The small Předbořice uranium deposit, Central Bohemia Region, Czech Republic (at ~49°32'57.590" N (latitude), 14°15'12.449" E (longitude)) is a famous mineral locality especially for its richness in rare selenides. It is the type-locality for four minerals, i.e.
, fischesserite Ag3
, hakite (Cu6
), milotaite PdSbSe, and permingeatite (Cu,Fe)4
. From 1961 to 1978 a total of 250 tons of uranium was mined out of over 100 low temperature hydrothermal veins between the small villages of Předbořice and Lašovice cutting through the krásnohorsko-sedlčanský metamorphic islet, close to its contact with granitoids of central bohemian pluton. Mineralized fissures are complicated veins up to 25−100 m long, 25−50 m high and up to 30 cm (max. 1 m) thick. The main ore mineral is uraninite, and the main gangue minerals are quartz, hematite-bearing calcite, and barite. Předbořice mineralization furthermore includes among others: aguilarite Ag4
SeS, athabascaite Cu5
, berzelianite, bukovite Tl2
, chaméanite (Cu,Fe)4
, chrisstanleyite Ag2
, clausthalite, eskebornite CuFeSe2
, eucairite AgCuSe, ferroselite FeSe2
, giraudite Cu6
, jolliffeite NiAsSe, krut’aite-trogtalite series, klockmannite, merenskyite (Pd)(Te,Se)2
, naumannite, telargpalite (Pd,Ag)3
(Te,Bi), tiemannite, tyrrellite, umangite, and native gold (chemical compositions above are given as ideal formulae). Further details of the low-temperature selenide association from the Předbořice deposit are provided by Johan (1989) [1
Petříčekite, CuSe2, has been identified in two specimens bought at a mineral fair. It generally occurs either as fractured inclusions in large eucairite grains closely associated with athabascaite/klockmannite and unknown selenide phases which are currently under investigation, or as fractured inclusions in tiemannite closely associated with eskebornite. Petříčekite was approved as a new mineral by the Commission of New Minerals, Nomenclature and Classification of IMA (2015-111). The mineral name honors Václav Petříček (b. 1948), Czech crystallographer (Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic), for his outstanding contributions to crystallography in general and mineralogical crystallography in particular. His pioneering studies of incommensurately modulated and composite structures and the development of the computer system JANA, have represented a milestone for all the members of the community working with the structural complexity of mineral structures. The holotype material from Předbořice is deposited in the reference collection of the Harvard Mineralogical and Geological Museum, reference number MGMH#2016.01.
In this study we report the description of the new mineral petříčekite, together with data on its crystal structure. In addition to the description of the new species from Předbořice, we provide the optical properties and compositional as well as structural data for petříčekite from a second occurrence, the El Dragón mine in Bolivia, and we infer the presence of petříčekite from a third occurrence, Sierra de Cacheuta in Argentina, on the basis of optical and chemical data.
2. Physical and Optical Properties
Petříčekite from Předbořice occurs as euhedral to subhedral grains (up to 150 μm in diameter) either solitary in the eucairite matrix or as angular fragments cemented prevalently by eucairite (Figure 1
and Figure 2
Petříčekite is black in color and shows a black streak. The mineral is opaque in transmitted light and exhibits a metallic luster. No cleavage is observed and the fracture is uneven. The calculated density (for Z = 2) for the empirical formula (see below) is 6.673 g/cm3. Unfortunately, the density could not be measured because of the small grain size. Micro-indentation measurements carried out with a VHN (Vickers Hardness Number) load of 15 g give a mean value of 33 kg/mm2 (range: 28–40) corresponding to a Mohs hardness of about 2–2½.
In plane-polarized incident light, petříčekite from the type locality is pale blue grey to pale pinkish, weakly pleochroic and weakly bireflectant from slightly blue-grey to slightly pinkish-grey. Between crossed polars, it is anisotropic with light grey-blue to light pink rotation tints. Internal reflections are absent and there is no optical evidence of oriented growth zonation. Notably, pleochroism and anisotropy increase significantly in near end-member petříčekite from El Dragón. Reflectance measurements for petříčekite from both occurrences were obtained 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 percentages for the four COM wavelengths (R1 and R2) for petřičekite from Předbořice and from El Dragón are: 42.35, 41.8 (470 nm), 42.0, 42.2 (546 nm), 41.9, 42.35 (589 nm), 42.05, 42.85 (650 nm) and 24.0, 36.3 (470 nm), 21.4, 28.2 (546 nm), 21.9, 25.1 (589 nm), 29.8, 22.9 (650 nm) respectively.
