Introduction

The area of the Lavrion mining district is well known for a large number of mineral species, among them more than 20 new minerals; for a short summary see Rieck et al. (2019; 2020; and references therein). Over the last 30 years a series of field trips by one of the autors (B.R.) in collaboration with colleagues from Vienna and Athens yielded a considerable amount of sampled material, resulting in the description of several new minerals: mereiterite [K2Fe2+(SO4)2·4H2O; Giester and Rieck 1995], niedermayr-ite [Cu4Cd(SO4)2(OH)6·4H2O; Giester et al. 1998], voudourisite [Cd(SO4)·H2O] and lazaridisite [Cd3(SO4)3·8H2O; Rieck et al. 2019], stergiouite [CaZn2(AsO4)2·4H2O; Rieck et al. 2020], and fabritzite, Zn9(SO4)2(OH)12Cl2·6H2O (Kolitsch and Giester 2013, Kolitsch et al. 2023).

The first specimens of the new mineral species katsarosite were discovered in April 2018 during a guided tour to the underground workings of the Esperanza Mine led by Īraklīs Katsaros and Vasilis Stergiou, both from Lavrion. During sample preparation it was noticed that the gentle removal of overlaying goslarite and/or epsomite by soaking in water revealed water-insoluble grains of what turned out to be a new mineral species. The authors decided to name the new mineral after Īraklīs Katsaros (ΗΡΑΚΛΗΣ ΚΑΤΣΑΡΟΣ, born December 22nd, 1969) in recognition of his contributions to the archeology/mining history and mineralogy/geology of the area. He has led (as a guide) a large number of scientific archaeological and mineralogical sampling tours through the ancient mining area of Lavrion. The mineral and name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association under number IMA 2020–014. The holotype, an untreated specimen together with a 2 mm sized group of compact crystals, uncovered by soaking with water, is in the collection of the Institut für Mineralogie und Kristallographie, University of Vienna (catalogue number HS13.977).

To date, two dozen oxalates have been described as valid mineral species, for an overview see for instance Echigo and Kimata (2010). Among them, the humboldtine group of metal oxalate dihydrates with currently known representatives glushinskite [Mg(C2O4)·2H2O; Wilson et al. 1980], lindbergite [Mn2+(C2O4)·2H2O; Atencio et al. 2004] and humboldtine [Fe2+(C2O4)·2H2O; Mazzi and Garavelli 1959; Carić 1959] is now extended by the new member katsarosite, ideally Zn(C2O4)·2H2O. Crystals of anthropogenic, Fe2+-bearing, yellow Zn(C2O4)·2H2O had already been observed in a void in Pb- and Ba-rich slags from Waitschach near Hüttenberg, Carinthia (Kolitsch 2013). Synthetic compounds M2+(C2O4)·2H2O with M = Mg, Mn, Fe, Co, Ni, and Zn have been characterized in a series of studies (e.g. Deyrieux et al. 1973; Dubernat and Pezerat 1974; Khan et al. 1976; Śledzińska et al. 1987; Kubacky-Beard et al. 1996; Giester 1997; Donkova et al. 2004; Li 2004; Bacsa et al. 2005; Echigo and Kimata 2008; Puzan et al. 2018; Müller et al. 2021 and others).

Occurrence and paragenesis

The Esperanza Mine (37.72547° N; 24.03247° E) belongs to the Kaminiza area of the Lavrion Mining District and shares many similarities with the better-known Km 3 area directly adjacent to the South and the Stefanie area to the North. Within the Lavrion Mining District it belongs to the areas richest in zinc and silver and was mined at least since 600 BC. The Lavrion area is part of the western Attic-Cycladic metamorphic belt, in the back-arc region of the active Hellenic subduction zone. From the Eocene until the Miocene, marbles and schists underwent several stages of metamorphism and deformation due to collision and collapse of the Cycladic belt. The Miocene exhumation featured the movement of a wide-ranging detachment fault system (Western Cycladic detachment system, WCDS), which also facilitated the emplacement of acid and intermediate magmatic rocks, leading to the formation of the rich Pb–Zn-(Ag) ore deposits of the Lavrion Mining District (cf. Voudouris et al. 2021 and references therein). The ore deposition occurred mainly within the marbles, at marble-schist contacts, below, within, or above the WCDS. The exact location of the katsarosite occurrence within the Esperanza Mine is a series of karst cavities that were opened by ancient mining. These cavities are located below (about 10–15 m elevation) the current entrance level of the Esperanza Mine and are closer to the northern side of the ridge (and thus formally already in the Stefanie area) than the southern side on which the current entrance is located. Archaeological work is currently being done to find a possible ancient entrance from the northern side of the ridge.

