Quijarroite, Cu6HgPb2Bi4Se12, a New Selenide from the El Dragón Mine, Bolivia

Quijarroite, ideally Cu6HgPb2Bi4Se12, is a new selenide species from the El Dragón mine, Department of Potosí, Bolivia. It most frequently occurs as lath-shaped thin plates (up to 150 μm in length and 20 μm in width) intimately (subparallel) intergrown with hansblockite, forming an angular network-like intersertal texture. Quijarroite is occasionally also present as subto anhedral grains up to 200 μm in length and 50 μm in width. It is non-fluorescent, black and opaque with a metallic luster and black streak. It is brittle, with an irregular fracture and no obvious cleavage and parting. In plane-polarized incident light, quijarroite is weakly pleochroic from cream to very slightly more brownish-cream, displaying no internal reflections. Between crossed polars, quijarroite is moderately anisotropic with pale orange-brown to blue rotation tints. Lamellar twinning on {110} is common; parquet twinning occurs rarely. The reflectance values in the air for the COM (Commission on Ore Mineralogy) standard wavelengths (R1 and R2) are: 46.7, 46.8 (470 nm), 47.4, 48.2 (546 nm), 47.1, 48.5 (589 nm), and 46.6, 48.7 (650 nm). Electron-microprobe analyses yielded a mean composition of Cu 13.34, Ag 1.02, Hg 7.67, Pb 16.87, Co 0.03, Ni 0.15, Bi 27.65, Se 33.52, total 100.24 wt %. The mean empirical formula, normalized to 25 apfu (atoms per formula unit), is (Cu5.84Ag0.26)Σ = 6.10(Hg1.06Ni0.07Co0.01)Σ = 1.14Pb2.27Bi3.68Se11.81 (n = 24). The simplified formula is Cu6HgPb2Bi4Se12. Quijarroite is orthorhombic, space group Pmn21, with a = 9.2413(8), b = 9.0206(7), c = 9.6219(8) Å, V = 802.1(1) Å3, Z = 1. The calculated density is 5.771 g·cm−3. The five strongest X-ray powder-diffraction lines (d in Å (I/I0) (hkl)) are: 5.36 (55) (111), 3.785 (60) (211), 3.291 (90) (022), 3.125 (100) (212), and 2.312 (50) (400). The crystal structure of quijarroite can be considered a galena derivative and could be derived from that of bournonite. It is a primary mineral, deposited from an oxidizing low-T hydrothermal fluid at a fSe2/ fS2 ratio greater than unity. The new species has been approved by the IMA-CNMNC (2016-052) and is named for the Quijarro Province in Bolivia, in which the El Dragón mine is located.

The new species quijarroite and its name have been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the IMA, proposal 2016-052. The holotype specimen, representing the X-rayed crystal, is deposited in the Florence Museum, catalogue number 3232/I. The polished section, from which the holotype crystal fragment was extracted, is housed in the collections of the Natural History Museum, London, catalogue number BM 2016, 26. The cotype material, consisting of a quijarroite-bearing section, is deposited within the Mineralogical State Collection Munich (Mineralogische Staatssammlung München, Museum "Reich der Kristalle"), inventory number MSM 73573.
The name is for the Quijarro Province in Bolivia, in which the El Dragón mine is located. Quijarro also hosts the world-known Porco Ag−Zn−Pb−Sn deposit, which has been in operation since the 1500s [8].

Geology
The El Dragón selenide occurrence is situated in southwestern Bolivia, in the Cordillera Oriental, some 30 km southwest of Cerro Rico de Potosí. The abandoned mine is located 19 • 49 23.90 S (latitude), 65 • 55 00.60 W (longitude), at an altitude of 4160 m above sea level. The adit of the El Dragón mine is on the orographic left side of the Rio Jaya Mayu, cutting through a series of thinly stratified, pyrite-rich black shales and reddish-grey, hematite-bearing siltstones of probably Devonian age, dipping 40 • to the north. The almost-vertical ore vein is located in the center of a 1.5-m-wide shear zone (average trend 135 degrees). In 1988, the selenium mineralization consisted of a single vein of small longitudinal extension (maximum 15-m-long gallery), ranging mostly from 0.5 to 2 cm in thickness.
The El Dragón mineralization represents a multi-phase assemblage of primary and secondary minerals, among which Se-bearing phases are the most prominent [1,2]. The full list of minerals recorded from El Dragón is given on [9]. A comprehensive survey of the mineralogy and origin of the El Dragón mineralization forms the subject of a companion study [10], in this issue.
More rarely, quijarroite also forms sub-to anhedral grains up to 200 µm in length and 50 µm in width, occurring either alone in the matrix or intergrown with watkinsonite, clausthalite, eldragónite, krut'aite-penroseite, CuSe 2 -NiSe 2 , eskebornite, CuFeSe 2 , klockmannite and umangite (Figures 2 and 3). Minerals occasionally being in grain-boundary contact encompass petrovicite, Cu 3 HgPbBiSe 5 , grundmannite, and native gold.            Quijarroite is black in color. The mineral is opaque in transmitted light and exhibits a metallic luster. No cleavage and parting is observed and the fracture is irregular. The density and Mohs hardness could not be measured owing to the small fragment size. The calculated density (for Z = 1) for the empirical formula (see below) and unit-cell parameters derived from X-ray single-crystal measurements is 5.771 g/cm 3 .
In plane-polarized incident light, quijarroite is weakly pleochroic from cream to very slightly more brownish-cream. The mineral does not show any internal reflections. Between crossed polars, quijarroite is moderately anisotropic with pale orange-brown to blue rotation tints (cf. Figure 3). Lamellar twinning on {110} is common; parquet twinning occurs rarely. Quantitative reflectance measurements were performed in air relative to a WTiC standard by means of a J & M TIDAS diode array spectrometer (J & M Analytik AG, Essingen, Germany) using ONYX software on a Zeiss Axioplan ore microscope (Carl Zeiss AG, Oberkochen, Germany) ( Table 1). Measurements were made on unoriented grains at extinction positions, leading to the designation of R 1 (minimum) and R 2 (maximum).
For the X-ray single-crystal diffraction study, a small crystal fragment (0.03 × 0.035 × 0.05 mm 3 ) was handpicked from a fragment of the holotype specimen. The crystal was preliminarily examined with a Bruker-Enraf MACH3 single-crystal diffractometer using graphite-monochromatized MoKα radiation. The data collection was then done with an Oxford Diffraction Xcalibur 3 diffractometer (Oxford Diffraction) (X-ray MoKα radiation, λ = 0.71073 Å) fitted with a Sapphire 2 CCD detector (Oxford Diffraction) (see Table 5 for details). Intensity integration and standard Lorentz-polarization corrections were done with the CrysAlis RED [12] software package. The program ABSPACK of the CrysAlis RED package [12] was used for the absorption correction. The merging R for the ψ-scan data set decreased from 0.165 before absorption correction to 0.041 after this correction. Reflections conditions (h0l: h + l = 2n; h00: h = 2n; 00l: l = 2n) were consistent with the space groups Pmn2 1 , P2 1 nm and Pmnm, and the statistical tests on the distribution of |E| values strongly indicated the absence of an inversion center (|E 2 -1| = 0.695). The structure solution was then initiated in the standard setting of space group Pmn2 1 . The refined unit-cell parameters are a = 9.2413(8), b = 9.0206(7), c = 9.6219(8) Å, V = 802.1(1) Å 3 , Z = 1.  The position of most of the atoms (i.e., Pb, Bi1, Bi2, Se2, Se3) was determined from the three-dimensional Patterson synthesis [13]. A least-squares refinement using these heavy-atom positions and isotropic temperature factors yielded an R factor of 10.4%. Three-dimensional difference Fourier synthesis yielded the position of the remaining metals and the two Se atoms. The full-matrix least-squares program SHELXL-97 [13] was used for the refinement of the structure. Site-scattering values were refined using scattering curves for neutral species [14] as follows: Hg vs. Cu, Pb vs. , Bi vs. and Se vs. , for the Cu, Pb, Bi and Se sites, respectively. Cu1 has a population of Cu 0.75 Hg 0.25 , whereas Cu2 is occupied at 75% (cf. Table 6). The Pb and Bi sites were found fully occupied. The Se sites were found to be fully occupied by Se. At the last stage, with anisotropic atomic displacement parameters for all atoms and no constraints, the residual value settled at R = 0.027 for 1523 independent observed reflections (4σ(F o ) level) and 69 parameters and at R = 0.028 for all 2195 independent reflections. Inspection of the difference Fourier map revealed that the maximum positive and negative peaks were 0.42 and 0.76 e − /Å 3 , respectively. Wyckoff positions, site occupation factors, fractional atomic coordinates, and equivalent isotropic displacement parameters (Å 2 ) are given in Table 6. The main interatomic distances (Å) are reported in Table 7.
The crystal structure of quijarroite ( Figure 4) can be considered a galena derivative. It can be derived from that of bournonite [15]. The bournonite isotypic series recently encompassed three members: bournonite, CuPbSbS 3 , seligmannite, CuPbAsS 3 , and součekite, CuPbBi(S,Se) 3 [16]. The crystal structure of the latter is unknown but the similarity of the unit-cell parameters and the diffraction pattern with bournonite and seligmannite indicates that it is likely a member of this group. Overall, in their structure (orthorhombic, space group Pn2 1 m), Pb forms 7,8-fold polyhedra, M 3+ (M = Sb, As, Bi) forms trigonal pyramids, and Cu exhibits a tetrahedral coordination. All these polyhedra share corners and edges to form a three-dimensional network. CuS 4 tetrahedra share corners to form chains parallel to [001]. In quijarroite, Bi fully occupies the two 2a Wyckoff positions usually occupied by As and Sb in seligmannite and bournonite, respectively, Cu enters the same tetrahedral 4b position (with an occupancy of three-fourths of the site; refined site population: Cu 0.75 0.25 ), and Se fully replaces S at all the available anion positions. The most striking difference is what occurs at the Pb positions: In quijarroite only one of the two Pb positions of bournonite and seligmannite (2b Wyckoff position) is occupied by Pb, whereas the second is vacant and replaced by a general (4b) position occupied by Cu and Hg (refined site population: Cu 0.75 Hg 0.25 ), showing an almost perfect linear coordination. A linear-coordinated mixed (Cu,Hg) site has been observed in some other phases, e.g., in the linearly coordinated Hg site of fettelite [17] or in rouxelite [18]. The linear-coordinated Cu/Hg atoms exhibit a mean bond distance of 2.342 Å, which leads to a bond valence sum (taking into account the parameters of Breese and O'Keeffe [19]) of 1.   [14], both drawn down [001]. In quijarroite, dark blue spheres indicate the linearly coordinated Cu/Hg atoms, whereas light blue tetrahedra refer to the partially occupied Cu position (Cu0.750.25). Light, dark green and red spheres refer to Bi, Pb and Se, respectively. In bournonite, light blue tetrahedra refer to Cu, whereas green, red and yellow spheres refer to Pb, As and S, respectively.

Discussion
The new selenium mineral resembles phase "A" of Paar et al. (2012), for which the empirical formula Cu5Pb2HgBi3Se10 (normalized to 22 apfu) was proposed [2,4]. The identity of quijarroite with phase "A", for which no structural data were provided by Paar et al. (2012), is well displayed in a (Cu + Ag)−Hg−Bi diagram, where the mean compositions of both species overlap within analytical error ( Figure 5). Note the correspondence of quijarroite with phase "A" and hansblockite with phase "B" within analytical error. Data sources: [4,5], this paper and [2], for phases "A" and "B".
The new mineral is chemically close to a species termed "Bi-rich petrovicite" [20] from the Schlema-Alberoda U-Se-polymetallic deposit (Erzgebirge, Germany). Normalized to 12 Se, it would  [14], both drawn down [001]. In quijarroite, dark blue spheres indicate the linearly coordinated Cu/Hg atoms, whereas light blue tetrahedra refer to the partially occupied Cu position (Cu 0.75 0.25 ). Light, dark green and red spheres refer to Bi, Pb and Se, respectively. In bournonite, light blue tetrahedra refer to Cu, whereas green, red and yellow spheres refer to Pb, As and S, respectively.

Discussion
The new selenium mineral resembles phase "A" of Paar et al. (2012), for which the empirical formula Cu 5 Pb 2 HgBi 3 Se 10 (normalized to 22 apfu) was proposed [2,4]. The identity of quijarroite with phase "A", for which no structural data were provided by Paar et al. (2012), is well displayed in a (Cu + Ag)−Hg−Bi diagram, where the mean compositions of both species overlap within analytical error ( Figure 5).

Discussion
The new selenium mineral resembles phase "A" of Paar et al. (2012), for which the empirical formula Cu5Pb2HgBi3Se10 (normalized to 22 apfu) was proposed [2,4]. The identity of quijarroite with phase "A", for which no structural data were provided by Paar et al. (2012), is well displayed in a (Cu + Ag)−Hg−Bi diagram, where the mean compositions of both species overlap within analytical error ( Figure 5). Note the correspondence of quijarroite with phase "A" and hansblockite with phase "B" within analytical error. Data sources: [4,5], this paper and [2], for phases "A" and "B".
The new mineral is chemically close to a species termed "Bi-rich petrovicite" [20] from the Schlema-Alberoda U-Se-polymetallic deposit (Erzgebirge, Germany). Normalized to 12 Se, it would Note the correspondence of quijarroite with phase "A" and hansblockite with phase "B" within analytical error. Data sources: [4,5], this paper and [2], for phases "A" and "B".
The new mineral is chemically close to a species termed "Bi-rich petrovicite" [20] from the Schlema-Alberoda U-Se-polymetallic deposit (Erzgebirge, Germany). Normalized to 12 Se, it would have the formula Cu 6.50 Hg 0.96 Pb 2.04 Bi 3.72 Se 12 . This phase has also an orthorhombic cell, but different unit-cell parameters. If the structural data are correct, it might be a polymorph of quijarroite.
As to the physico-chemical environment of the formation of quijarroite, its commonly associated minerals provide little if any p−T−x information. The stability relations of hansblockite and watkinsonite are largely unconstrained, and clausthalite has a broad stability field. Luckily, inferences could be made from the Cu selenides, with which quijarroite is occasionally intergrown. Thus, its association with umangite and klockmannite implies high selenium fugacities, above values defined by the umangite-klockmannite univariant reaction. At T = 100 • C and an elevated oxygen fugacity defined by the magnetite-hematite buffer, this relation would be consistent with a minimum log f Se 2 of roughly −14.5 [21]. The absence of end-member krut'aite and sulfides (chalcopyrite, pyrite) defines the maximum log f S 2 to roughly −19.