Introduction

During some of the authors’ (UK, BR, KHF) ongoing studies of the mineralogy and mines of the famous Lavrion mining district in Greece (Bonsall et al. 2011; Scheffer et al. 2019 and references therein), we encountered, in 2016, a colourless to white supergene mineral on weathered lead–zinc ore, whose thin tabular, six-sided crystal morphology did not remind us of any known species. In fact, a full characterisation of the mineral, including a determination of its crystal structure, showed that it is a new, unusual hydrated oxysalt species containing Ca, Sb(V) and As(III). The mineral and its name were subsequently approved by the International Mineralogical Association’s Commission on New Minerals, Nomenclature and Classification (IMA 2018-081).

The mineral is named after Dr Ivan Prachař (b. 1957) of Prague, Czech Republic. Dr Prachař has, in close collaboration with one of the authors (KHF), studied the mineralogy of the numerous Lavrion mines for more than 20 years and helped to carefully survey and document all the underground workings (ancient to modern). Several discoveries of rare and unusual minerals in Lavrion are due to his work. The name is pronounced pra-har̝̊-ait.

The present contribution provides a full characterisation of prachařite and a comparison with related species. Cotype material is deposited in the Natural History Museum, Vienna, Austria (one cotype specimen and the crystal used for the crystal-structure solution; catalogue no. NHMW-MIN-O357), in the National Museum, Prague, Czech Republic (one cotype specimen, catalogue no. P1P 15/2018), and in the Natural History Museum of Los Angeles County, USA (one cotype specimen, catalogue no. 67242).

Occurrence and paragenesis

Prachařite was found in September 2016 in underground workings of the Plaka Mine No. 80, Plaka, Lavrion Mining District, Lavreotiki, Attica, Greece. The Plaka Mine No. 80 is located in the northern part of the famous Lavrion mining district (Leleu 1966, 1969; Skarpelis 2007; Voudouris et al. 2008; Skarpelis and Argyraki 2009; Bonsall et al. 2011; Scheffer et al. 2019). This mine is known for late-stage hydrothermal As-Sb-Pb–Zn(-Ag) ore veins with assemblages of primary silver minerals and a large variety of supergene phases, mainly arsenates (Wendel and Markl 1996; Rieck and Rieck 1999; Rieck et al. 2018, 2020, 2022; Frenzel et al. 2022). About 133 mineral species are currently (May 2023) known from the mine.

On the collected material, prachařite is accompanied by pharmacolite, a few tiny sulphur crystals and very rare smamite {Ca2Sb(OH)4[H(AsO4)2]·6H2O} (the latter only as a unique crystal, adjacent to a prachařite crystal, on a single, very small specimen; Rieck et al. 2022). These supergene phases crystallised on an ore matrix composed of dark brownish sphalerite, galena and carbonate gangue (dolomite and calcite). The vein-type primary ore mineralisation at this locality is composed of sphalerite, galena, pyrite, arsenic, stibarsen, stibnite and other phases. Typical secondary minerals include pharmacolite, picropharmacolite (very common), various Ca arsenates [including the unnamed phase Ca5(AsO4)2(HAsO4)2·5H2O, Kolitsch et al. 2014], antimony oxides, sulphur and others.

Morphological, physical and optical properties

Prachařite forms colourless to white, thin tabular hexagonal, in general sharp crystals up to 2.5 mm in diameter, but mostly 1–1.5 mm (Figs. 1 and 2). Parts of the crystals may appear somewhat crumbly, with recognisable cleavage cracks. The colourless platelets may show a frosted tabular face and a very thin white opaque rim. Prachařite is translucent to transparent, with a glassy lustre, white streak, a good cleavage parallel to {0001} and a distinct cleavage parallel to {10\(\overline{1 }\)0}. It is non-luminescent, brittle, and has an uneven fracture and a Mohs hardness of 2–2.5. Optically, it is uniaxial negative, with ω = 1.619(1) and ε = 1.553(1) (determined in white light) and nonpleochroic. Mass density was not measured directly; the X-ray density Dx is 2.848 g/cm3 (based on single-crystal data measured in Vienna—see below). For the ideal composition and the Vienna single-crystal data Dcalc. = 2.838 g/cm3; for the measured compositions (see following section), Dcalc. = 2.836–2.853 g/cm3.

