Occurrence

Škáchaite has been found in samples from the hydrothermal vein B117 between the 5^th–6^th level (depth 250–300 m below surface) of shaft No. 6 – Brod southeast of Příbram. In the period 1950–1991, this mine was one of the shafts open to the shallow parts of the Brod deposit, which belongs to the uranium and base-metal Příbram ore district, central Bohemia, Czech Republic (Komínek, Reference Komínek1995). The GPS coordinates for the occurrence of škáchaite are 49°40′5.094″N, 14°1′13.809″E. The uranium and base-metal Příbram ore district and the neighbouring Březové Hory ore district form the two main parts of the Příbram ore area.

The Příbram uranium and base-metal district represents the most significant accumulation of vein-type hydrothermal U ores in the Czech Republic. The hydrothermal U-mineralisation of late Variscan age is related to a 1–2 km wide and almost 25 km long zone formed by a strongly tectonised series of Upper Proterozoic sedimentary rocks along the contact with granitoids of the Devonian–Carboniferous Central Bohemian Plutonic Complex (Litochleb et al., Reference Litochleb, Černý, Litochlebová, Sejkora and Šreinová2003). A complete list of over 320 minerals identified to date from this area is given on mindat.org at http://www.mindat.org/loc-779.html.

The holotype polished section of the new mineral was taken from a museum specimen (National Museum Prague, catalogue number P1N 49.308) with dimensions 45 × 42 × 13 cm, which represents the complete profile through a 22 cm thick symmetrical vein filling, also including surrounding rock (Fig. 1). This unique sample of bonanza-type silver mineralisation contains abundant elongated native silver aggregates up to 6 cm in length in gangue formed by various types of carbonates: calcite, siderite, spherocobaltite, ankerite, kutnohorite, minrecordite and members of the dolomite–škáchaite series. Other associated minerals are safflorite, löllingite, cobaltpentlandite, Co-analogue of heazlewoodite, siegenite, carrollite, mckinstryite, jalpaite, chalcocite, tetrahedrite-(Zn), argentotetrahedrite-(Fe), not yet approved ‘argentotennantite-(Fe)’ (IMA2023-126 under voting), hematite, sphalerite, galena, chalcopyrite, pyrite and hisingerite (Litochleb et al., Reference Litochleb, Sejkora and Šrein2002). Škáchaite is of low-temperature hydrothermal origin.

Figure 1.

Overall photo of symmetrical vein filling with abundant elongated native silver aggregates up to 6 cm in length in gangue formed by various types of carbonates – calcite, siderite, spherocobaltite, ankerite, kutnohorite, minrecordite and members of the dolomite–škáchaite series (catalogue number P1N 49.308, mineralogical collection of the National Museum Prague); vein B117, Brod deposit, the uranium and base-metal Příbram ore district (Czech Republic). The holotype of škáchaite was taken from a central part of the sample with bright pink carbonate aggregates.

Physical and optical properties

Škáchaite occurs as anhedral grains up to 50 μm in size and growth zones in škáchaite–dolomite zoned crystals (Fig. 2) with thickness up to 100 μm but usually only 5–20 μm. Škáchaite is pale to bright pink in colour; tiny fragments are translucent. The mineral has a white streak and vitreous lustre. It does not fluoresce under either short- or long-wave ultraviolet light. The mineral is brittle, cleavage on {10$\bar{1}$1} is perfect; the Mohs hardness cannot be measured due to the small size of škáchaite grains and may be close to 3½–4, similar to other members of the dolomite group. The density could not be measured directly because of the small amount of available material. The calculated density is 3.140 g/cm³ based on the empirical formula and unit-cell volume from single-crystal data.

Figure 2.

Back-scattered electron image of holotype material (P1P 52/2022) – zoned škáchaite-dolomite crystals. Škáchaite zones are white, whereas dolomite (with variable Mg/Co/Fe) ratios) is grey. The red box indicates the area used for the single-crystal X-ray diffraction study.

