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Redetermination of conichalcite, CaCu(AsO4)(OH)

aDepartment of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721-0077, USA
*Correspondence e-mail: hyang@u.arizona.edu

(Received 2 July 2008; accepted 29 July 2008; online 6 August 2008)

The crystal structure of conichalcite [calcium copper(II) arsenate(V) hydroxide], with ideal formula CaCu(AsO4)(OH), was redetermined from a natural twinned specimen found in the Maria Catalina mine (Chile). In contrast to the previous refinement from photographic data [Qurashi & Barnes (1963[Qurashi, M. M. & Barnes, W. H. (1963). Can. Mineral. 7, 561-577.]). Can. Mineral. 7, 561–577], all atoms were refined with anisotropic displacement parameters and with the H atom located. Conichalcite belongs to the adelite mineral group. The Jahn–Teller-distorted [CuO6] octa­hedra share edges, forming chains running parallel to [010]. These chains are cross-linked by eight-coordinate Ca atoms and by sharing vertices with isolated AsO4 tetra­hedra. Of five calcium arsenate minerals in the adelite group, the [MO6] (M = Cu, Zn, Co, Ni and Mg) octa­hedron in conichalcite is the most distorted, and the donor–acceptor O—H⋯O distance is the shortest.

Related literature

For background on the adelite mineral family, see: Qurashi & Barnes (1963[Qurashi, M. M. & Barnes, W. H. (1963). Can. Mineral. 7, 561-577.], 1964[Qurashi, M. M. & Barnes, W. H. (1964). Can. Mineral. 8, 23-39.]); Qurashi et al. (1953[Qurashi, M. M., Barnes, W. H. & Berry, L. G. (1953). Am. Mineral. 38, 557-559.]). For structure refinements in the adelite group, see: Effenberger et al. (2002[Effenberger, H., Krause, W. & Bernhardt, H. J. (2002). Exp. Miner. Petrol. Geochem. Abstr. 9, 30.]) for adelite, CaMgAsO4(OH); Clark et al. (1997[Clark, L. A., Pluth, J. J., Steele, I., Smith, J. V. & Sutton, S. R. (1997). Mineral. Mag. 61, 677-683.]) and Giuseppetti & Tadini (1988[Giuseppetti, G. & Tadini, C. (1988). Neues Jahrb. Mineral. Monatsh. 1988, 159-166.]) for austinite, CaZnAsO4(OH); Yang et al. (2007[Yang, H., Costin, G., Keogh, J., Lu, R. & Downs, R. T. (2007). Acta Cryst. E63, i53-i55.]) for cobaltaustinite, CaCoAsO4(OH); Cesbron et al. (1987[Cesbron, F., Ginderow, D., Giraud, R., Pelisson, P. & Pillard, F. (1987). Can. Mineral. 25, 401-407.]) for nickelaustinite, CaNiAsO4(OH). Correlations between O—H streching frequencies and O—H⋯O donor–acceptor distances are given by Libowitzky (1999[Libowitzky, E. (1999). Monatsh. Chem. 130, 1047-1059.]). Raman spectroscopic data on some minerals of the adelite group have been reported by Martens et al. (2003[Martens, W., Frost, R. L. & Williams, P. A. (2003). J. Raman Spectrosc. 34, 104-111.]); for general background, see: Robinson et al. (1971[Robinson, K., Gibbs, G. V. & Ribbe, P. H. (1971). Science, 172, 567-570.]).

Experimental

Crystal data
  • CaCu(AsO4)(OH)

  • Mr = 259.57

  • Orthorhombic, P 21 21 21

  • a = 7.3822 (2) Å

  • b = 5.8146 (2) Å

  • c = 9.2136 (3) Å

  • V = 395.49 (2) Å3

  • Z = 4

  • Mo Kα radiation

  • μ = 15.03 mm−1

  • T = 293 (2) K

  • 0.06 × 0.05 × 0.04 mm

Data collection
  • Bruker APEXII CCD diffractometer

  • Absorption correction: multi-scan (TWINABS; Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]) Tmin = 0.492, Tmax = 0.585 (expected range = 0.461–0.548)

  • 7088 measured reflections

  • 1602 independent reflections

  • 1487 reflections with I > 2σ(I)

  • Rint = 0.023

Refinement
  • R[F2 > 2σ(F2)] = 0.018

  • wR(F2) = 0.038

  • S = 1.03

  • 1602 reflections

  • 79 parameters

  • All H-atom parameters refined

  • Δρmax = 0.63 e Å−3

  • Δρmin = −0.49 e Å−3

  • Absolute structure: Flack (1983[Flack, H. D. (1983). Acta Cryst. A39, 876-881.]), 644 Friedel pairs

