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This study presents the first structural report of iranite, ideally CuPb10(CrO4)6(SiO4)2(OH)2 [copper deca­lead hexa­chromate bis­(orthosilicate) dihydroxide], based on single-crystal X-ray diffraction data. Iranite is isomorphous with hemihedrite, with substitution of Cu for Zn and OH for F. The Cu atom is situated at the special position with site symmetry \overline{1}. The CrO4 and SiO4 tetra­hedra and CuO4(OH)2 octa­hedra form layers that are parallel to (120) and are linked together by five symmetrically independent Pb2+ cations displaying a rather wide range of bond distances. The CuO4(OH)2 octa­hedra are corner-linked to two CrO4 and two SiO4 groups, while two additional CrO4 groups are isolated. The mean Cr-O distances for the three nonequivalent CrO4 tetra­hedra are all slightly shorter than the corresponding distances in hemihedrite, whereas the CuO4(OH)2 octa­hedron is more distorted than the ZnO4F2 octa­hedron in hemihedrite in terms of octa­hedral quadratic elongation.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270107053449/iz3032sup1.cif
Contains datablocks I, global

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270107053449/iz3032Isup2.hkl
Contains datablock I

Comment top

Iranite was first discovered in Sébarz, Anarak (central Iran), and incorrectly described as a lead chromate with chemical formula PbCrO4·H2O (Bariand & Herpin, 1963). Adib & Ottemann (1970) reported several new lead chromates from Iran, one of which was called khuniite, with chemical formula (Pb1.6Zn0.2Cu0.2)CrO5. However, further studies of khuniite by Adib et al. (1972) suggested a new chemical formula, Pb5(Cu,Zn)(CrO4)3SiO4, for this mineral. Concurrently, the new mineral hemihedrite, ZnPb10(CrO4)6(SiO4)2F2, was discovered (Williams & Anthony, 1970) and its crystal structure reported (McLean & Anthony, 1970). Fleischer (1973) noted that the chemical formula given by Adib et al. (1972) for khuniite was not charge-balanced and that this mineral might be isostructural with hemihedrite, on the basis of similar X-ray powder diffraction data. A re-examination of the type mineral of iranite by Williams (1974) showed that it was probably the Cu analog of hemihedrite and that khuniite was misidentified and is actually iranite. A synthetic hydroxide analogue of iranite and synthetic hemihedrite were found to have formulae CuPb10(CrO4)6(SiO4)2(OH)2 and ZnPb10(CrO4)6(SiO4)2(OH)2, respectively, and analyses of natural materials indicated that there is probably a solid solution between the two minerals (Cesbron & Williams, 1980; Bariand & Poullen, 1980). While the crystal structure of hemihedrite was determined by McLean & Anthony (1970), the crystal structure of iranite remained undetermined until now. This paper reports the first structural refinement of natural iranite based on single-crystal X-ray diffraction data.

