Redetermination of clinobarylite, BaBe2Si2O7

Clinobarylite, ideally BaBe2Si2O7 (chemical name barium diberyllium disilicate), is a sorosilicate mineral and dimorphic with barylite. It belongs to a group of compounds characterized by the general formula BaM 2+ 2Si2O7, with M 2+ = Be, Mg, Fe, Mn, Zn, Co, or Cu, among which the Be-, Fe-, and Cu-members have been found in nature. The crystal structure of clinobarylite has been re-examined in this study based on single-crystal X-ray diffraction data collected from a natural sample from the type locality (Khibiny Massif, Kola Peninsula, Russia). The structure of clinobarylite can be considered as a framework of BeO4 and SiO4 tetrahedra, with one of the O atoms coordinated to two Be and one Si, one coordinated to two Si, and two O atoms coordinated to one Si and one Be atom. The BeO4 tetrahedra share corners, forming chains parallel to the c axis, which are interlinked by the Si2O7 units oriented parallel to the a axis. The Ba2+ cations (site symmetry m..) are in the framework channels and are coordinated by eleven O atoms in form of an irregular polyhedron. The Si—Obr (bridging O atom, at site symmetry m..) bond length, the Si—Onbr (non-bridging O atoms) bond lengths, and the Si—O—Si angle within the Si2O7 unit are in marked contrast to the corresponding values determined in the previous study [Krivovichev et al. (2004 ▶). N. Jb. Miner. Mh. pp. 373–384].

Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Xtal-Draw (Downs & Hall-Wallace, 2003); software used to prepare material for publication: publCIF (Westrip, 2010  . It belongs to a group of compounds characterized by the general formula BaM 2+ 2 Si 2 O 7 , with M 2+ = Be, Mg, Fe, Mn, Zn, Co, or Cu (Yang et al., 2012). In addition to the Be-member, the Fe-and Cu-members of this group have also been found in nature, and are known as andrémeyerite and scottyite, respectively. The BaM 2 Si 2 O 7 compounds have been the subject of numerous investigations for scientific and industrial interests. For example, the materials with M = Be, Mg, and Zn are suitable hosts for luminescent activating ions. In particular, Pb 2+ -doped BaBe 2 Si 2 O 7 is used commercially as an UV-emitting material in moth-killing lamps and (Eu 2+ /Mn 2+ )-doped BaMg 2 Si 2 O 7 is a deep-red luminescent emitter through effective energy transfers from Eu 2+ to Mn 2+ (Barry, 1970;Yao et al., 1998). Moreover, the compounds with M = Cu, Co, and Mn are ideal prototypical quasi-one-dimensional quantum spin (S = 1/2, 3/2, and 5/2, respectively) Heisenberg antiferromagnets with adjustable superexchange interactions, vital for our understanding of high-Tc superconductivity (e.g., Adams & Layland, 1996;Lu et al., 2000;Yamada et al., 2001;Ohta et al., 2004;Bertaina & Hayn, 2006;Zvyagin, 2006;Zheludev et al., 2007). However, Krivovichev et al. (2004) examined the monoclinic structure reported by  and noted that a shift of 0.0088 Å along b in all the atomic positions would result in a change of symmetry from monoclinic Pm to orthorhombic Pmn2 1 . They subsequently collected single-crystal X-ray diffraction data from a new sample, and found that systematic intensity absences were consistent with space group Pmn2 1 and a racemic twinning model (R1 = 0.030). Yet, they were unable to obtain positive definite anisotropic displacement parameters for Be and O atoms, and attributed that to the effect of the dominant Ba scattering factor. The resulting geometric parameters, such as bond lengths and angles, matched those obtained from the Pm structural model determined by . An examination of the clinobarylite structure reported by Krivovichev et al. (2004), nevertheless, reveals a rather peculiar feature: The Si-O br (bridging O atom) distance (1.597 Å) is significantly shorter than the Si-O nbr (non-bridging O atoms) distances (1.619-1.631 Å). This contradicts the previous observations specifically for disilicate compounds (e.g., Lin et al., 1999;Fleet & Liu, 2001;Kolitsch et al., 2009), including all other compounds in the BaM 2+ 2 Si 2 O 7 group (Yang et al., 2012). The present study was conducted to clarify this controversy. Si-O, Be-O, and Ba-O bond lengths in clinobarylite are 1.630, 1.941, and 2.825 Å, respectively. Our study confirmed the space group Pmn2 1 for clinobarylite, as determined by Krivovichev et al. (2004), but revealed different bond length from those given by Krivovichev et al. (2004): The Si-O br bond length [1.6566 (10) Å] is, in fact, substantially longer than the Si-O nbr bond lengths [1.6065 (13) -1.6315 (14) Å], in marked contrast to the corresponding values of 1.597 (4) Å, 1.619 (6)-1.631 (7) Å as reported by Krivovichev et al. (2004). Moreover, the Si-O br -Si angle within the Si 2 O 7 disilicate unit from our study is 128.82 (13)°, in contrast to 138.5° (Krivovichev et al., 2004).
The Raman spectrum of clinobarylite is plotted in Fig. 3, along with that of barylite (R060620 from the RRUFF Project) for comparison. Evidently, the two Raman spectra are quite similar. In general, they can be divided into four regions.
Region 1, between 800 and 1100 cm -1 , contains bands attributable to the Si-O symmetric and anti-symmetric stretching vibrations (ν 1 and ν 3 modes) within the SiO 4 tetrahedra. Region 2, between 660 and 700 cm -1 , includes bands resulting from the Si-O br -Si bending vibrations within the Si 2 O 7 tetrahedral dimers. Major bands in region 3, ranging from 420 to 660 cm -1 , are ascribed to the O-Si-O symmetric and anti-symmetric bending vibrations (ν 2 and ν 4 modes) within the SiO 4 tetrahedra. The bands in region 4, below 420 cm -1 , are mainly associated with the rotational and translational modes of SiO 4 tetrahedra, as well as the Be-O interactions and lattice vibrational modes. Fig. 3 is that the wavenumbers of the bands due to the Si-O br -Si bending mode for barylite and clinobarylite are nearly identical (~685 cm -1 ), indicating that the Si-O br bond lengths and the Si-O br -Si angles in these two minerals are rather comparable. This is indeed the case. The Si-O br distance and the Si-O br -Si angle are 1.657 Å and 128.59°, respectively, in barylite (Robinson & Fang, 1977), and 1.6566 (10) Å and 128.82 (13)° in clinobarylite.

Experimental
The clinobarylite sample used in this study is from the type locality Yukspor Mountain, Khibiny  The Raman spectrum of clinobarylite was collected from a randomly oriented crystal at 100% power on a Thermo Almega microRaman system, using a solid-state laser with a wavelength of 532 nm, and a thermoelectrically cooled CCD detector. The laser is partially polarized with 4 cm -1 resolution and a spot size of 1 µm.

Refinement
For better comparison with the previous determination of barylite (Krivovichev et al., 2004), the same atom numbering was used, along with the given coordinates as starting parameters for the subsequent refinement. An inversion twin was introduced in the refinement, giving a twin ratio of 0.502 (12):0.498 (12). All atoms were refined with anisotropic displacement parameters. The ideal chemistry, BaBe 2 Si 2 O 7 , was assumed during the final refinement. The highest residual peak (0.47 e -/Å 3 ) in the difference Fourier maps was located at (0, 0.7651, 0.2479), 0.75 Å from Ba, and the deepest hole (-0.42 e -/Å 3 ) at (0, 0.3845, 0.0984), 1.75 Å from O4. 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).    Raman Spectrum of clinobarylite, along with that of barylite for comparison. The spectra are shown with vertical offset for more clarity.  (12) 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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.