Fig. 1
figure 1

Colourless thin tabular hexagonal crystal of prachařite. Area on co-type specimen P1P 15/2018. White-light photomicrograph Jiří Sejkora

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Fig. 2
figure 2

Colourless thin tabular hexagonal crystal of prachařite associated with white sprays of indistinct acicular pharmacolite crystals. Area on co-type specimen NHMW-MIN-O357. White-light photomicrograph Harald Schillhammer

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Chemical composition

Chemical analyses of prachařite were carried out with two electron probe micro-analysers, with very similar results for the standard polished mounts. The cotype material in Prague was quantitatively analysed with a Cameca SX 100 instrument (wavelength-dispersive mode, 15 kV, 4 nA, 10 μm defocussed beam diameter). Contents of Al, Bi, Cl, Co, Cu, Fe, K, Mn, Na, Ni, P, Pb, S, Si, V and Zn were all below detection limits. The following X-ray lines and reference materials were used: (i) Kα lines: Ca (wollastonite), Mg (synthetic Mg2SiO4), Lα line: As (lammerite); (ii) Lβ line: Sb (synthetic Sb). Counting times were 20 s on peak and 10 s on each background position. The raw intensities were converted to the concentrations automatically using the PAP (Pouchou and Pichoir 1985) matrix-correction software. The H2O content could not be determined directly, because of the very meagre amount of pure material. The H2O content was therefore calculated on the basis of 10 H2O per formula unit (pfu) from the crystal-structure analysis. Results are given in Table 1. The high analytical totals after the addition of the calculated H2O content (114.83) are caused by partial dehydration of the sample (about half of the total water content) under the vacuum conditions of the instrument chamber and due to visible beam damage; dehydration is reflected by abundant dehydration fractures in the crystal grains. We also tried to vary measurement conditions, using a beam current of up to 1 nA and a beam diameter of 20 μm, however, with the same results. Finally, we note that the observation of variable transparency of the crystals may indicate possible prior partial dehydration of those crystals that are translucent or not transparent. The material in Vienna was quantitatively analysed with a JEOL “Hyperprobe” JXA 8530F field-emission gun electron probe micro-analyser (EPMA), employing JEOL and “Probe for EPMA” (Probe Software, Inc.) analysis software (wavelength-dispersive mode, 10 kV, 20 nA, 20 μm defocussed beam diameter, counting time 10 s on peak and 5 s on background positions). The following natural reference materials and X-ray lines were used: wollastonite (CaKα), kermesite (SbLα, SKα), scorodite (AsLα) and synthetic ZnO (ZnLα). If the analytical data were processed assuming an ideal formula, totals of ca. 114–117% were obtained, and the Ca, Sb and As values were all too high (especially As). If a loss of a third of all water molecules (i.e. only the hydrogen-bonded ones, see below) is assumed (“unprocessed” sum is 88%), then the total would be 101% and the Ca:Sb:As-ratio would be nearly perfect.

Table 1 Results of EPMA chemical analyses of prachařite
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The calculated empirical formulae, based on O = 22 atoms per formula unit (apfu), are very similar for the two datasets: (Ca0.93Mg0.02)Σ0.95Sb2.02(AsO3)4.00·10H2O (Prague material) and Ca1.04Sb2.03(AsO3)3.92·10H2O (Vienna material). The ideal formula is CaSb5+2(As3+O3)4·10H2O, which we reformulated as CaSb5+2O2(As3+2O5)2·10H2O to emphasise the presence of As2O5 dimers in the crystal structure (see below). The ideal formula requires CaO 5.87, Sb2O5 33.86, As2O3 41.41, H2O 18.86, total 100.00 wt%.

The calculated Gladstone-Dale compatibility index (1-KP/KC; Mandarino 1981, 2007) is 0.065 (fair) using the empirical formula and single-crystal data from Vienna, and 0.071 (fair) using the empirical formula and single-crystal data from Prague. It is 0.067 using the ideal formula and the single-crystal data from Vienna. The following observation might explain the “fair” category: Prachařite contains corner-shared SbO6 octahedra; Shannon and Fischer (2016) noticed that corner-shared octahedral network and chain structures such as perovskites, tungsten bronzes and titanite-related structures showed systematic deviations between observed and calculated polarisabilities (polarisability analysis is a more reliable measure of the compatibility of a mineral’s refractive index, composition and crystal structure) and thus were excluded from their regression analysis.