At El Dragón, petříčekite occurs in euhedral to subhedral crystals and partly oriented crystal aggregates forming skeletal and/or myrmekitic aggregates of up to 200 μm (Figure 3
). Petříčekite aggregates cement (usually together with krut’aite and needle-like crystals of a yet undescribed anisotropic mineral resembling the ideal composition Cu3
) shrinkage cracks and any kind of pores and vugs or fill interstices in brecciated krut’aite-penroseite. It is locally also present as euhedral to subhedral crystals up to 25 μm in width, occurring either isolated, rarely twinned, or intergrown with krut’aite and Cu3
, formed as late-stage replacement product of clausthalite and/or krut’aite-penroseite (Figure 4
). Homogeneous grains are exceptional; typically, they are replacement remnants constituting intimate intergrowths with krut’aite and other primary and secondary species down to the nm-scale, including native selenium and goethite (Figure 5
). The El Dragón petříčekite itself is usually partly or completely replaced by krut’aite. Minerals that are occasionally in grain-boundary contact are klockmannite, watkinsonite and native selenium. Other minerals adjacent to petříčekite encompass quartz, calcite, barite, covellite, goethite, lepidocrocite, chalcomenite, molybdomenite, olsacherite, schmiederite, ahlfeldite, favreauite, felsőbányaite and allophane. Petříčekite from El Dragón is non-fluorescent, black and opaque with a metallic luster and black streak. It is brittle with an irregular fracture and no obvious parting or cleavage.
Optical properties of end-member petříčekite from El Dragón are displayed in Figure 3
. In plane-polarized incident light, it is violet in color, slightly bireflectant and strongly pleochroic from violet to blue. The mineral does not show any internal reflections. Between partly crossed polars, petříčekite is strongly anisotropic, with copper-red to light grey rotations tints.
Finally, petříčekite of end-member composition (CuSe2
) is likely present also at the selenium mineralization near Cerro de Cacheuta (= Sierra de Cacheuta), Luján de Cuyo Department, Mendoza, Argentina (Figure 6
The Sierra de Cacheuta deposit (at ~33°01'18" S (latitude), 69°07'40.50" W (longitude)) is at an altitude of about 1580 m above sea level and is the type locality for achávalite (Fe,Cu)Se and molybdomenite PbSeO3. In the 1860s, the selenium mineralization was discovered in calcitic veinlets in porphyry forming a fine-grained mixture of clausthalite, naumannite, klockmannite, umangite, berzelianite, eucairite, tyrrellite and eskebonite. The petříčekite-bearing specimen from Cacheuta described herein was purchased in the late 1970s from an American mineral dealer at the Munich Mineral Fair.
Despite detailed investigations during the last years, petříčekite, of nearly end-member composition from El Dragón and Sierra de Cacheuta obviously remained undetected probably to have been classified as umangite because of very similar optical properties.
3. Chemical Data
Quantitative chemical analyses were performed using a JEOL Hyperprobe JXA-8500F electron-microprobe (Akishima, Tokyo, Japan), operating in WDS (Wavelength Dispersive Spectrometry) mode. The experimental conditions were: accelerating voltage 20 kV, beam current 20 nA, beam size 1 μm. No elements other than those indicated below with atomic number greater than 5 were detected. Standards are (element, emission line): naumannite (Ag, Se L
α), Cu metal (Cu K
α), cinnabar (Hg L
α), clausthalite (Pb M
α), pyrite (Fe K
α), Pd metal (Pd L
α), and sphalerite (Zn K
α). The crystal fragment is homogeneous within analytical errors. Table 2
gives analytical data (average of nine spot analyses) for the grains from which the structural data were obtained.
The empirical formula of holotype petříčekite from Předbořice (based on three atoms pfu, Me + 2(Se + S)) is (Cu0.53Fe0.48)Σ1.01(Se1.98S0.01)Σ1.99. The simplified formula is CuSe2, which requires Cu 28.69 wt % and Se 71.31 wt %, total 100.00 wt %.
The formula of the Cu-richest grain additionally studied from a structural point of view corresponds to (Cu0.70Fe0.30)Σ1.00Se2.00 (using the same instrument and the same experimental conditions as above). The domain closest to end-member composition measured in petříčekite from the type sample has the composition (Cu0.74Fe0.27Ag0.01)Σ1.02Se1.98.
Petříčekite from Předbořice displays a wider range in composition than indicated in Table 2
. Compositional data (n
= 66) are summarized in Table 3
. The mean formula is (Cu0.58
. Concentrations of “minor” elements (in wt %) maximize to: Ag = 1.5, Hg = 3.6, Ni = 1.8, Pd = 9.6. The composition of the most Pd-rich grain is (Cu0.49
Petříčekite from El Dragón and from Sierra de Cacheuta has practically end-member composition (Cu0.99
). Elements other than Cu and Se detected by microprobes (Ag, Co, Ni, S) occur at concentrations <0.1 wt % (cf. Table 3
). Associated krut’aite is also devoid of non-stoichiometric elements (Cu1.00
); a composition not yet reported from any other occurrence.
