Cerromojonite , CuPbBiSe 3 , from El Dragón ( Bolivia ) : A New Member of the Bournonite Group

Cerromojonite, ideally CuPbBiSe3, represents a new selenide from the El Dragón mine, Department of Potosí, Bolivia. It either occurs as minute grains (up to 30 μm in size) in interstices of hansblockite/quijarroite intergrowths, forming an angular network-like intersertal texture, or as elongated, thin-tabular crystals (up to 200 μm long and 40 μm wide) within lath-shaped or acicular mineral aggregates (interpreted as pseudomorphs) up to 2 mm in length and 200 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, cerromojonite is grey to cream-white, and weakly pleochroic, showing no internal reflections. Between crossed polarizers, cerromojonite is weakly anisotropic, with rotation tints in shades of brown and grey. Lamellar twinning on {110} is common. The reflectance values in air for the COM standard wavelengths (R1 and R2) are: 48.8 and 50.3 (470 nm), 48.2 and 51.8 (546 nm), 47.8 and 52.0 (589 nm), and 47.2 and 52.0 (650 nm). Electron-microprobe analyses yielded a mean composition of: Cu 7.91, Ag 2.35, Hg 7.42, Pb 16.39, Fe 0.04, Ni 0.02, Bi 32.61, Se 33.37, total 100.14 wt %. The empirical formula (based on 6 atoms pfu) is (Cu0.89Hg0.11)Σ = 1.00(Pb0.56Ag0.16Hg0.15 Bi0.11Fe0.01)Σ = 0.99Bi1.00Se3.01. The ideal formula is CuPbBiSe3. Cerromojonite is orthorhombic (space group Pn21m), with a = 8.202(1) Å, b = 8.741(1) Å, c = 8.029(1) Å, V = 575.7(1) Å3, Z = 4. Calculated density is 7.035 g·cm−3. The five strongest measured X-ray powder diffraction lines (d in Å (I/I0) (hkl)) are: 3.86 (25) (120), 2.783 (100) (122), 2.727 (55) (212), 2.608 (40) (310), and 1.999 (25) (004). Cerromojonite is a new member of the bournonite group, representing the Se-analogue of součekite, CuPbBi(S,Se)3. It is deposited from strongly oxidizing low-T hydrothermal fluids at a f Se2/f S2 ratio >1, both as primary and secondary phase. The new species has been approved by the IMA-CNMNC (2018-040) and is named for Cerro Mojon, the highest mountain peak closest to the El Dragón mine.

This paper provides a description of a new species in the Cu-Hg-Pb-Bi-Se system, cerromojonite, ideally CuPbBiSe 3 , from El Dragón.This new species and its name have been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the IMA, proposal 2018-040.The X-rayed crystal is preserved by one of the authors (L.B.) at the Dipartimento di Scienze della Terra, Università degli Studi di Firenze.The polished section, from which the crystal was extracted (holotype), is housed in the collections of the Natural History Museum, London, with the catalogue number BM 2018, 11.Cotype material, consisting of another cerromojonite-bearing polished section, is deposited within the Mineralogical State Collection Munich (Mineralogische Staatssammlung München, Museum "Reich der Kristalle"), with the inventory number MSM 73583.
The species is named for Cerro Mojon, the highest mountain peak nearest to El Dragón (4292 m above sea level), located about 800 m northeast of the mine.
Unnamed phase "C", described at El Dragón already in 2016 [3], for which no structural data were obtained, compositionally resembles cerromojonite.A species similar to cerromojonite was speculated to occur in carbonate veins in the Schlema-Alberoda U-Se-polymetallic deposit (Erzgebirge, Germany), forming tiny inclusions which are intimately intergrown with berzelianite [9,10].However, neither compositional data of pure material nor structural data were provided for this material.

Geology
The abandoned El Dragón mine is situated in the Cordillera Oriental (southwestern Bolivia), about 30 km southwest of Cerro Rico de Potosí.It is located at 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 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, 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.