Iranite is isotypic with hemihedrite, with the substitution of Cu for Zn and OH for F. The view down the [211] direction (Fig. 1) demonstrates that iranite can be regarded as a layered structure of CrO4 and SiO4 tetrahedra along with CuO4(OH)2 octahedra that lie parallel to (120) and are linked together by five symmetrically independent Pb2+ cations. Within a polyhedral layer, each CuO4(OH)2 octahedron shares opposite corners with two SiO4 tetrahedra as well as opposite corners with two Cr3O4 tetrahedra, while the two OH groups are oriented towards the Pb layer. The Cr1O4 and Cr2O4 tetrahedra are isolated in the polyhedral layer (Fig. 2). Owing mostly to the Jahn–Teller effect of Cu2+, the Cu1—O11 bond length [2.290 (5) Å] in iranite is longer than the Zn—O11 length (2.17 Å) in hemihedrite, whereas the Cu1—O17 bond distance [1.950 (5) Å] is shorter than the Zn—F distance (2.05 Å). As a consequence, the CuO4(OH)2 octahedron in iranite is more distorted (1.0150) than the ZnO4F2 octahedron (1.0075) in hemihedrite, in terms of the octahedral quadratic elongation (Robinson et al., 1971). The mean Cr—O distances for the Cr1, Cr2 and Cr3 tetrahedra are 1.640, 1.650 and 1.648 Å, respectively, which are all slightly shorter than the corresponding mean distances (1.659, 1.662 and 1.658 Å) in hemihedrite (McLean & Anthony, 1970), but are comparable to most values in the literature, such as those reported for tarapacaite (K2CrO4; 1.643 Å; McGinnety, 1972), lopezite (K2Cr2O7; ~1.65 Å; Brunton, 1973 or 1983??), dietzeite [Ca2(IO3)2CrO4·H2O; 1.647 Å; Burns & Hawthorne, 1993] and edoylerite (Hg3CrO4S2; 1.643 Å; Burns, 1999). Within 3.2 Å, the coordination numbers of the five nonequivalent Pb2+ cations vary from 7 to 9.

There have been considerable discussions about the effects of the F–OH substitution on crystal structures and properties of minerals (e.g. Groat et al., 1990; Cooper & Hawthorne, 1995; Yang et al., 2007). For some minerals, the OH and F members may form a complete solid solution, such as for the amblygonite [LiAl(PO4)F]–montebrasite [LiAl(PO4)(OH)] and fluorapatite [Ca5(PO4)3F]–hydroxylapatite [Ca5(PO4)3(OH)] series. However, there are also many examples in which the F–OH substitution results in structural transformations or symmetry changes, such as the cases between C2/c tilasite [CaMg(AsO4)F] and P212121 adelite [CaMg(AsO4)(OH)], and between C2/c triplite [Mn2(PO4)F] and P21/c triploidite [Mn2(PO4)(OH)]. Although a number of studies (e.g. Bariand & Poullen, 1980; Cesbron & Williams, 1980) have shown that the OH-rich iranite and hemihedrite constitute a continuous series varying the Zn/Cu ratio, Frost (2004) reported that the Raman spectra of the F-rich iranite and hemihedrite are remarkably different in the Cr—O stretching region (between 750 and 900 cm-1) and suggested that the two minerals may not be homologous. The presence of Pb atoms in iranite prevented the determination of the H-atom location. However, bond valence considerations show that only atom O17 can accommodate H as an OH group. The O17 site is also where the F atom is located in hemihedrite. It appears that atom O17 may form an O—H···O linkage with one of the two closest O atoms, O6 (2.852 Å away from O17) or O7 (2.881 Å away from O17). The possibility of the O17—H···O7 linkage can be ruled out on the basis of the Pb3—O17—O7 angle being equal to 53°. When the O17—H···O6 linkage is considered, then the environment around atom O17 is tetrahedral with Pb3—O17—O6 = 117.6°, Pb2—O17—O6 = 108.6°, Cu1—O17—O6 = 104.4° and an average angle of 110°. According to Libowitzky (1999), an O—H···O distance of 2.85 Å would correspond to an O—H stretching frequency of ~3399 cm-1, which is close to what we measured (~3387 cm-1) for our sample with Raman spectroscopy (https://rruff.info). A comparison of the environment around atom O17 in iranite with that around the F atom in hemihedrite indicates that the bonding topologies of these two ions are similar, leading us to suggest that there is no impediment to a complete solid solution between OH and F in the hemihedrite–iranite series.

Related literature top

For related literature, see: Adib et al. (1972); Bariand & Herpin (1963); Bariand & Poullen (1980); Burns (1999); Burns & Hawthorne (1993); Cesbron & Williams (1980); Cooper & Hawthorne (1995); Fleischer (1973); Frost (2004); Groat et al. (1990); Libowitzky (1999); McGinnety (1972); McLean & Anthony (1970); Williams (1974); Williams & Anthony (1970); Yang et al. (2007).