  • Flack parameter: 0.00 (2)

Table 1
Selected bond lengths (Å)

Ca—O5i 2.3626 (13)
Ca—O3ii 2.3995 (17)
Ca—O4iii 2.4818 (16)
Ca—O2iv 2.5178 (17)
Ca—O4v 2.5281 (16)
Ca—O1iv 2.5462 (14)
Ca—O3 2.5786 (17)
Ca—O2 2.6264 (17)
Cu—O5 1.8850 (15)
Cu—O5vi 1.8855 (16)
Cu—O1vi 2.0666 (16)
Cu—O1 2.0688 (15)
Cu—O3vii 2.2976 (15)
Cu—O4viii 2.3882 (14)
As—O4 1.6749 (16)
As—O3 1.6779 (16)
As—O2 1.6796 (16)
As—O1 1.7099 (13)
Symmetry codes: (i) [-x+{\script{1\over 2}}, -y+1, z-{\script{1\over 2}}]; (ii) [x+{\script{1\over 2}}, -y+{\script{3\over 2}}, -z]; (iii) x, y+1, z; (iv) [-x+1, y+{\script{1\over 2}}, -z+{\script{1\over 2}}]; (v) [x+{\script{1\over 2}}, -y+{\script{1\over 2}}, -z]; (vi) [-x, y-{\script{1\over 2}}, -z+{\script{1\over 2}}]; (vii) [x-{\script{1\over 2}}, -y+{\script{1\over 2}}, -z]; (viii) [-x+{\script{1\over 2}}, -y, z+{\script{1\over 2}}].

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA D—H⋯A
O5—H1⋯O2ix 0.86 (4) 1.91 (4) 2.678 (2) 149 (3)
Symmetry code: (ix) x-1, y, z.

Data collection: APEX2 (Bruker, 2003[Bruker (2003). SMART. Bruker AXS Inc., Madison, Wisconsin, USA.]); cell refinement: SAINT (Bruker, 2005[Bruker (2005). SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.]); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); molecular graphics: XtalDraw (Downs & Hall-Wallace, 2003[Downs, R. T. & Hall-Wallace, M. (2003). Am. Mineral. 88, 247-250.]); software used to prepare material for publication: SHELXTL (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]).

Supporting information


Comment top

Minerals of the adelite group crystallize with orthorhombic symmetry in space group P212121 (Qurashi & Barnes, 1963, 1964) and have a general chemical formula A+,2+M2+,3+(X4+,5+,6+O4)(OH), where A = Na, Ca, Pb, M = Al, Mg, Zn, Mn, Fe, Co, Cu, Ni, and X = Si, P, V, As. There are five calcium arsenates in this group: adelite CaMgAsO4(OH), austinite CaZnAsO4(OH), conichalcite CaCuAsO4(OH), nickelaustinite CaNiAsO4(OH), and cobaltaustinite, CaCoAsO4(OH). All structures of these calcium arsenate minerals have been determined previously (Qurashi & Barnes, 1963; Cesbron et al., 1987; Giuseppetti & Tadini, 1988; Clark et al., 1997; Effenberger et al., 2002; Yang et al., 2007). However, in our efforts to understand the relationships between the hydrogen bonding schemes and Raman spectra of hydrous minerals, we noted that the structural information of conichalcite needs to be improved, because this structure was refined by Qurashi & Barnes (1963) with X-ray intensity data collected by Qurashi et al. (1953) from precession photographs without anisotropic displacement parameters and localisation of the H atom position.

Conichalcite can be compared with the other Ca-arsenate minerals in the adelite group. The distorted [CuO6] octahedra (i.e. elongated tetragonal bipyramids) share edges to form chains running parallel to [010], which are cross-linked by Ca atoms and by sharing vertices with isolated AsO4 tetrahedra (Fig. 1). The principal difference among the five calcium arsenates in the group is manifested in the bonding environments around the octahedrally coordinated M cations. The average M—O bond lengths appear to decrease from <Zn—O> (= 2.106 Å) in austinite (Clark et al., 1997), to <Cu—O> (= 2.099 Å) in conichalcite, <Co—O> (= 2.092 Å) in cobaltaustinite (Yang et al., 2007), <Ni—O> (= 2.085 Å) in nickelaustinite (Cesbron et al., 1987), and to <Mg—O> (= 2.075 Å) in adelite (Effenberger et al., 2002). Of these [MO6] octahedra, the Cu-octahedron, due to its strong Jahn-Teller effect, displays the greatest distortion in terms of the tetragonal elongation and angle variance (Robinson et al., 1971), which are 1.0229 and 23.58, respectively.