Physical and Optical Properties
Cerromojonite occurs within two different mineral assemblages, which are interpreted as representing genetically distinct types.
Cerromojonite is black in color and possesses a black streak.The mineral is opaque in transmitted light, exhibits a metallic luster, and is non-fluorescent.No cleavage and parting is observed, and the fracture is irregular.Density and Mohs hardness could not be measured owing to the small crystal size.The calculated density is 7.035 g/cm 3 (for Z = 4), based on the empirical formula (see below) and the unit-cell parameters derived from X-ray single-crystal refinement.
Minerals 2018, 8, x FOR PEER REVIEW 3 of 14 3 minerals are themselves altered by late klockmannite, fracture-filling chalcopyrite, covellite, goethite, endmember petříčekite and krut'aite (CuSe2), and native selenium (Figure 5).Cerromojonite is black in color and possesses a black streak.The mineral is opaque in transmitted light, exhibits a metallic luster, and is non-fluorescent.No cleavage and parting is observed, and the fracture is irregular.Density and Mohs hardness could not be measured owing to the small crystal size.The calculated density is 7.035 g/cm 3 (for Z = 4), based on the empirical formula (see below) and the unit-cell parameters derived from X-ray single-crystal refinement.minerals are themselves altered by late klockmannite, fracture-filling chalcopyrite, covellite, goethite, endmember petříčekite and krut'aite (CuSe2), and native selenium (Figure 5).Cerromojonite is black in color and possesses a black streak.The mineral is opaque in transmitted light, exhibits a metallic luster, and is non-fluorescent.No cleavage and parting is observed, and the fracture is irregular.Density and Mohs hardness could not be measured owing to the small crystal size.The calculated density is 7.035 g/cm 3 (for Z = 4), based on the empirical formula (see below) and the unit-cell parameters derived from X-ray single-crystal refinement.In plane-polarized incident light, cerromojonite is grey to cream-white.In the assemblage with klockmannite and watkinsonite, it is weakly pleochroic or bireflectant.The mineral does not show any internal reflections.Between crossed polarizers, cerromojonite is weakly anisotropic, with rotation tints in shades of brown and grey (Figure 6).Twinning of cerromojonite is expressed either by distinct sharp polysynthetic lamellae (Figure 7) or by finely divided twinning in fan-shaped aggregates.In plane-polarized incident light, cerromojonite is grey to cream-white.In the assemblage with klockmannite and watkinsonite, it is weakly pleochroic or bireflectant.The mineral does not show any internal reflections.Between crossed polarizers, cerromojonite is weakly anisotropic, with rotation tints in shades of brown and grey (Figure 6).Twinning of cerromojonite is expressed either by distinct sharp polysynthetic lamellae (Figure 7) or by finely divided twinning in fan-shaped aggregates.In plane-polarized incident light, cerromojonite is grey to cream-white.In the assemblage with klockmannite and watkinsonite, it is weakly pleochroic or bireflectant.The mineral does not show any internal reflections.Between crossed polarizers, cerromojonite is weakly anisotropic, with rotation tints in shades of brown and grey (Figure 6).Twinning of cerromojonite is expressed either by distinct sharp polysynthetic lamellae (Figure 7) or by finely divided twinning in fan-shaped aggregates.Quantitative reflectance measurements were performed in air relative to a WTiC standard (Zeiss number 314) by means of a J & M TIDAS diode array spectrometer (J & M Analytik AG, Essingen, Germany), running ONYX software (Version 1.1, Cavendish Instruments Ltd., Sheffield, UK) on a Zeiss Axioplan ore microscope (Carl Zeiss AG, Oberkochen, Germany) (Table 1, Figure 8).Measurements were made on unoriented grains at extinction positions leading to the designation of R1 (minimum) and R2 (maximum).Quantitative reflectance measurements were performed in air relative to a WTiC standard (Zeiss number 314) by means of a J & M TIDAS diode array spectrometer (J & M Analytik AG, Essingen, Germany), running ONYX software (Version 1.1, Cavendish Instruments Ltd., Sheffield, UK) on a Zeiss Axioplan ore microscope (Carl Zeiss AG, Oberkochen, Germany) (Table 1, Figure 8).Measurements were made on unoriented grains at extinction positions leading to the designation of R 1 (minimum) and R 2 (maximum).