Experimental top

The iranite specimen used in this study is from Chapacase mine, Sierra Cerillos district, Tocopilla, Chile, and is in the collection of the RRUFF project (deposition No. R060781; https://rruff.info), donated by Mike Scott. The average chemical composition of the studied sample, CuPb10{(Cr0.99[]0.01)Σ=1[O3.82(OH)0.18]Σ=4}6(SiO4)2(OH)2, was determined with a CAMECA SX50 electron microprobe (https://rruff.info).

Refinement top

The H atoms were not located in the final difference Fourier syntheses. The chemical analysis showed a little deficiency in Cr when compared with the ideal value of six per chemical formula, but the refinement assumed an ideal chemistry, as the overall effects of such a small amount of vacancy on the final structure results are negligible. The highest residual peak in the difference Fourier map was located at (0.6733, 0.4075, 0.7771), 0.71 Å from atom Pb4, and the deepest hole at (0.2382, 0.0680, 0.5726), 0.88 Å from Pb2.

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, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XtalDraw (Downs & Hall-Wallace, 2003); software used to prepare material for publication: SHELXTL (Bruker, 1997).

Figures top
[Figure 1] Fig. 1. The crystal structure of iranite viewed down [211]. The spheres represent Pb atoms. Atom Cu1 is in an octahedral coordination, and Si1, Cr1, Cr2 and Cr3 are in tetrahedral coordination.
[Figure 2] Fig. 2. The polyhedral layer in iranite. The Cu1 atoms are in octahedra that are corner-linked to the Si1O4 and Cr3O4 tetrahedra. The Cr1O4 and Cr2O4 tetrahedra are isolated. The spheres represent the OH groups.
copper decalead hexachromate bis(orthosilicate) dihydroxide top
Crystal data top
CuPb10(CrO4)6(SiO4)2(OH)2Z = 1
Mr = 3049.64F(000) = 1295
Triclinic, P1Dx = 6.492 Mg m3
Hall symbol: -P 1Mo Kα radiation, λ = 0.71073 Å
a = 9.5416 (4) ÅCell parameters from 6213 reflections
b = 11.3992 (5) Åθ = 5.4–69.6°
c = 10.7465 (4) ŵ = 56.58 mm1
α = 120.472 (2)°T = 293 K
β = 92.470 (2)°Block, brown
γ = 55.531 (2)°0.05 × 0.05 × 0.04 mm
V = 780.08 (6) Å3
Data collection top
Bruker APEXII CCD area-detector
diffractometer
6319 independent reflections
Radiation source: fine-focus sealed tube5022 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.036
Phi and ω scanθmax = 34.7°, θmin = 2.3°
Absorption correction: multi-scan
(SADABS; Sheldrick, 2005)
h = 1215
Tmin = 0.083, Tmax = 0.104k = 1718
14248 measured reflectionsl = 1017
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullH-atom parameters not defined
R[F2 > 2σ(F2)] = 0.034 w = 1/[σ2(Fo2) + (0.019P)2 + 4.6078P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.070(Δ/σ)max = 0.001
S = 1.01Δρmax = 3.68 e Å3
6319 reflectionsΔρmin = 3.50 e Å3
242 parametersExtinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraintsExtinction coefficient: 0.00147 (6)
Primary atom site location: structure-invariant direct methods
Crystal data top
CuPb10(CrO4)6(SiO4)2(OH)2γ = 55.531 (2)°
Mr = 3049.64V = 780.08 (6) Å3
Triclinic, P1Z = 1
a = 9.5416 (4) ÅMo Kα radiation
b = 11.3992 (5) ŵ = 56.58 mm1
c = 10.7465 (4) ÅT = 293 K
α = 120.472 (2)°0.05 × 0.05 × 0.04 mm
β = 92.470 (2)°
Data collection top
Bruker APEXII CCD area-detector
diffractometer
6319 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 2005)
5022 reflections with I > 2σ(I)
Tmin = 0.083, Tmax = 0.104Rint = 0.036
14248 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0340 restraints
wR(F2) = 0.070H-atom parameters not defined
S = 1.01Δρmax = 3.68 e Å3
6319 reflectionsΔρmin = 3.50 e Å3
242 parameters
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
Pb10.25932 (4)0.10817 (4)0.26084 (3)0.01674 (7)
Pb20.26267 (4)0.08803 (4)0.65782 (3)0.01224 (6)
Pb30.92955 (4)0.24266 (4)0.02886 (3)0.01330 (7)
Pb40.73129 (4)0.41520 (4)0.74861 (4)0.01951 (7)
Pb50.31800 (4)0.45125 (4)0.53230 (3)0.01563 (7)
Cu10.00000.50000.00000.0073 (2)
Cr10.95691 (16)0.07568 (16)0.35431 (14)0.0129 (2)
Cr20.56240 (15)0.17381 (14)0.15417 (13)0.0085 (2)
Cr30.45357 (15)0.32313 (15)0.83512 (13)0.0094 (2)
Si10.0238 (2)0.4530 (2)0.66184 (19)0.0016 (3)
O10.7526 (8)0.2258 (8)0.4793 (7)0.0273 (14)
O20.1016 (8)0.0791 (8)0.4400 (7)0.0241 (13)
O30.9960 (8)0.1191 (7)0.7373 (6)0.0179 (12)
O40.9711 (9)0.1113 (9)0.2286 (8)0.0276 (14)
O50.5094 (7)0.1359 (7)0.2693 (6)0.0157 (11)
O60.4274 (8)0.2011 (8)0.0550 (7)0.0239 (13)
O70.7703 (7)0.0099 (7)0.0328 (6)0.0166 (11)
O80.5351 (8)0.3564 (7)0.2680 (7)0.0193 (12)
O90.6089 (8)0.2855 (8)0.9127 (7)0.0239 (13)
O100.4636 (9)0.3961 (9)0.7413 (7)0.0255 (14)
O110.2494 (7)0.4823 (7)0.9729 (6)0.0159 (11)
O120.4831 (7)0.1376 (7)0.7178 (7)0.0165 (11)
O130.2111 (7)0.3011 (7)0.5135 (6)0.0129 (10)
O140.0385 (7)0.3927 (6)0.7755 (6)0.0104 (10)
O150.9898 (7)0.3729 (7)0.2577 (6)0.0148 (11)
O160.8491 (7)0.4798 (7)0.6138 (6)0.0146 (11)
O170.1390 (7)0.2594 (6)0.9361 (6)0.0097 (10)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Pb10.01577 (14)0.01461 (14)0.01353 (14)0.00899 (12)0.00408 (11)0.00552 (12)
Pb20.01004 (12)0.01029 (13)0.01122 (13)0.00534 (10)0.00213 (10)0.00470 (11)
Pb30.01343 (13)0.01210 (13)0.01257 (13)0.00789 (11)0.00281 (10)0.00636 (11)
Pb40.02207 (15)0.02192 (16)0.02042 (16)0.01419 (13)0.01093 (12)0.01546 (14)
Pb50.01874 (14)0.02070 (15)0.01581 (14)0.01476 (13)0.01030 (11)0.01279 (13)
Cu10.0094 (5)0.0051 (5)0.0061 (5)0.0042 (4)0.0025 (4)0.0030 (4)
Cr10.0137 (5)0.0133 (6)0.0116 (6)0.0101 (5)0.0042 (5)0.0054 (5)
Cr20.0070 (5)0.0062 (5)0.0077 (5)0.0024 (4)0.0009 (4)0.0036 (4)
Cr30.0085 (5)0.0096 (5)0.0107 (5)0.0059 (4)0.0035 (4)0.0061 (5)
Si10.0025 (7)0.0026 (7)0.0022 (7)0.0010 (6)0.0011 (6)0.0018 (6)
O10.021 (3)0.021 (3)0.023 (3)0.014 (3)0.001 (3)0.003 (3)
O20.029 (3)0.026 (3)0.027 (3)0.022 (3)0.022 (3)0.016 (3)
O30.019 (3)0.017 (3)0.015 (3)0.014 (2)0.004 (2)0.005 (2)
O40.030 (3)0.038 (4)0.037 (4)0.025 (3)0.017 (3)0.031 (4)
O50.016 (3)0.016 (3)0.016 (3)0.010 (2)0.003 (2)0.010 (2)
O60.021 (3)0.031 (3)0.019 (3)0.016 (3)0.012 (3)0.015 (3)
O70.011 (2)0.013 (3)0.014 (3)0.001 (2)0.002 (2)0.007 (2)
O80.019 (3)0.017 (3)0.026 (3)0.012 (2)0.008 (2)0.014 (3)
O90.018 (3)0.028 (3)0.027 (3)0.013 (3)0.016 (3)0.019 (3)
O100.037 (4)0.033 (4)0.027 (3)0.027 (3)0.013 (3)0.022 (3)
O110.014 (3)0.010 (2)0.014 (3)0.005 (2)0.000 (2)0.004 (2)
O120.012 (2)0.012 (3)0.020 (3)0.008 (2)0.006 (2)0.006 (2)
O130.010 (2)0.011 (2)0.010 (2)0.006 (2)0.001 (2)0.003 (2)
O140.013 (2)0.008 (2)0.006 (2)0.004 (2)0.0009 (19)0.005 (2)
O150.024 (3)0.012 (3)0.011 (3)0.012 (2)0.005 (2)0.007 (2)
O160.016 (3)0.016 (3)0.017 (3)0.009 (2)0.010 (2)0.013 (2)
O170.012 (2)0.005 (2)0.007 (2)0.004 (2)0.0003 (19)0.0022 (19)
Geometric parameters (Å, º) top
Pb1—O132.308 (5)Pb4—O6vi3.049 (6)
Pb1—O52.570 (5)Pb5—O16vi2.289 (5)
Pb1—O12i2.598 (5)Pb5—O132.371 (5)
Pb1—O15ii2.652 (5)Pb5—O52.637 (6)
Pb1—O4ii2.739 (6)Pb5—O15vi2.657 (5)
Pb1—O22.746 (6)Pb5—O102.747 (6)
Pb1—O7iii2.810 (6)Pb5—O82.896 (6)
Pb1—O6iii3.113 (6)Pb5—O8vi3.135 (6)
Pb2—O142.368 (5)Cu1—O17x1.950 (5)
Pb2—O172.433 (5)Cu1—O17vii1.950 (5)
Pb2—O122.453 (5)Cu1—O14vii2.007 (5)
Pb2—O3ii2.488 (5)Cu1—O14x2.007 (5)
Pb2—O5i2.731 (5)Cu1—O11vii2.290 (5)
Pb2—O22.735 (6)Cu1—O11x2.290 (5)
Pb2—O1i3.196 (6)Cr1—O41.626 (6)
Pb3—O152.392 (6)Cr1—O11.628 (6)
Pb3—O17iv2.413 (5)Cr1—O2ix1.634 (6)
Pb3—O7v2.414 (5)Cr1—O3xi1.672 (6)
Pb3—O11vi2.597 (5)Cr2—O61.630 (6)
Pb3—O3vii2.624 (6)Cr2—O71.638 (5)
Pb3—O4v3.067 (7)Cr2—O81.651 (6)
Pb3—O43.085 (6)Cr2—O51.678 (5)
Pb3—O9vii3.124 (6)Cr3—O91.616 (6)
Pb3—O10vi3.182 (7)Cr3—O101.632 (6)
Pb4—O162.463 (5)Cr3—O121.671 (6)
Pb4—O8vi2.493 (6)Cr3—O111.674 (5)
Pb4—O12.509 (7)Si1—O16ii1.628 (6)
Pb4—O11viii2.647 (6)Si1—O131.636 (5)
Pb4—O102.679 (6)Si1—O15vi1.637 (5)
Pb4—O32.783 (6)Si1—O141.646 (5)
Pb4—O14ix2.810 (5)
O17x—Cu1—O17vii180.0O2ix—Cr1—O3xi111.5 (3)
O17x—Cu1—O14vii95.5 (2)O6—Cr2—O7107.9 (3)
O17vii—Cu1—O14vii84.5 (2)O6—Cr2—O8109.5 (3)
O17x—Cu1—O14x84.5 (2)O7—Cr2—O8111.0 (3)
O17vii—Cu1—O14x95.5 (2)O6—Cr2—O5110.5 (3)
O14vii—Cu1—O14x180.0 (4)O7—Cr2—O5111.5 (3)
O17x—Cu1—O11vii85.9 (2)O8—Cr2—O5106.4 (3)
O17vii—Cu1—O11vii94.1 (2)O9—Cr3—O10109.7 (3)
O14vii—Cu1—O11vii90.5 (2)O9—Cr3—O12109.0 (3)
O14x—Cu1—O11vii89.5 (2)O10—Cr3—O12110.9 (3)
O17x—Cu1—O11x94.1 (2)O9—Cr3—O11109.6 (3)
O17vii—Cu1—O11x85.9 (2)O10—Cr3—O11106.5 (3)
O14vii—Cu1—O11x89.5 (2)O12—Cr3—O11111.1 (3)
O14x—Cu1—O11x90.5 (2)O16ii—Si1—O13112.6 (3)
O11vii—Cu1—O11x180.0O16ii—Si1—O15vi113.8 (3)
O4—Cr1—O1110.5 (3)O13—Si1—O15vi103.5 (3)
O4—Cr1—O2ix107.7 (3)O16ii—Si1—O14104.7 (3)
O1—Cr1—O2ix110.3 (3)O13—Si1—O14109.6 (3)
O4—Cr1—O3xi108.5 (3)O15vi—Si1—O14112.8 (3)
O1—Cr1—O3xi108.3 (3)
Symmetry codes: (i) x+1, y, z+1; (ii) x1, y, z; (iii) x+1, y, z; (iv) x+1, y, z1; (v) x+2, y, z; (vi) x+1, y+1, z+1; (vii) x, y, z1; (viii) x+1, y+1, z+2; (ix) x+1, y, z; (x) x, y+1, z+1; (xi) x+2, y, z+1.

Experimental details

Crystal data
Chemical formulaCuPb10(CrO4)6(SiO4)2(OH)2
Mr3049.64
Crystal system, space groupTriclinic, P1
Temperature (K)293
a, b, c (Å)9.5416 (4), 11.3992 (5), 10.7465 (4)
α, β, γ (°)120.472 (2), 92.470 (2), 55.531 (2)
V3)780.08 (6)
Z1
Radiation typeMo Kα
µ (mm1)56.58
Crystal size (mm)0.05 × 0.05 × 0.04
Data collection
DiffractometerBruker APEXII CCD area-detector
diffractometer
Absorption correctionMulti-scan
(SADABS; Sheldrick, 2005)
Tmin, Tmax0.083, 0.104
No. of measured, independent and
observed [I > 2σ(I)] reflections
14248, 6319, 5022
Rint0.036
(sin θ/λ)max1)0.800
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.034, 0.070, 1.01
No. of reflections6319
No. of parameters242
H-atom treatmentH-atom parameters not defined
Δρmax, Δρmin (e Å3)3.68, 3.50

Computer programs: APEX2 (Bruker, 2003), SAINT (Bruker, 2005), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), XtalDraw (Downs & Hall-Wallace, 2003), SHELXTL (Bruker, 1997).

 

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