In thin section, škáchaite is pink with a distinct rhombohedral cleavage. It is optically uniaxial (–), with ω = 1.741(3) and ɛ = 1.535(3) (measured at 589 nm). Škáchaite is not pleochroic and has hints of undulatory extinction. The Gladstone–Dale compatibility index, 1 – (K _P/K _C), is –0.050 for the empirical formula and unit-cell parameters from single-crystal data, indicating good compatibility (Mandarino, Reference Mandarino1981).

Chemical composition

Chemical analyses were performed using a Cameca SX100 electron microprobe operating in wavelength-dispersive mode (15 kV, 10 nA and 3 μm beam size). The following standards and X-ray lines were used to minimise line overlaps: Co (CoKα), diopside (MgKα), hematite (FeKα), rhodonite (MnKα), wollastonite (CaKα) and ZnO (ZnKα). Peak counting times were 20 s for all elements and 10 s for each background. Contents of other measured elements (Al, Ba, Cu, Na, Ni, P, Pb, Si and Sr) were always below detection limits. Matrix correction using the PAP algorithm (Pouchou and Pichoir, Reference Pouchou and Pichoir1985) was applied to the data (measured oxides and calculated amount of CO₂ resting to analytical sum of 100 wt.%). Carbonate content could not be analysed directly because of the minute amount of material available; it was confirmed by Raman spectroscopy and single-crystal structure study and calculated by stoichiometry based on the ideal content of two CO₃ groups in the crystal structure of dolomite-group minerals.

The chemical composition (mean of eleven analyses) of the škáchaite grain used for the single-crystal study (holotype polished section) is given in Table 1. Individual point analyses (Supplementary Table S1) show a Co-dominant character (0.41–0.49 apfu) with Mg (0.35–0.41 apfu) and minor Fe (0.04–0.06 apfu), Mn (0.03–0.05 apfu) and Zn (up to 0.01 apfu) On the basis of Σcations = 2 atoms per formula unit (apfu), the average empirical formula is Ca_1.00(Co_0.45Mg_0.38Ca_0.08Fe_0.05Mn_0.03Zn_0.01)_Σ1.00(CO₃)₂. The ideal formula is CaCo(CO₃)₂, which requires (in wt.%) CaO 25.60, CoO 34.21, CO₂ 40.19, a total 100.00.

Table 1.

Chemical composition (in wt.%) of škáchaite

* CO₂ content was calculated on the basis of two CO₃ groups present in the crystal structure of dolomite-group minerals.

S.D. – standard deviation

Škáchaite was also determined in another 26 polished sections (taken from the same museum sample) usually in thin (units to tens μm) zones in škáchaite–dolomite grains. In the chemical analyses (Supplementary Table S2), the predominant Co (0.35–0.59 apfu) is accompanied by Mg (0.17–0.44 apfu), Fe (0.03–0.31 apfu), Mn (0.01–0.12 apfu) and minor Ni and Zn up to 0.06 and 0.03 apfu, respectively.

Raman spectroscopy

The Raman spectra of škáchaite were collected in the range 50–4000 cm^–1 using a DXR dispersive Raman Spectrometer (Thermo Scientific) mounted on a confocal Olympus microscope. The Raman signal was excited by an unpolarised red 633 nm He–Ne gas laser and detected by a CCD detector. The experimental parameters were: 100× objective, 10 s exposure time, 100 exposures, 50 μm pinhole spectrograph aperture and 8 mW laser power level. The eventual thermal damage of the measured points was excluded by visual inspection of the excited surface after measurement, observation of possible decay of spectral features at the start of excitation and checking for thermal downshift of Raman lines. The instrument was set up by a software-controlled calibration procedure using multiple neon emission lines (wavelength calibration), multiple polystyrene Raman bands (laser frequency calibration) and standardised white-light sources (intensity calibration). Spectral manipulations were performed using Omnic 9 software (Thermo Scientific).

