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

Abstract

Quijarroite, ideally Cu₆HgPb₂Bi₄Se₁₂, is a new selenide species from the El Dragόn 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 sub- to 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 (Cu_5.84Ag_0.26)_{Σ = 6.10}(Hg_1.06Ni_0.07Co_0.01)_{Σ = 1.14}Pb_2.27Bi_3.68Se_11.81 (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) ų, 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 $f_{{Se}_2}$/$f_{S_2}$ 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.

1. Introduction

The Andes of Bolivia host two selenide occurrences of mineralogical interest: Pacajake, District of Hiaco de Charcas, and El Dragόn, Province of Antonio Quijarro, both in the Department of Potosí. The geology and ore mineralization of the El Dragόn mine was first explored by Grundmann et al. (1990) [1]. It is the type locality of eldragόnite, Cu₆BiSe₄(Se₂) [2]; favreauite, PbBiCu₆O₄(SeO₃)₄(OH)∙H₂O [3]; grundmannite, CuBiSe₂ [4]; hansblockite, (Cu,Hg)(Bi,Pb)Se₂ [5]; and alfredopetrovite, Al₂(Se⁴⁺O₃)₃∙6H₂O [6]. This Se mineralization also contains the recently discovered orthorhombic modification of CuSe₂, petříčekite, as a late-stage phase [7].

This paper provides the description of a new species in the Cu−Hg−Pb−Bi−Se system, quijarroite, ideally Cu₆HgPb₂Bi₄Se₁₂, from El Dragόn. The most frequently associated minerals include hansblockite, unnamed phase “C” [4], empirical formula Cu₄HgPb₂Bi₄Se₁₁ (normalized to 22 atoms per formula unit, apfu), clausthalite, PbSe, and watkinsonite, Cu₂PbBi₄Se₈.

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].

2. 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 Dragόn 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 Dragόn 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 Dragόn is given on [9]. A comprehensive survey of the mineralogy and origin of the El Dragόn mineralization forms the subject of a companion study [10], in this issue.

3. Physical and Optical Properties

Quijarroite most frequently occurs as lath-shaped thin plates (up to 150 µm in length and 20 µm in width) intimately (subparallel) intergrown with hansblockite and, rarely, unnamed phase “C” [4], forming an angular network-like intersertal texture (Figure 1). Phase “C” occurs in the interstices of the quijarroite/hansblockite intersertal intergrowths, often together with Co-rich penroseite, NiSe₂, umangite, Cu₃Se₂, klockmannite, CuSe, watkinsonite, and clausthalite.

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, CuSe₂-NiSe₂, eskebornite, CuFeSe₂, klockmannite and umangite (Figure 2 and Figure 3). Minerals occasionally being in grain-boundary contact encompass petrovicite, Cu₃HgPbBiSe₅, 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/cm³.

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. Reflectance data for quijarroite.

λ (nm)	R1 (%)	R2 (%)	λ (nm)	R1 (%)	R2 (%)
400	45.1	45.6	560	47.30	48.4
420	45.3	45.8	580	47.2	48.5
440	45.7	46.1	600	47.0	48.6
460	46.4	46.6	620	46.8	48.7
480	46.9	47.0	640	46.6	48.7
500	47.3	47.5	660	46.5	48.7
520	47.7	47.8	680	46.4	48.7
540	47.4	48.1	700	46.3	48.8

). Measurements were made on unoriented grains at extinction positions, leading to the designation of R₁ (minimum) and R₂ (maximum).

Reflectance percentages for the four COM (Commission on Ore Mineralogy) wavelengths (R₁ and R₂) are: 46.7, 46.8 (470 nm); 47.4, 48.2 (546 nm); 47.1, 48.5 (589 nm); 46.6, 48.7 (650 nm).