The donor-acceptor O5—H···O2 distance in conichalcite is 2.678 (2) Å, which is the shortest of all five Ca-arsenates in the adelite group [2.723 (2) Å in austinite (Clark et al., 1997), 2.721 (7) Å in cobaltaustinite (Yang et al., 2007), 2.73 (1) Å in nickelaustinite (Cesbron et al., 1987), and 2.766 (2) Å in adelite (Effenberger et al., 2002)]. As the O—H stretching frequencies (νOH) increase with the O—H···O distance (Libowitzky,1999), we should expect the smallest νOH value for conichalcite and the largest for adelite among the five calcium arsenates in the adelite group. Indeed, the major νOH band positions determined from Raman spectra for conichalcite and adelite are, respectively, 3158 and 3550 cm-1 from Martens et al. (2003), or 3161 and 3423 cm-1 from the RRUFF project (http://rruff.info), with intermediate νOH values for the other three minerals (austinite, cobaltaustinite, and nickelaustinite).

Related literature top

For background on the adelite mineral family, see: Qurashi & Barnes (1963, 1964); Qurashi et al. (1953). For structure refinements in the adelite group, see: Effenberger et al. (2002) for adelite, CaMgAsO4(OH); Clark et al. (1997) and Giuseppetti & Tadini (1988) for austinite, CaZnAsO4(OH); Yang et al. (2007) for cobaltaustinite, CaCoAsO4(OH); Cesbron et al. (1987) for nickelaustinite, CaNiAsO4(OH). Correlations between O—H streching frequencies and O—H···O donor–acceptor distances are given by Libowitzky (1999). Raman spectroscopic data on some minerals of the adelite group have been reported by Martens et al. (2003); for general background, see: Robinson et al. (1971).

Experimental top

The conichalcite crystal used in this study is from Maria Catalina mine, Pampa Larga Mining District, Tierra Amarilla, Chile, and is a sample from the RRUFF project (deposition No. R070430; http//rruff.info). The chemical composition, Ca(Cu0.99Zn0.01)(AsO4)(OH), was determined with a CAMECA SX50 electron microprobe (http//rruff.info).

Refinement top

The final refinement assumed a full occupancy of the metal site by Cu only, as the overall effects of the trace amount of Zn on the final structure results are negligible. In the final stages of the refinement it turned out that the measured crystal was racemically twinned with an approximate twin fraction of 4:1 (BASF = 0.21). The H atom was located from difference Fourier maps and its position was refined freely. The highest residual peak in is located 1.60 Å from the H atom, and the deepest hole is 0.63 Å from the Ca atom.

Computing details top

Data collection: APEX2 (Bruker, 2003); cell refinement: SAINT (Bruker, 2005); data reduction: SAINT (Bruker, 2005); 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: SHELXTL (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. The crystal structure of conichalcite. Green octahedra, yellow tetrahedra, grey large sphares, and red small spheres represent [CuO6], [AsO4], Ca, and H, respectively. Hydrogen bonding is indicated with blue lines.
calcium copper(II) arsenate(V) hydroxide top
Crystal data top
CaCu(AsO4)(OH)F(000) = 492
Mr = 259.57Dx = 4.359 Mg m3
Orthorhombic, P212121Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2abCell parameters from 3371 reflections
a = 7.3822 (2) Åθ = 3.6–34.0°
b = 5.8146 (2) ŵ = 15.03 mm1
c = 9.2136 (3) ÅT = 293 K
V = 395.49 (2) Å3Euhedral, equant, green
Z = 40.06 × 0.05 × 0.04 mm
Data collection top
Bruker APEX2 CCD
diffractometer
1602 independent reflections
Radiation source: fine-focus sealed tube1487 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.023
ϕ and ω scansθmax = 34.0°, θmin = 3.5°
Absorption correction: multi-scan
(TWINABS; Sheldrick, 2008)
h = 911
Tmin = 0.492, Tmax = 0.585k = 99
7088 measured reflectionsl = 1414
Refinement top
Refinement on F2Hydrogen site location: difference Fourier map
Least-squares matrix: fullAll H-atom parameters refined
R[F2 > 2σ(F2)] = 0.018 w = 1/[σ2(Fo2) + (0.0151P)2 + 0.1227P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.038(Δ/σ)max < 0.001
S = 1.03Δρmax = 0.63 e Å3
1602 reflectionsΔρmin = 0.49 e Å3
79 parametersExtinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraintsExtinction coefficient: 0.0029 (5)
Primary atom site location: structure-invariant direct methodsAbsolute structure: Flack (1983), 644 Friedel pairs
Secondary atom site location: difference Fourier mapAbsolute structure parameter: 0.00 (2)
Crystal data top
CaCu(AsO4)(OH)V = 395.49 (2) Å3
Mr = 259.57Z = 4
Orthorhombic, P212121Mo Kα radiation
a = 7.3822 (2) ŵ = 15.03 mm1
b = 5.8146 (2) ÅT = 293 K
c = 9.2136 (3) Å0.06 × 0.05 × 0.04 mm
Data collection top
Bruker APEX2 CCD
diffractometer
1602 independent reflections
Absorption correction: multi-scan
(TWINABS; Sheldrick, 2008)
1487 reflections with I > 2σ(I)
Tmin = 0.492, Tmax = 0.585Rint = 0.023
7088 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.018All H-atom parameters refined
wR(F2) = 0.038Δρmax = 0.63 e Å3
S = 1.03Δρmin = 0.49 e Å3
1602 reflectionsAbsolute structure: Flack (1983), 644 Friedel pairs
79 parametersAbsolute structure parameter: 0.00 (2)
0 restraints
Special details top