The Raman spectrum of škáchaite is given in Fig. 3. The main bands observed are (in wavenumbers): 1748, 1437, 1094, 723, 294 and 173 cm^–1. The experimental spectrum of škáchaite agrees very well with published Raman spectra of dolomite (e.g. Gillet et al., Reference Gillet, Biellmann, Reynard and McMillan1993; Perrin et al., Reference Perrin, Vielzeuf, Laporte, Ricolleau, Rossman and Floquet2016). The strong band at 1094 cm^–1 is attributed to the symmetric stretching vibration (ν₁) and the band at 1437 cm^–1 to the asymmetric stretching vibration (ν₃) of (CO₃)^2– groups. A band at 723 cm^–1 is assigned to the in-plane bending vibration (ν₄). The out-of-plane bending vibration (ν₂) is inactive in the Raman spectrum, while its overtone (2ν₂) is observed at 1748 cm^–1. The bands below 300 cm^–1 (294 and 173 cm^–1) are attributed to lattice modes.

Figure 3.

Raman spectrum of škáchaite.

X-ray diffraction data

Single-crystal X-ray studies were carried out on a thin growth zone retrieved from a larger fragment identified by wavelength-dispersive spectroscopy (WDS), which was selected to be tested on a Rigaku SuperNova single-crystal diffractometer equipped with the Atlas S2 CCD detector (microfocused MoKα radiation). The approximate dimensions of the fragment are 0.06 × 0.01 × 0.01 mm and the diffraction experiment yielded the following data: trigonal space group R $\bar{3}$ (no. 148), a = 4.8177(18), c = 16.093(7) Å, V = 323.5(2) ų and Z = 3. The c:a ratio calculated from unit cell parameters is 3.3404. The intensity data for škáchaite were processed using the Rigaku CrysAlis package, including applying an empirical multi-scan absorption correction (Rigaku, 2019). The structure was solved using the intrinsic phasing algorithm of the SHELXT program (Sheldrick, Reference Sheldrick2015). Refinement proceeded by full-matrix least-squares on F ² using Jana2020 (Petříček et al., Reference Petříček, Palatinus, Plášil and Dušek2023). A reverse–obverse twinning (Table 2) was taken into account during the refinement in Jana (see Petříček et al., Reference Petříček, Dušek and Plášil2016). The anisotropic structural model converged to R ₁ = 0.0304 for 94 reflections with F _o > 3σ(F _o) for 20 refined parameters, including site-occupancy refinement for Mg/Co. The final refinement returned site occupancies of Co_0.503 and Mg_0.497; the corresponding site scattering 19.5 e ^– is in line with those obtained from the WDS (20.7 e ^–). Details of data collection and refinement are given in Table 2. Fractional atomic coordinates and equivalent isotropic displacement parameters are reported in Table 3 and selected bond-distances and polyhedral volumes in Table 4. Anisotropic displacement parameters are reported in the crystallographic information file (cif) deposited with the Principal Editor of Mineralogical Magazine and available as Supplementary material (see below).

Table 2.

Crystal data and refinement details for škáchaite.

Table 3.

Atom coordinates, equivalent atomic displacement parameters (in Ų) and site-occupancies for škáchaite.

* BVS – sum of the bond valences (in valence units) incident upon the atomic site

Table 4.

Bond-distances (in Å) and polyhedral volumes (in ų) for škáchaite in comparison with selected members of the dolomite–škáchaite series.

Note: CD1 0.00 apfu Co, CD6 0.30 apfu Co.

*General formula AB(CO₃)₂, where A = Ca and B = Co and Mg.

Powder X-ray diffraction data could not be collected due to the paucity of available material. The experimental pseudo-Gandolfi scan was attempted to be collected, but the observed intensities were extremely weak. This is most probably due to the small volume of diffracting material (see the crystal dimensions). Consequently, powder X-ray diffraction data, given in Table 5, were calculated using the software PowderCell 2.3 (Kraus and Nolze, Reference Kraus and Nolze1996) and based on the structural model given in Tables 2 and 3.

Table 5.

Calculated powder X-ray diffraction data for škáchaite.*

*Intensity and d _hkl (in Å) were calculated using the software PowderCell2.3 (Kraus and Nolze, Reference Kraus and Nolze1996) and based on the structural model given in Tables 2 and 3. Only reflections with I _calc > 1 are listed. The six strongest reflections are given in bold.