4. Chemical Data

Quijarroite was checked for concentrations of Cu, Ag, Pb, Hg, Fe, Co, Ni, As, Sb, Bi, S and Se. Quantitative chemical analyses were conducted in wavelength-dispersive (WDS) mode, using a JEOL thermal field-emission-type electron probe X-ray microanalyzer (FE-EPMA) JXA-8500F (JEOL Ltd., Akishima, Japan). The probe was operated at 20 kV, 20 nA; the beam size was 1–2 µm. The counting time on the peak was 20 s, with half that time on the background on both sites of the peak. The following standards, emission lines and analyzing crystals (in parentheses) were used: Cu—synthetic Cu-metal, Kα (LIF); Ag—naumannite, Lα (PETJ); Pb—clausthalite, Mα (PETH); Hg—cinnabar, Lα (LIF); Fe—pyrite, Kα (LIF); Co—skutterudite, Kα (LIF); Ni—pentlandite, Kα (LIF); As—skutterudite, Lα (TAP); Sb—stibnite, Lα (PETJ); Bi—synthetic Bi₂Se₃, Mα (PETH); S—sphalerite, Kα (PETJ); Se—naumannite, Kα (LIF). The CITZAF routine in the JEOL software, which is based on the ф(ϱZ) method [11], was used for data processing. The composition of the grain used for the structural study corresponds chemically to the other grains analyzed by the microprobe. All grains are homogeneous within analytical error.

Table 2. Chemical data for quijarroite.

Element	Mean	Range	SD
Cu (wt %)	13.34	12.50–13.86	0.28
Ag	1.02	0.42–1.76	0.28
Hg	7.67	7.31–8.16	0.22
Pb	16.87	16.55–17.11	0.13
Co	0.03	0.00–0.11	0.04
Ni	0.15	0.00–0.45	0.15
Bi	27.65	27.37–28.05	0.19
Se	33.52	33.19–34.00	0.20
Total	100.24	99.64–100.65	0.30

Note: SD = 1σ standard deviation.

compiles the analytical data for quijarroite (mean of 24 spot analyses; range, and standard deviation).

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

Element	d.l. (ppm)	1	2	3	4	5	6
Cu (wt %)	250	13.79	13.33	12.88	13.42	12.50	13.36
Ag	200	0.42	1.05	1.57	0.71	1.76	1.09
Hg	1100	7.56	7.96	7.40	7.31	7.75	8.16
Pb	400	16.86	16.90	16.76	16.93	16.76	16.79
Co	200	0	0.11	0.06	0	0	0
Ni	200	0.04	0.11	0.09	0.45	0.14	0
Bi	300	27.49	27.46	27.80	27.46	27.51	27.69
Se	800	33.86	33.39	33.19	33.36	33.60	33.49
Total		100.02	100.30	99.76	99.64	100.03	100.57
Cu (apfu)		6.02	5.84	5.69	5.88	5.51	5.85
Ag		0.11	0.27	0.41	0.18	0.46	0.28
Hg		1.04	1.10	1.04	1.02	1.08	1.13
Pb		2.26	2.27	2.27	2.28	2.27	2.25
Co			0.05	0.03
Ni		0.02	0.05	0.04	0.21	0.07
Bi		3.65	3.66	3.73	3.66	3.69	3.69
Se		11.90	11.76	11.79	11.77	11.92	11.80

Note: d.l. = detection limit.

provides a selection of microprobe spot analysis on quijarroite, together with the elemental detection limits (d.l.).

The empirical formula of quijarroite (based on 25 apfu) is (Cu_05.84Ag_0.26)_{Σ = 6.10}(Hg_1.06Ni_0.07Co_0.01)_{Σ = 1.14}Pb_2.27Bi_3.68Se_11.81. The ideal formula is Cu₆HgPb₂Bi₄Se₁₂, which requires (in wt %) Cu 13.72, Hg 7.22, Pb 14.91, Bi 30.06, Se 34.09, sum 100.00.