X-ray Crystallography and Description of the Crystal Structure
X-ray powder diffraction data (Table 4) were obtained from the same fragment used for the single-crystal study (see below), with an Oxford Diffraction Excalibur PX Ultra diffractometer (Oxford Diffraction, Oxford, UK), fitted with a 165 mm diagonal Onyx CCD detector, and using copper radiation (CuKα, λ = 1.54138Å).The working conditions were 40 kV and 40 mA, with 1 hour of exposure; the detector-to-sample distance was 7 cm.The program Crysalis RED [14] was used to convert the observed diffraction rings to a conventional powder diffraction pattern.Least squares refinement gave the following orthorhombic unit-cell values: a = 8.2004(6) Å, b = 8.7461(5) Å, c = 8.0159(5) Å, V = 574.91(5)Å 3 , and Z = 4.
A small crystal (0.040 × 0.055 × 0.060 mm 3 ) of type-II cerromojonite was handpicked from the holotype specimen (it is registered under the number #123 in the mineralogical collection of one of the Authors, G.G.).The crystal was preliminarily examined with a Bruker-Enraf MACH3 single-crystal diffractometer (Bruker, Karlsruhe, Germany), using graphite-monochromatized MoKα radiation.Single-crystal X-ray diffraction intensity data were collected using an Oxford Diffraction Xcalibur diffractometer equipped with an Oxford Diffraction CCD detector, with graphite-monochromatized MoKα radiation (λ = 0.71073 Å).The data were integrated, and corrected for standard Lorentz and polarization factors, with the CrysAlis RED package [12].The program ABSPACK in CrysAlis RED [12] was used for the absorption correction.Table 5 reports details of the selected crystal, data collection, and refinement.Statistical tests (|E 2 − 1| = 0.821) and systematic absences agreed with the acentric space group Pn2 1 m.The crystal structure was refined starting from the atomic coordinates of bournonite [15].Given the observed larger unit-cell volume of cerromojonite (i.e., 575.7 Å 3 ) compared to bournonite (i.e., 552.3 Å 3 ; [15]), the site occupancy factor (s.o.f.) at the crystallographic sites was allowed to vary (Pb vs. Ag and Bi vs. Ag for the Pb and Bi sites; Cu vs. Hg for the Cu site; Se vs. S for the anionic site), using scattering curves for neutral atoms taken from the International Tables for Crystallography [16].After several cycles of anisotropic refinement, a final R 1 = 0.0256 for 701 reflections with F o > 4 σ(F o ) and 68 refined parameters was achieved (0.0315 for all 1359 reflections).Atomic coordinates, site occupancies, and equivalent isotropic displacement parameters are listed in Table 6, whereas anisotropic displacement parameters are given in Table 7. Selected bond distances and bond-valence sums are provided in Table 8.The Crystallographic Information File (CIF) is available as Supplementary Material.

Discussion
Cerromojonite is a new member of the bournonite group and represents the Se-analogue of součekite, CuPbBi(S,Se)3 [12,18].Interestingly, whereas previously analyzed součekite always contained appreciable amounts of Se (together with minor Te) substituting for S, cerromojonite from El Dragón is practically devoid of S, containing S at concentrations below its detection limit of ~200 ppm.Moreover, součekite is characterized by an almost ideal occupancy of the Cu-, Pb-, and Bi-sites, in contrast to cerromojonite, where significant amounts of other cations, in particular Ag and Hg, The overall crystallochemical formula, as obtained through the single-crystal X-ray diffraction study, is [Cu 0.880 Hg 0.120 ]Bi[Pb 0.560 Ag 0.155 Hg 0.142 Bi 0.107 0.036 ]Se 3 (Z = 4), and is in excellent agreement with that obtained from electron microprobe data.

Discussion
Cerromojonite is a new member of the bournonite group and represents the Se-analogue of součekite, CuPbBi(S,Se) 3 [12,18].Interestingly, whereas previously analyzed součekite always contained appreciable amounts of Se (together with minor Te) substituting for S, cerromojonite from El Dragón is practically devoid of S, containing S at concentrations below its detection limit of ~200 ppm.Moreover, součekite is characterized by an almost ideal occupancy of the Cu-, Pb-, and Bi-sites, in contrast to cerromojonite, where significant amounts of other cations, in particular Ag and Hg, entered the structure.
Conclusions on the physico-chemical environment of cerromojonite formation could be drawn from the associated Cu selenides.It is apparent that type-I cerromojonite crystallized together with umangite and klockmannite, implying that selenium fugacities (f Se2 ) fluctuated around values defined by the umangite-klockmannite univariant reaction.Type-II cerromojonite precipitated in equilibrium with klockmannite, outside the stability fields of umangite and krut'aite.At T = 100 • C, a temperature typical for the formation of vein-type selenide deposits, and an elevated oxygen fugacity defined by the magnetite-hematite buffer, these paragenetic relations are consistent with the range of log f Se2 between −14.6 and −11.6 [19].The absence of krut'aite and sulfides (chalcopyrite, pyrite) define the maximum log of sulfur fugacity (f S2 ) to be roughly −19.

Figure 4 .
Figure 4. BSE image of parallel-intergrown type-II cerromojonite (ce) grains with darker domains of an unknown Cu-(Ag)-Hg-Pb-Bi selenide.The biggest grain of this potentially new species is marked by a question mark.

Figure 4 .
Figure 4. BSE image of parallel-intergrown type-II cerromojonite (ce) grains with darker domains of an unknown Cu-(Ag)-Hg-Pb-Bi selenide.The biggest grain of this potentially new species is marked by a question mark.

Figure 4 .
Figure 4. BSE image of parallel-intergrown type-II cerromojonite (ce) grains with darker domains of an unknown Cu-(Ag)-Hg-Pb-Bi selenide.The biggest grain of this potentially new species is marked by a question mark.

Figure 9 .
Figure 9.The crystal structure of cerromojonite projected down [001] (six unit-cells).The unit-cell and orientation of the figure are outlined.Symbols: Bi = green dots, Pb = blue dots, Cu = light blue dots, Se = orange dots.

Figure 10 .
Figure 10.The crystal structure of cerromojonite projected down [100] (six unit-cells).Symbols as in Figure 9. CuSe 4 tetrahedra are depicted as light blue polyhedra.The unit-cell and orientation of the figure are outlined.

Author
Contributions: G.G. collected the samples and manufactured the polished sections; H.-J.F. and G.G. found the new mineral; H.-J.F.conducted the electron-microprobe analyses; L.B. performed the X-ray structural investigations; C.J.S. and G.G. determined the optical and physical properties; H.-J.F.wrote the paper.Funding: This research received no external funding.

Table 3 .
Representative results of electron-microprobe spot analyses of cerromojonite.

Table 4 .
Measured and calculated X-ray powder diffraction data (d in Å) for cerromojonite.The strongest measured diffraction lines are given in bold.Note: calculated diffraction pattern obtained with the atom coordinates reported in Table6(only reflections with I rel ≥ 3 are listed).

Table 5 .
Data and experimental details for the selected cerromojonite crystal.

Table 7 .
Anisotropic displacement parameters (U) of the atoms for the selected cerromojonite crystal.

Table 8 .
Bond distances (in Å) and bond valence sums (BVS in valence units) in the structure of cerromojonite.