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 (Å2) top
xyzUiso*/Ueq
Ca0.61727 (5)0.72961 (8)0.07340 (4)0.01133 (12)
Cu0.00416 (4)0.00002 (6)0.25029 (4)0.00898 (8)
As0.36728 (2)0.26438 (4)0.08118 (2)0.00768 (6)
O10.18844 (17)0.2450 (3)0.19847 (15)0.0135 (3)
O20.5395 (2)0.3313 (3)0.19256 (18)0.0187 (4)
O30.3514 (2)0.4927 (3)0.02947 (17)0.0157 (3)
O40.3885 (2)0.0147 (3)0.00782 (15)0.0145 (4)
O50.13880 (18)0.2539 (3)0.31777 (14)0.0113 (3)
H10.229 (5)0.232 (6)0.260 (3)0.055 (11)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Ca0.01169 (18)0.0119 (2)0.01041 (18)0.00064 (17)0.00072 (13)0.00020 (18)
Cu0.00885 (11)0.00621 (12)0.01190 (12)0.00029 (9)0.00211 (8)0.00121 (8)
As0.00801 (8)0.00659 (9)0.00845 (9)0.00006 (8)0.00059 (7)0.00014 (9)
O10.0141 (6)0.0107 (7)0.0157 (6)0.0025 (7)0.0035 (5)0.0006 (7)
O20.0146 (7)0.0212 (9)0.0205 (8)0.0029 (6)0.0048 (6)0.0004 (7)
O30.0201 (7)0.0107 (7)0.0164 (7)0.0011 (7)0.0044 (7)0.0035 (6)
O40.0181 (8)0.0096 (7)0.0158 (8)0.0018 (6)0.0022 (7)0.0018 (6)
O50.0106 (5)0.0102 (6)0.0132 (6)0.0000 (8)0.0002 (5)0.0003 (6)
Geometric parameters (Å, º) top
Ca—O5i2.3626 (13)Cu—O5vi1.8855 (16)
Ca—O3ii2.3995 (17)Cu—O1vi2.0666 (16)
Ca—O4iii2.4818 (16)Cu—O12.0688 (15)
Ca—O2iv2.5178 (17)Cu—O3vii2.2976 (15)
Ca—O4v2.5281 (16)Cu—O4viii2.3882 (14)
Ca—O1iv2.5462 (14)As—O41.6749 (16)
Ca—O32.5786 (17)As—O31.6779 (16)
Ca—O22.6264 (17)As—O21.6796 (16)
Cu—O51.8850 (15)As—O11.7099 (13)
O5i—Ca—O3ii75.88 (5)O4v—Ca—O277.15 (5)
O5i—Ca—O4iii73.62 (5)O1iv—Ca—O279.00 (5)
O3ii—Ca—O4iii89.42 (5)O3—Ca—O261.06 (5)
O5i—Ca—O2iv151.07 (5)O5—Cu—O5vi177.68 (6)
O3ii—Ca—O2iv108.51 (6)O5—Cu—O1vi98.01 (6)
O4iii—Ca—O2iv77.79 (5)O5vi—Cu—O1vi84.26 (6)
O5i—Ca—O4v74.41 (5)O5—Cu—O184.21 (6)
O3ii—Ca—O4v76.55 (5)O5vi—Cu—O193.52 (6)
O4iii—Ca—O4v147.36 (3)O1vi—Cu—O1177.66 (7)
O2iv—Ca—O4v134.48 (5)O5—Cu—O3vii91.88 (6)
O5i—Ca—O1iv141.59 (5)O5vi—Cu—O3vii88.82 (6)
O3ii—Ca—O1iv73.13 (5)O1vi—Cu—O3vii84.84 (6)
O4iii—Ca—O1iv127.50 (5)O1—Cu—O3vii95.85 (6)
O2iv—Ca—O1iv62.85 (5)O5—Cu—O4viii84.76 (6)
O4v—Ca—O1iv76.76 (5)O5vi—Cu—O4viii94.77 (6)
O5i—Ca—O372.94 (5)O1vi—Cu—O4viii89.81 (6)
O3ii—Ca—O3147.75 (2)O1—Cu—O4viii89.67 (5)
O4iii—Ca—O374.21 (5)O3vii—Cu—O4viii173.23 (6)
O2iv—Ca—O395.19 (5)O4—As—O3113.27 (7)
O4v—Ca—O3102.41 (5)O4—As—O2115.43 (8)
O1iv—Ca—O3138.68 (5)O3—As—O2103.94 (8)
O5i—Ca—O2117.86 (6)O4—As—O1108.94 (8)
O3ii—Ca—O2145.22 (5)O3—As—O1112.45 (8)
O4iii—Ca—O2124.48 (5)O2—As—O1102.33 (7)
O2iv—Ca—O275.44 (3)
Symmetry codes: (i) x+1/2, y+1, z1/2; (ii) x+1/2, y+3/2, z; (iii) x, y+1, z; (iv) x+1, y+1/2, z+1/2; (v) x+1/2, y+1/2, z; (vi) x, y1/2, z+1/2; (vii) x1/2, y+1/2, z; (viii) x+1/2, y, z+1/2.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O5—H1···O2ix0.86 (4)1.91 (4)2.678 (2)149 (3)
Symmetry code: (ix) x1, y, z.

Experimental details

Crystal data
Chemical formulaCaCu(AsO4)(OH)
Mr259.57
Crystal system, space groupOrthorhombic, P212121
Temperature (K)293
a, b, c (Å)7.3822 (2), 5.8146 (2), 9.2136 (3)
V3)395.49 (2)
Z4
Radiation typeMo Kα
µ (mm1)15.03
Crystal size (mm)0.06 × 0.05 × 0.04
Data collection
DiffractometerBruker APEX2 CCD
diffractometer
Absorption correctionMulti-scan
(TWINABS; Sheldrick, 2008)
Tmin, Tmax0.492, 0.585
No. of measured, independent and
observed [I > 2σ(I)] reflections
7088, 1602, 1487
Rint0.023
(sin θ/λ)max1)0.787
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.018, 0.038, 1.03
No. of reflections1602
No. of parameters79
H-atom treatmentAll H-atom parameters refined
Δρmax, Δρmin (e Å3)0.63, 0.49
Absolute structureFlack (1983), 644 Friedel pairs
Absolute structure parameter0.00 (2)

Computer programs: APEX2 (Bruker, 2003), SAINT (Bruker, 2005), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), XtalDraw (Downs & Hall-Wallace, 2003), SHELXTL (Sheldrick, 2008).

Selected bond lengths (Å) top
Ca—O5i2.3626 (13)Cu—O5vi1.8855 (16)
Ca—O3ii2.3995 (17)Cu—O1vi2.0666 (16)
Ca—O4iii2.4818 (16)Cu—O12.0688 (15)
Ca—O2iv2.5178 (17)Cu—O3vii2.2976 (15)
Ca—O4v2.5281 (16)Cu—O4viii2.3882 (14)
Ca—O1iv2.5462 (14)As—O41.6749 (16)
Ca—O32.5786 (17)As—O31.6779 (16)
Ca—O22.6264 (17)As—O21.6796 (16)
Cu—O51.8850 (15)As—O11.7099 (13)
Symmetry codes: (i) x+1/2, y+1, z1/2; (ii) x+1/2, y+3/2, z; (iii) x, y+1, z; (iv) x+1, y+1/2, z+1/2; (v) x+1/2, y+1/2, z; (vi) x, y1/2, z+1/2; (vii) x1/2, y+1/2, z; (viii) x+1/2, y, z+1/2.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O5—H1···O2ix0.86 (4)1.91 (4)2.678 (2)149 (3)
Symmetry code: (ix) x1, y, z.
 

Acknowledgements

The authors gratefully acknowledge support of this study by the RRUFF project.

References

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