5. X-ray Crystallography and Crystal Structure

Powder X-ray data (Cu Kα radiation) were collected with an automated CCD-equipped Oxford Diffraction Xcalibur PX single-crystal diffractometer (Oxford Diffraction, Oxford, UK) using a Cu Kα radiation (Gandolfi-type data collection). The measured and calculated (using the software PowderCell 2.3 [6]) powder diffraction patterns are given in

Table 4. Measured and calculated X-ray powder diffraction data (d in Å) for quijarroite. The strongest diffraction lines are given in bold.

hkl	dmeas	Imeas	dcalc	Icalc
010	-	-	9.0206	47
101	6.65	20	6.6651	23
111	5.36	55	5.3606	50
002	4.82	20	4.8109	18
020	-	-	4.5103	12
012	-	-	4.2450	5
210	-	-	4.1125	6
120	-	-	4.0533	5
211	3.785	60	3.7816	58
202	3.331	40	3.3325	45
022	3.291	90	3.2904	95
220	3.228	25	3.2276	29
212	3.125	100	3.1260	100
122	-	-	3.0998	5
221	3.059	25	3.0600	30
030	-	-	3.0069	8
113	2.871	15	2.8723	16
031	-	-	2.8700	7
130	-	-	2.8593	5
222	-	-	2.6803	9
032	2.551	10	2.5498	12
213	-	-	2.5291	10
230	-	-	2.5202	11
014	-	-	2.3243	11
400	2.312	50	2.3103	46
040	2.256	10	2.2552	13
232	2.233	10	2.2325	15
033	-	-	2.1936	11
313	-	-	2.1572	6
024	-	-	2.1225	9
214	2.078	35	2.0764	40
241	-	-	1.9831	15
233	1.981	30	1.9816	29
332	-	-	1.9642	7
224	1.926	10	1.9287	12
422	1.888	30	1.8908	35
242	1.865	20	1.8677	22
050	-	-	1.8041	6
215	-	-	1.7430	9
234	-	-	1.7401	7
432	-	-	1.7121	6
414	-	-	1.6385	7
440	-	-	1.6138	8
433	-	-	1.5908	6
252	-	-	1.5865	6
424	-	-	1.5630	6
612	-	-	1.4479	7
614	-	-	1.2839	6
264	-	-	1.2290	6

Note: Calculated diffraction pattern obtained with the atom coordinates reported in Table 6 (only reflections with I_rel ≥ 5 are listed).

. Unit-cell parameters refined from the collected data are as follows: a = 9.2376(8), b = 9.0176(7), c = 9.6198(8) Å, V = 801.34(8) ų, Z = 1.

For the X-ray single-crystal diffraction study, a small crystal fragment (0.03 × 0.035 × 0.05 mm³) 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. Crystal data and details of data collection and refinement for quijarroite.

Crystal Data
space group	Pmn21
cell parameters	a = 9.2413(8) (Å)
	b = 9.0206(7) (Å)
	c = 9.6219(8) (Å)
	V = 802.1(1) (Å3)
Z	1
crystal color	black
crystal shape	block
crystal size (mm3)	0.030 × 0.035 × 0.050
Data Collection
diffractometer	Oxford Diffraction Xcalibur III
radiation type	MoKα (λ = 0.71073 Å)
monochromator	oriented graphite (002)
scan mode	ϕ/ω
temperature (K)	293
detector to sample distance (cm)	5
number of frames	702
rotation width per frame (°)	0.15
measuring time (s)	100
maximum covered 2θ (°)	60.02
range of h, k, l	−13 ≤ h ≤ 13, −12 ≤ k ≤ 12, −13 ≤ l ≤ 13
collected reflections	9312
Rint before absorption correction	0.0840
Rint after absorption correction	0.0235
Refinement
refinement coefficient	F2
No. of refl. in refinement	2195
No. of observed refl.	1523
No. of refined parameters	69
weighting scheme	w = 1/[σ2(I) + (0.044 × I)2]
R † (obs)/R † (all)	0.0276/0.0281
wR2 † (obs)/wR2 † (all)	0.0242/0.0258
Flack parameter	−0.002(9)
diff. Fourier (e−/Å3)	[−0.42, 0.76]

Note: ^† R = Σ||F_o| − |F_c||/|F_o|. wR² = [Σ w (|F_o|² − |F_c|²)²/Σ w (|F_o|⁴)]^1/2.

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 Pmn2₁, P2₁nm and Pmnm, and the statistical tests on the distribution of |E| values strongly indicated the absence of an inversion center (|E² – 1| = 0.695). The structure solution was then initiated in the standard setting of space group Pmn2₁. The refined unit-cell parameters are a = 9.2413(8), b = 9.0206(7), c = 9.6219(8) Å, V = 802.1(1) ų, 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_0.75Hg_0.25, whereas Cu2 is occupied at 75% (cf.