4. X-Ray Crystallography
For the X-ray single-crystal diffraction study, the intensity data were collected using an Oxford Diffraction Xcalibur 3 diffractometer (Oxford Diffraction, Oxford, UK), equipped with a Sapphire 2 CCD area detector, with Mo K
α radiation. The detector to crystal working distance was 6 cm. Intensity integration and standard Lorentz-polarization corrections were performed with the CrysAlis
] software package. The program ABSPACK in CrysAlis
] was used for the absorption correction. Tests on the distribution of |E
| values agree with the occurrence of an inversion centre (|E2
− 1| = 0.967). This information, together with the observed systematic absences, suggested the space group Pnnm
. The refined unit-cell parameters are a
= 4.918(2) Å, b
= 6.001(2) Å, c
= 3.670(1) Å, V
= 108.31(1) Å3
The crystal structure was refined with Shelxl
] starting from the atomic coordinates of synthetic CuSe2
]. The site occupancy factors (s.o.f.) were refined using the scattering curves for neutral atoms given in the International Tables for Crystallography
]. Crystal data and details of the intensity data collection and refinement are reported in Table 4
After several cycles of isotropic refinement, the R1
converged to 0.13, thus confirming the correctness of the structural model. The refinement of the mean electron number at the Cu site (against structural vacancy) produced an occupancy of Cu = 95.1%, thus indicating a population that can be written as Cu0.53
. With the introduction of an anisotropic model for all the atoms, the R1
dropped to 0.034. Atom coordinates and isotropic displacement parameters are given in Table 5
The refined crystal chemical formula (Cu0.53Fe0.47)Se2 is in perfect agreement with that derived from the electron microprobe data, i.e., (Cu0.53Fe0.48)Σ1.01(Se1.98S0.01)Σ1.99.
Powder X-ray data (Cu K
α radiation) were collected with an automated CCD-equipped Oxford Diffraction Xcalibur PX single-crystal diffractometer using a Cu K
α radiation (Gandolfi-type data collection). The measured and calculated (using the software PowderCell
]) powder diffraction patterns are given in Table 6
. Unit-cell parameters refined from the collected data are as follows: a
= 4.9072(3) Å, b
= 6.0116(4) Å, c
= 3.6671(5) Å, V
= 108.180(7) Å3
Powder X-ray data (Cu K
α radiation) were also collected with the same instrument on an end-member petříčekite grain intermixed with krut’aite of ideal stoichiometry and goethite from El Dragón. The fragment consisted of roughly 60% petřičekite, 25% krut’aite, and 15% goethite. The number of diffraction lines surely attributable to petřičekite was 8, which gave the following unit-cell values: with a
= 5.014(1) Å, b
= 6.203(1) Å, c
= 3.740(1) Å, V
= 116.31(5) Å3
. These values are in excellent agreement with those observed for pure synthetic CuSe2
= 5.0226(7) Å, b
= 6.1957(7) Å, c
= 3.7468(6) Å, V
= 116.59(2) Å3
]). Unfortunately, the crystal structure was not refined.
5. Results and Discussion
The crystal structure of petříčekite (Figure 7
) belongs to the marcasite-type structure. It consists of edge-sharing chains of CuSe6
octahedra parallel to  linked by sharing Se2
dimers. The Se–Se bonds are all parallel to (001). The structure determination and refinement of pure synthetic CuSe2
has been previously published [4
The chemistry obtained with electron microprobe is in perfect agreement with the results of the structure refinement. Indeed, the mean electron number at the metal site taking into account the electron microprobe data is 27.9, in good agreement with that obtained from the structure refinement, i.e.
, 27.6. The mean bond distance observed for the (Cu,Fe)-octahedron (2.473 Å) matches very well that calculated from the weighted bond distance 0.53(Cu–Se) + 0.47(Fe–Se) = 2.474 Å from the ideal (pure) end-members [4
A second crystal from Předbořice was studied by single-crystal X-ray diffraction, but, unfortunately, the diffraction quality did not allow a full data collection to refine the structure. However, the refined unit-cell parameters are: a
= 4.96(1), b
= 6.07(1), c
= 3.70(1) Å. The chemistry of such a grain is (Cu0.70
. Interestingly, if we plot the unit-cell parameters of the two studied petříčekite crystals from Předbořice together with the two synthetic end-members (FeSe2
]) against the Cu contents in atoms per formula unit (Figure 8
), the trend is totally obeyed.
Petříčekite from Předbořice and El Dragón are distinct with respect to their temporal position within the selenide assemblage. In Předbořice, it constitutes an early formed species, precipitated together with eucairite, but slightly predates the associated minerals athabascaite, klockmannite, permingeatite, chameanite, and others. Its association with klockmannite implies high selenium fugacities during formation, hence the above values defined by the umangite–klockmannite reaction [8
]. At El Dragón, petříčekite is late-stage, postdating the bulk of selenides, such as penroseite-krut’aite, eldragónite, petrovicite, grundmannite, tiemannite, and several others [9
]. Its association with klockmannite, krut’aite, and native selenium suggests similar, if not higher selenium fugacities as prevailed during the precipitation of petříčekite at Předbořice, possibly representing values above the klockmannite−krut’aite univariant reaction.