X-ray crystallography

X-ray powder diffraction

Powder diffraction data were obtained using two different methods. The first data were measured in Los Angeles using a Rigaku R-Axis Rapid II curved imaging plate microdiffractometer with monochromatised MoKα radiation. A Gandolfi-like motion on the φ and ω axes was used to randomise the sample. Observed d values and intensities were derived by profile fitting using JADE 2010 software (Materials Data, Inc.). Data (in Å for MoKα) are given in Table 2 (“data 2”). Unit-cell parameters refined from the powder data using JADE 2010 with whole pattern fitting are: a = 13.952(6), c = 19.918(10) Å, V = 3358(3) Å3.

Table 2 Powder X-ray diffraction data (d in Å) for prachařite
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The second data, also given in Table 2 (“data 1”), were measured using a Bruker D8 Advance powder diffractometer equipped with solid-state LynxEye detector and secondary monochromator producing CuKα radiation housed at the Department of Mineralogy and Petrology, National Museum, Prague, Czech Republic. The instrument was operated at 40 kV and 40 mA. In order to minimise the background, the powder sample was placed on the surface of a flat silicon wafer in ethanol suspension. The powder pattern was collected in the Bragg–Brentano geometry in the range 3–70° 2θ, with a step size of 0.01° and a counting time of 30 s per step (the total duration of the experiment was about three days). The positions and intensities of the diffraction maxima were determined and refined using the Pearson VII profile-shape function of the ZDS program package (Ondruš 1993) and the following unit-cell parameters were refined by the least-squares program of Burnham (1962): a = 13.9541(5), c = 19.795(1) Å, V = 3338.0(3) Å3.

Single-crystal X-ray diffraction

An Ewald sphere of intensity data from a fragment of a well-developed transparent prachařite crystal was collected in Vienna with a Nonius KappaCCD four-circle single-crystal diffractometer (see Table 3 for details on data collection and refinement). The crystal structure was solved and refined in space group P\(\overline{3 }\)c1 (no. 165) using SHELX-97 (Sheldrick 2008) to R1 = 2.3% and wR2all = 5.8%.

Table 3 Crystal data, data collection information and refinement details for prachařite
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An additional intensity dataset was collected in Prague using a Rigaku SuperNova single-crystal diffractometer (microfocus X-ray tube, MoKa X-radiation, Atlas S2 charge-coupled device detector, 293 K, 2θmax = 59.08°, completeness 0.98, Rint = 5.1%). The model obtained is basically identical to the originally obtained model (albeit with standard uncertainties higher by a factor of about 1.5 to 2) and will not be discussed here except in case of notable differences. The unit-cell parameters, a = 13.9545(4) Å, c = 19.7896(5) Å, V = 3337.33(16) Å3, are similar except for a slightly decreased length of the c axis (a smaller c-axis length is also seen in the powder diffraction data of the corresponding material). This is tentatively attributed to variable water contents (see discussion below).

Table 4 Fractional atomic coordinates and displacement parameters (in Å2) for prachařite
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Table 5 Selected bond distances (Å) and bond angles (°), and calculated bond-valence sums (in valence units, v.u.) for the coordination polyhedra in prachařite
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Table 4 gives final atomic coordinates and displacement parameters for prachařite, while Table 5 provides selected bond lengths and angles, including calculated bond-valence sums. A CIF-file is available in the Supplement. The asymmetric unit contains two Ca, one Sb, two As, 13 O and 14 H sites, nine of which could be located and refined with the following restraints: O–H = 0.90(2) Å; H…H = 1.50(5) Å. Seven of the 13 O sites represent water molecules. One water molecule site (Ow13), located in an interlayer space, is a partially occupied split-site (occupancy ~ 0.40); the refined formula is CaSb5+2(As3+O3)4· ~ 10.1H2O. The aforementioned second dataset gave a slightly decreased Ow13 occupancy, ~ 0.25, which might explain the equally slightly decreased length of the c axis of the fragment used for that dataset.

Prachařite has a layered structure based on two different layers (Figs. 3, 4 and 5). The first one comprises a six-membered ring of corner-sharing SbO6 octahedra (< Sb–O >  = 1.970 Å, Table 5). The eight-coordinated Ca1 atom occupies the centre of the ring (< Ca1–O >  = 2.474 Å). The Ca1O8 polyhedron is a hexagonal dipyramid that shares each of its horizontal edges with one of the six symmetrically equivalent SbO6 octahedra. Thus, six ligands are O atoms (O6) and two are water molecules (Ow7). Attached to the SbO6 octahedra by vertices are AsO3 groups (< As1–O >  = 1.779 Å, < As2–O >  = 1.781 Å). The (As1O3)3− and (As2O3)3− groups are corner-linked to form a (As2O5)4− diarsenite group in an eclipsed configuration; thus, the structural formula was formulated as CaSb5+2O2(As3+2O5)2·10H2O. The heteropolyhedral Ca-Sb-As-O-cluster represents the fundamental building unit of prachařite (Figs. 3 and 4).

Fig. 3
figure 3

The fundamental building unit in the crystal structure of prachařite: a heteropolyhedral cluster composed of corner-sharing SbO6 octahedra (yellow), As2O5 dimers (with eclipsed configuration) built of AsO3 groups (orange) and a central Ca1O8 polyhedron (dark blue). Oxygen ligands are shown as small pale blue circles. Drawing done with ATOMS V. 6.3.1 (Dowty 2006)

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Fig. 4
figure 4

The crystal structure of prachařite in a view along the c-axis. Ca-O polyhedra are dark blue, SbO6 octahedra are yellow, AsO3 groups are orange, fully occupied Ow atoms are pale blue spheres, and partly occupied Ow13 atoms are green spheres. H atoms have been omitted for clarity. The unit cell is outlined. Drawing done with ATOMS V. 6.3.1 (Dowty 2006)

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The second Ca atom, Ca2, is located between two heteropolyhedral Ca1-Sb-As-O layers (Fig. 5). The Ca2 atom shows seven-fold coordination (< Ca2–O >  = 2.405 Å); all ligands are water molecules (Ow10 × 3, Ow9 × 3 and Ow8). Also in the interlayer are located two fully occupied, only hydrogen-bonded water molecules, represented by Ow11 and Ow12, and the already mentioned partially occupied water molecule, Ow13 (Figs. 4 and 5). The latter is close to a symmetrically equivalent position [Ow13…Ow13’ = 1.08(2) Å] and shows an increased Uiso value (~ 0.13 Å2). Hydrogen bonds are more or less weak, with donor…acceptor distances ranging between 2.74 and 3.22 Å (Table 5). The bond-valence sums, calculated using the recently improved parameters of Gagné and Hawthorne (2015), are in very good agreement with the ideal valencies (Table 5).

Fig. 5
figure 5

The crystal structure of prachařite in a view along the a-axis, showing the interlayer content and the eclipsed configuration of the As2O5 dimer. Legend as in Fig. 4. Drawing done with ATOMS V. 6.3.1 (Dowty 2006)

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Raman spectroscopy

Raman spectra (a representative one is shown in Fig. 6) were collected on a crystal lying on its (0001) face in the range 4500–10 cm–1 using a DXR dispersive Raman Spectrometer (Thermo Scientific) attached to an Olympus microscope. The Raman signal was excited by the green emission (532 nm) of a diode-pumped solid-state laser and analysed by a charge-coupled device detector. The experimental parameters were: 50 × objective (numerical aperture 0.50), 10 s exposure time, 100 exposures and 1 mW laser power. With a 400 lines/mm diffraction grating in the optical pathway, the spectral resolution was in the range 4.4–9.7 cm–1. The spectra were repeatedly acquired from different crystals in order to obtain a representative spectrum with the best signal-to-noise ratio. A possible thermal damage of the measured spots was excluded by visual inspection of each spot after the measurement, by 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 wavelength calibration) and standardised white-light sources (intensity calibration). Spectral manipulations were performed using the Omnic 9 software (Thermo Scientific). The lateral resolution was ~ 0.7 μm in confocal mode.

Fig. 6
figure 6

Raman spectrum of prachařite (split at 2000 cm–1)

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The main bands observed are (in cm–1, strong bands underlined): 3620, 3470, 3383, 3288, 1655, 851, 840, 830, 792, 541, 458, 358, 297, 273, 253, 233, 185, 129, 116, 63 and 24. The dominant bands in the 600–200 cm–1 region can be assigned to δ As–O–As and AsO2 vibrations of the (As2O5)4− diarsenite group (see above description of crystal structure). Weaker bands in the range 900–750 cm–1 may be related to stretching (symmetric and antisymmetric) vibrations of As–O bonds, as well as to Sb–O vibrations of the SbO6 octahedron (Vandenborre et al. 1980; Baran and Botto 1981; Devi and Vidyasagar 1998; Castro et al. 2009; Bahfenne and Frost 2010; Glamazda et al. 2017). The bands in the 200–10 cm–1 region are caused by lattice modes. The presence of water is documented by a broad OH-stretching band running from 3800 to 3100 cm–1 with recognisable components at 3620, 3470, 3383 and 3288 cm–1; at least four distinct components in this area indicate several structurally non-equivalent water molecules, in agreement with the crystal-structure determination. Following the correlation curve of Libowitzky (1999), the band component at 3288 cm–1 probably corresponds to the hydrogen bond Ow7–H71···Ow13 [2.741(11) Å] (Table 5), while the 3470 and 3383 cm–1 components would correspond to at least five different H bonds with O···O distances between 2.80 and 2.89 Å. The band component at 3620 cm–1 is probably related to the very weak hydrogen bond Ow11–H111···O5. A weak band at 1655 cm–1 is assigned to the ν2 (δ) bending vibrations of water molecules.

Discussion

Relation to other mineral species

No close relationship with other minerals is apparent. Specifically, there is neither a known mineral species nor a known synthetic compound that both contains Sb5+ and As3+. Interestingly, whitecapsite, H16Fe52+Fe143+Sb63+(AsO4)18O16·120H2O (Pekov et al. 2014) contains both Sb3+ and As5+; its structure is based on a complex heteropolyhedral [(□,Fe2+)6Fe3+7Sb3O8(AsO4)9(H2O)30] (□ = vacancy) cluster. Smamite, triclinic Ca2Sb(OH)4[H(AsO4)2]·6H2O (Plášil et al. 2020), also contains both Sb3+ and As5+. Its atomic arrangement is based upon {Ca2(H2O)6Sb(OH)4[H(AsO4)2]} infinite chains consisting of edge-sharing dimers of Ca(H2O)3O2(OH)2 polyhedra which share edges with Sb(OH)4O2 octahedra. A recently described mineral containing both Sb3+ and As3+ is lepageite, Mn2+3(Fe3+7Fe2+4)O3[Sb3+5As3+8O34] (Pieczka et al. 2019), which was found in the Szklary pegmatite (Lower Silesia, Poland). Seven other minerals are presently known to contain diarsenite groups (listed in chronological order of published crystal-structure determination): paulmooreite, Pb2As2O5; gebhardite, Pb8(As2O5)2OCl6; fetiasite, (Fe3+,Fe2+,Ti)3(As2O5)O2; vajdakite, [(Mo6+O2)2(H2O)2As3+2O5]·3H2O; schneiderhöhnite, Fe2+Fe3+3As3+5O13; karibibite, Fe3+3(As3+O2)4(As3+2O5)(OH); and bianchiniite, Ba2(TiV)(As2O5)2OF. Among these, vajdakite is the only other hydrated phase.

We are not aware of the presence of any other isolated SbO6-based Sb6O30 rings in either minerals or synthetic compounds. Although rings of edge-sharing SbO6 octahedra exist in rosiaite, PbSb2O6 (Basso et al. 1996), and isotypic compounds, they share edges with neighbouring rings to form an interrupted gibbsite-like layer.

Stability and occurrence

Prachařite has formed by weathering of a hydrothermal As-Sb-Pb–Zn(-Ag) ore vein in an estimated temperature range of 10–18 °C (based on long-term observations of the underground microclimate during different seasons) and under slightly acidic and slightly oxidising conditions. The Eh (redox potential) range must be fairly narrow since Sb5+ and As3+ must be simultaneously (meta-?)stable. Although thermodynamically, a mineral containing both Sb5+ and As3+ should not be stable, in reality, this is considered very possible because the stability boundaries Sb3+/Sb5+ and As3+/As5+ are very close and other arsenite minerals exist that should not be thermodynamically stable, e.g. tooeleite, Fe3+6(As3+O3)4(SO4)(OH)4·4H2O (Juraj Majzlan, pers. comm. to U.K.). Furthermore, it was reported that in a relatively reducing sediment impacted by As- and Sb-rich mining waste, most of the As is reduced to As3+, while Sb5+ bound to the oxygen phase persists and represents 58% of the total Sb (Fawcett and Jamieson 2011). Other researchers made similar observations (Mitsunobu et al. 2006; Ritchie et al. 2013; Fawcett et al. 2015). Conditions that would favour the crystallisation of prachařite are assumed to occur also in geologically similar deposits such as the Sainte-Marie-aux-Mines ore district, Haut-Rhin department, France (von Eller and Weil 1966; Fluck 1968), which is, as mentioned above, the type locality of the chemically similar mineral smamite, Ca2Sb(OH)4[H(AsO4)2]·6H2O, the latter also occuring on one of the studied prachařite specimens.