Table 6. Wyckoff positions, site occupation factors (s.o.f.), fractional atomic coordinates (x,y,z), and equivalent isotropic displacement parameters (U_iso, Ų) for the selected quijarroite crystal.

Atom	Wyckoff	s.o.f.	x	y	z	Uiso
Cu1	4b	Cu0.75(1)Hg0.25	0.2311(3)	0.2658(4)	0.8228(4)	0.0381(11)
Pb	2a	Pb1.00	½	0.4971(3)	0.1768(2)	0.0424(6)
Bi1	2a	Bi1.00	½	0.0703(2)	0.0560(3)	0.0410(5)
Bi2	2a	Bi1.00	0	0.5064(2)	0.1495(2)	0.0369(5)
Cu2	4b	Cu0.75(2)☐0.25	0.2437(6)	0.2757(9)	0.4230(11)	0.0430(2)
Se1	2a	Se1.00	0	0.2400(6)	0.2794(6)	0.0342(11)
Se2	2a	Se1.00	½	0.2319(6)	0.2990(6)	0.0340(11)
Se3	4b	Se1.00	0.2373(4)	0.0861(5)	0.6428(5)	0.0362(8)
Se4	4b	Se1.00	0.2685(4)	0.5532(5)	0.4925(5)	0.0374(8)

). 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⁻/ų, respectively. Wyckoff positions, site occupation factors, fractional atomic coordinates, and equivalent isotropic displacement parameters (Ų) are given in Table 6. The main interatomic distances (Å) are reported in

Table 7. Main interatomic distances (Å) for the studied quijarroite crystal.

Cu1-Se4 i	2.309(6)	Bi2-Se4 vi	2.673(4)
Cu1-Se3	2.373(6)	Bi2-Se4 ii	2.673(4)
Cu1-Pb i	3.333(4)	Bi2-Se1	2.708(6)
Pb-Se2	2.665(6)	Cu2-Se4	2.601(9)
Pb-Se4 ii	3.083(4)	Cu2-Se1	2.662(7)
Pb-Se4 iii	3.083(4)	Cu2-Se2	2.681(7)
Pb-Cu iii	3.333(4)	Cu2-Se3	2.72(1)
Bi1-Se3 iv	2.738(4)
Bi1-Se3 v	2.738(4)
Bi1-Se2	2.755(6)

Note: Symmetry codes: (i) −x + 1/2, −y + 1, z + 1/2; (ii) −x + 1/2, −y + 1, z − 1/2; (iii) x + 1/2, −y + 1, z − 1/2; (iv) −x + 1/2, −y, z − 1/2; (v) x + 1/2, −y, z − 1/2; (vi) x − 1/2, −y + 1, z − 1/2.

.

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, CuPbSbS₃, seligmannite, CuPbAsS₃, 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 Pn2₁m), Pb forms 7,8-fold polyhedra, M³⁺ (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. CuS₄ 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_0.75Hg_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 1.33 v.u. (vs. the ideal value of 1.25). The two Bi positions lead to 2.61 and 3.06 v.u. (for Bi1 and Bi2, respectively) and the Pb positions to a value of 1.74 v.u. The tetrahedral site exhibits a mean bond distance of 2.666 Å, which results in a bond valence sum of 0.73 v.u. (vs. the ideal value of 0.75, given the structural vacancy present at this site). If we compare the formula of quijarroite to that of the members of the bournonite group, we should write [Cu_0.75☐_0.25][Cu_0.75Hg_0.25]Pb_0.5BiSe₃, which leads to Cu_1.50Hg_0.25Pb_0.5BiSe₃ (with Z = 4), or Cu₆HgPb₂Bi₄Se₁₂ (with Z = 1). Quijarroite shows a strong enlargement of the unit-cell volume with respect to the members of the bournonite group (802.1(1) ų vs. 538.8, 551.7 and 559.7 ų, in seligmannite, bournonite and součekite, respectively). This increase is mainly due to the presence of Hg and Se replacing S.

6. 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).

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_6.50Hg_0.96Pb_2.04Bi_3.72Se₁₂. 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.