of the [VO 5 ] chains around [010]. A similar behavior was reported for brackebuschite Pb 2 Mn 3+ (VO 4 ) 2 (OH) (Lafuente & Downs, 2016) where the [Mn 3+ (VO 4 ) 2 OH] chains oscillating about an axis.
Numerical data of the hydrogen-bonding scheme in metarossite are presented in Table 1. The bond-valence calculations (Brown, 2002) with the parameters given by Brese & O'Keeffe (1991) confirm that OW3 and OW8 correspond to the two H 2 O molecules ( Table 2). The low bond-valence sum for O5 is because it is an acceptor for three hydrogen atoms (H2, H3 and H4; Table 1). In fact, all acceptor O atoms involved in hydrogen bonding are from VO 5 polyhedra,   Photograph of the metarossite specimen analyzed in this study. Table 1 Hydrogen-bond geometry (Å , ).

Raman spectrum
The Raman spectrum of metarossite ( Fig. 4) is comparable with the data recorded by Frost et al. (2005) below 1000 cm À1 , but is different in the O-H stretching region between 2800 and 3700 cm À1 (Frost et al., 2004). Indeed, they recorded only three Raman bands (at 3177, 3401 and 3473 cm À1 ), whereas with the present data, it is possible to distinguish four to five bands depending on the orientation (2904, 2954, 3189, 3240 and 3398 cm À1 ), along with a broad shoulder around 3415-3480 cm À1 (Fig. 4). According to Libowitzky (1999), the band at 3398 cm À1 can be attributed to the OW8-H4 vibration, and the broad shoulder around 3415-3480 cm À1 may correspond to the OW8-H3 and OW3-H2 vibrations ( Table 3). The last vibration (OW3-H1) cannot be seen on Fig. 4, but since the frequency currently accepted for free OH À ion is 3560 cm À1 (Lutz, 1995), it can be associated with the IR band at 3526 cm À1 observed by Frost et al. (2004).

Synthesis and crystallization
The natural sample used in this study is from the Blue Cap mine, San Juan County of Utah, USA ( Fig. 1) Table 3 OÁ Á ÁO measured distances (Å ), Raman stretching frequencies (cm À1 ) calculated using the correlation for d < 3.2 Å and samples without Cu (Libowitzky, 1999), and comparison with OÁ Á ÁO calculated by Frost et al. (2004)

Figure 4
Raman spectrum of metarossite collected with a 532 nm laser. Only the band at 3400 cm À1 can be clearly assigned to hydrogen stretching vibrations (OW8-H4) but the broad shoulder discernible around 3415-3480 cm À1 corresponds probably to OW8-H3 and OW3-H2 vibrations.
partially polarized with 4 cm À1 resolution and a spot size of 1 mm.

Transformation of metarossite
When a small piece of metarossite (edge length in all dimensions 0.1 mm) was placed under a full power laser (150 mW, 532 nm), a change in its Raman spectrum was observed (Fig. 5). In particular, all bands originating from O-H stretching vibrations disappeared, suggesting a complete dehydration of the sample. Moreover, the spectrum below 1200 cm À1 was found to match that of synthetic CaV 2 O 6 ( Baran et al., 1987). In addition, we observed similar Raman spectra collected from a metarossite fragment that was heated in air in an oven at 373 K for 12 h. Single crystal X-ray diffraction analysis on the heated crystal revealed monoclinic symmetry with unit cell parameters a = 10.0 (1), b = 3.6 (2), c = 6.9 (6) Å , = 105 (6) , which match those reported for brannerite (Szymanski & Scott, 1982). However, we were unable to obtain more detailed structure information for the heated sample due to its poor crystallinity (caused probably by dehydration). A number of synthetic metavanadates, such as those with formula M 2+ V 2 O 6 where M = Cu, Cd, Mg or Mn, are found to be isostructural with brannerite (Baran et al., 1987;Mü ller-Buschbaum & Kobel, 1991). There are also many hydrated forms of these compounds, including synthetic CuV 2 O 6 Á2H 2 O (Leblanc & Ferey, 1990), and CdV 2 O 6 Á2H 2 O (Ulická , 1988), as well as natural dickthomssenite MgV 2 O 6 Á7H 2 O (Hughes et al., 2001) or ansermetite MnV 2 O 6 Á4H 2 O (Brugger et al., 2003). Because tetrahydrated or dihydrated forms of these materials have structures related to rossite or metarossite, it is likely, then, that natural equivalents of the synthetic metavanadates

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 4. The electron microprobe analysis revealed traces of Sr in our sample. The empirical formula shows a little deficiency for Ca and excess for V. For simplicity, the ideal chemical formula CaV 2 O 6 Á2H 2 O was assumed during the refinement. Kelsey & Barnes (1960) underline that {101} is often a twin-plane in metarossite, but the crystal used for this X-ray analysis did not show twinning. Atomic coordinates of the previous study were taken as starting parameters for refinement. The H atoms were located from difference Fourier syntheses and their positions refined with fixed isotropic displacement parameters (U iso = 0.04 Å 2 ). The maximum residual electron density in the difference Fourier maps was located at 0.86 Å from O7 and the minimum density at 1.39 Å from Ca. Only H-atom coordinates refined Á max , Á min (e Å À3 ) 0.77, À0.58 Computer programs: APEX2 and SAINT (Bruker, 2004), SHELXT (Sheldrick, 2015a), SHELXL2014 (Sheldrick, 2015b), XtalDraw (Downs & Hall-Wallace, 2003) and publCIF (Westrip, 2010).

Figure 5
Raman spectrum of metarossite after heated by a full power 532 nm laser (red curve) and comparison with an initial metarossite spectrum (black curve).

Calcium vanadium (V) oxide dihydrate
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )