Acta Cryst. (2008). E64, i53-i54 [ doi:10.1107/S1600536808024173 ]
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). 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.
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).
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.
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).
![[Figure 1]](./wm2185fig1thm.gif)
| 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. |
| CaCu(AsO4)(OH) | F000 = 492 |
| Mr = 259.57 | Dx = 4.359 Mg m−3 |
| Orthorhombic, P212121 | Mo Kα radiation λ = 0.71073 Å |
| Hall symbol: P 2ac 2ab | Cell parameters from 3371 reflections |
| a = 7.3822 (2) Å | θ = 3.6–34.0º |
| b = 5.8146 (2) Å | µ = 15.03 mm−1 |
| c = 9.2136 (3) Å | T = 293 (2) K |
| V = 395.49 (2) Å3 | Euhedral, equant, green |
| Z = 4 | 0.06 × 0.05 × 0.04 mm |
| Bruker APEX2 CCD diffractometer | 1602 independent reflections |
| Radiation source: fine-focus sealed tube | 1487 reflections with I > 2σ(I) |
| Monochromator: graphite | Rint = 0.023 |
| T = 293(2) K | θmax = 34.0º |
| φ and ω scans | θmin = 3.5º |
| Absorption correction: multi-scan (TWINABS; Sheldrick, 2008) | h = −9→11 |
| Tmin = 0.492, Tmax = 0.585 | k = −9→9 |
| 7088 measured reflections | l = −14→14 |
| Refinement on F2 | All H-atom parameters refined |
| Least-squares matrix: full | w = 1/[σ2(Fo2) + (0.0151P)2 + 0.1227P] where P = (Fo2 + 2Fc2)/3 |
| R[F2 > 2σ(F2)] = 0.018 | (Δ/σ)max < 0.001 |
| wR(F2) = 0.038 | Δρmax = 0.63 e Å−3 |
| S = 1.03 | Δρmin = −0.49 e Å−3 |
| 1602 reflections | Extinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
| 79 parameters | Extinction coefficient: 0.0029 (5) |
| Primary atom site location: structure-invariant direct methods | Absolute structure: Flack (1983), 644 Friedel pairs |
| Secondary atom site location: difference Fourier map | Flack parameter: 0.00 (2) |
| Hydrogen site location: difference Fourier map |
| CaCu(AsO4)(OH) | V = 395.49 (2) Å3 |
| Mr = 259.57 | Z = 4 |
| Orthorhombic, P212121 | Mo Kα |
| a = 7.3822 (2) Å | µ = 15.03 mm−1 |
| b = 5.8146 (2) Å | T = 293 (2) K |
| c = 9.2136 (3) Å | 0.06 × 0.05 × 0.04 mm |
| Bruker APEX2 CCD diffractometer | 1602 independent reflections |
| Absorption correction: multi-scan (TWINABS; Sheldrick, 2008) | 1487 reflections with I > 2σ(I) |
| Tmin = 0.492, Tmax = 0.585 | Rint = 0.023 |
| 7088 measured reflections |
| R[F2 > 2σ(F2)] = 0.018 | All H-atom parameters refined |
| wR(F2) = 0.038 | Δρmax = 0.63 e Å−3 |
| S = 1.03 | Δρmin = −0.49 e Å−3 |
| 1602 reflections | Absolute structure: Flack (1983), 644 Friedel pairs |
| 79 parameters | Flack parameter: 0.00 (2) |
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. |
| x | y | z | Uiso*/Ueq | ||
| Ca | 0.61727 (5) | 0.72961 (8) | 0.07340 (4) | 0.01133 (12) | |
| Cu | −0.00416 (4) | −0.00002 (6) | 0.25029 (4) | 0.00898 (8) | |
| As | 0.36728 (2) | 0.26438 (4) | 0.08118 (2) | 0.00768 (6) | |
| O1 | 0.18844 (17) | 0.2450 (3) | 0.19847 (15) | 0.0135 (3) | |
| O2 | 0.5395 (2) | 0.3313 (3) | 0.19256 (18) | 0.0187 (4) | |
| O3 | 0.3514 (2) | 0.4927 (3) | −0.02947 (17) | 0.0157 (3) | |
| O4 | 0.3885 (2) | 0.0147 (3) | −0.00782 (15) | 0.0145 (4) | |
| O5 | −0.13880 (18) | 0.2539 (3) | 0.31777 (14) | 0.0113 (3) | |
| H1 | −0.229 (5) | 0.232 (6) | 0.260 (3) | 0.055 (11)* |
| U11 | U22 | U33 | U12 | U13 | U23 | |
| Ca | 0.01169 (18) | 0.0119 (2) | 0.01041 (18) | −0.00064 (17) | −0.00072 (13) | 0.00020 (18) |
| Cu | 0.00885 (11) | 0.00621 (12) | 0.01190 (12) | 0.00029 (9) | 0.00211 (8) | −0.00121 (8) |
| As | 0.00801 (8) | 0.00659 (9) | 0.00845 (9) | 0.00006 (8) | 0.00059 (7) | 0.00014 (9) |
| O1 | 0.0141 (6) | 0.0107 (7) | 0.0157 (6) | −0.0025 (7) | 0.0035 (5) | −0.0006 (7) |
| O2 | 0.0146 (7) | 0.0212 (9) | 0.0205 (8) | −0.0029 (6) | −0.0048 (6) | 0.0004 (7) |
| O3 | 0.0201 (7) | 0.0107 (7) | 0.0164 (7) | 0.0011 (7) | 0.0044 (7) | 0.0035 (6) |
| O4 | 0.0181 (8) | 0.0096 (7) | 0.0158 (8) | 0.0018 (6) | 0.0022 (7) | −0.0018 (6) |
| O5 | 0.0106 (5) | 0.0102 (6) | 0.0132 (6) | 0.0000 (8) | 0.0002 (5) | −0.0003 (6) |
| Ca—O5i | 2.3626 (13) | Cu—O5vi | 1.8855 (16) |
| Ca—O3ii | 2.3995 (17) | Cu—O1vi | 2.0666 (16) |
| Ca—O4iii | 2.4818 (16) | Cu—O1 | 2.0688 (15) |
| Ca—O2iv | 2.5178 (17) | Cu—O3vii | 2.2976 (15) |
| Ca—O4v | 2.5281 (16) | Cu—O4viii | 2.3882 (14) |
| Ca—O1iv | 2.5462 (14) | As—O4 | 1.6749 (16) |
| Ca—O3 | 2.5786 (17) | As—O3 | 1.6779 (16) |
| Ca—O2 | 2.6264 (17) | As—O2 | 1.6796 (16) |
| Cu—O5 | 1.8850 (15) | As—O1 | 1.7099 (13) |
| O5i—Ca—O3ii | 75.88 (5) | O4v—Ca—O2 | 77.15 (5) |
| O5i—Ca—O4iii | 73.62 (5) | O1iv—Ca—O2 | 79.00 (5) |
| O3ii—Ca—O4iii | 89.42 (5) | O3—Ca—O2 | 61.06 (5) |
| O5i—Ca—O2iv | 151.07 (5) | O5—Cu—O5vi | 177.68 (6) |
| O3ii—Ca—O2iv | 108.51 (6) | O5—Cu—O1vi | 98.01 (6) |
| O4iii—Ca—O2iv | 77.79 (5) | O5vi—Cu—O1vi | 84.26 (6) |
| O5i—Ca—O4v | 74.41 (5) | O5—Cu—O1 | 84.21 (6) |
| O3ii—Ca—O4v | 76.55 (5) | O5vi—Cu—O1 | 93.52 (6) |
| O4iii—Ca—O4v | 147.36 (3) | O1vi—Cu—O1 | 177.66 (7) |
| O2iv—Ca—O4v | 134.48 (5) | O5—Cu—O3vii | 91.88 (6) |
| O5i—Ca—O1iv | 141.59 (5) | O5vi—Cu—O3vii | 88.82 (6) |
| O3ii—Ca—O1iv | 73.13 (5) | O1vi—Cu—O3vii | 84.84 (6) |
| O4iii—Ca—O1iv | 127.50 (5) | O1—Cu—O3vii | 95.85 (6) |
| O2iv—Ca—O1iv | 62.85 (5) | O5—Cu—O4viii | 84.76 (6) |
| O4v—Ca—O1iv | 76.76 (5) | O5vi—Cu—O4viii | 94.77 (6) |
| O5i—Ca—O3 | 72.94 (5) | O1vi—Cu—O4viii | 89.81 (6) |
| O3ii—Ca—O3 | 147.75 (2) | O1—Cu—O4viii | 89.67 (5) |
| O4iii—Ca—O3 | 74.21 (5) | O3vii—Cu—O4viii | 173.23 (6) |
| O2iv—Ca—O3 | 95.19 (5) | O4—As—O3 | 113.27 (7) |
| O4v—Ca—O3 | 102.41 (5) | O4—As—O2 | 115.43 (8) |
| O1iv—Ca—O3 | 138.68 (5) | O3—As—O2 | 103.94 (8) |
| O5i—Ca—O2 | 117.86 (6) | O4—As—O1 | 108.94 (8) |
| O3ii—Ca—O2 | 145.22 (5) | O3—As—O1 | 112.45 (8) |
| O4iii—Ca—O2 | 124.48 (5) | O2—As—O1 | 102.33 (7) |
| O2iv—Ca—O2 | 75.44 (3) |
| Symmetry codes: (i) −x+1/2, −y+1, z−1/2; (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, y−1/2, −z+1/2; (vii) x−1/2, −y+1/2, −z; (viii) −x+1/2, −y, z+1/2. |
| D—H···A | D—H | H···A | D···A | D—H···A |
| O5—H1···O2ix | 0.86 (4) | 1.91 (4) | 2.678 (2) | 149 (3) |
| Symmetry codes: (ix) x−1, y, z. |
| Ca—O5i | 2.3626 (13) | Cu—O5vi | 1.8855 (16) |
| Ca—O3ii | 2.3995 (17) | Cu—O1vi | 2.0666 (16) |
| Ca—O4iii | 2.4818 (16) | Cu—O1 | 2.0688 (15) |
| Ca—O2iv | 2.5178 (17) | Cu—O3vii | 2.2976 (15) |
| Ca—O4v | 2.5281 (16) | Cu—O4viii | 2.3882 (14) |
| Ca—O1iv | 2.5462 (14) | As—O4 | 1.6749 (16) |
| Ca—O3 | 2.5786 (17) | As—O3 | 1.6779 (16) |
| Ca—O2 | 2.6264 (17) | As—O2 | 1.6796 (16) |
| Cu—O5 | 1.8850 (15) | As—O1 | 1.7099 (13) |
| Symmetry codes: (i) −x+1/2, −y+1, z−1/2; (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, y−1/2, −z+1/2; (vii) x−1/2, −y+1/2, −z; (viii) −x+1/2, −y, z+1/2. |
| D—H···A | D—H | H···A | D···A | D—H···A |
| O5—H1···O2ix | 0.86 (4) | 1.91 (4) | 2.678 (2) | 149 (3) |
| Symmetry codes: (ix) x−1, y, z. |
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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).