Katsarosite is a product of the alteration of sphalerite; possibly a biomineral formed by the action of lichens (e.g. Acarospora smaragdula, Aspicila alpina, Lecidea lactea; cf. Burford et al. 2003). The karst cavities that contain this mineralization have in some places small tree-roots hanging from the ceiling, so a (non-anthropogenic) biology-induced genesis of katsarosite is possible or even likely. Associated minerals are sphalerite, pyrite, gypsum, hydrozincite, goslarite, epsomite, chalcanthite, humboldtine (Zn-bearing), ammoniojarosite, jarosite, and natrojarosite.

Chemical composition

Chemical analyses were performed on ten samples (five each originating from two different specimens) with a Thermo iCAP 6000 ICP-OES (Thermo Fisher Scientific). The mean results (ranges in parentheses) are C2O3 38.32 (38.21–38.40); ZnO 38.99 (38.70–39.36); FeO 1.92 (1.53–2.40); MgO 0.64 (0.00–1.01); MnO 0.74 (0.00–1.01); H2O 19.04 wt% (the latter calculated from the crystal-structure structure refinement); total 99.65 wt%. The empirical formula calculated on the basis of one C2O4 group per formula unit is (Zn0.90Fe0.05Mg0.03Mn0.02)1.00(C2O4)·1.99H2O. The simplified formula is (Zn,Fe)(C2O4)·2H2O and the ideal formula is Zn(C2O4)·2H2O, which requires ZnO 42.96, C2O3 38.00, and H2O 19.04 wt%. The mineral was found to be soluble in dilute acids.

Morphological, physical and optical properties

Morphological features were studied by secondary electron (SE) and back-scattered electron (BSE) imaging in a Phenom GSR Desktop scanning electron microscope. Crystal aggregates are mostly fine granular to earthy. The compact crystals (Figs. 1 and 2) are tiny, usually rounded with an average diameter of 30 µm, sometimes prismatic along [001] or platy, exhibiting the indistinct forms {100}, {001}, {110}, and {101}. No twinning was visually observed, but evaluation of X-ray diffraction data of the studied crystal indicated twinning, in agreement with pervasive polysynthetic twinning reported for humboldtine-group minerals. Katsarosite exhibits perfect {110} cleavage; on synthetic crystals (Giester 1997) an imperfect cleavage on {100} and {010} is also visible and the crystal forms {100}, {001}, {110}, and {101} are more pronounced. Fracture is uneven in all other directions. The mineral is malleable with a Mohs hardness of 1½ – 2. Mass density was measured using micro-pycnometry (CHBr3-filled capillary with an inner diameter of 0.5 mm and a length of 10 cm) to be 2.50(2) g cm−3. Considering the limited amount of pure material available for testing, the above value compares well with the calculated mass density of 2.508 g cm−3, based on the empirical formula and unit-cell dimensions obtained from single-crystal X-ray diffraction. The mineral is white, shows transparent to translucent diaphaneity and has a resinous luster. No luminescence was observed under long-wave and short-wave UV radiation. Katsarosite is optically biaxial (+). Refractive indices measured at a wavelength of 589 nm on a synthetic sample are nα = 1.488(2), nβ = 1.550(2), nγ = 1.684(2), with 2Vobs = 71(3)°, 2Vcalc = 73°. The orientation is X = a and Y = b. Calculating Kc for the empirical formula, using the Gladstone-Dale relationship (Gladstone and Dale 1863) and based on the empirical values of Mandarino (1976), revealed a compatibility index 1 – (Kp/Kc) = 0.031, which is rated as excellent (Mandarino 1981). There is no discernible pleochroism, and the optical dispersion is very weak, with nred < nviolet.

Fig. 1
figure 1

Indistinct crystals and crystal aggregates of white katsarosite grown directly on sphalerite. An overlaying epsomite layer has been removed by dissolution in deionized water

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Fig. 2
figure 2

BSE image showing a cluster of tiny katsarosite crystals (outlined in red; Kat) embedded in a mixture of fine-grained hydrozincite (Hznc) and platy natrojarosite (Njrs). Note that the pockmarked surface of hydrozincite results from the dissolution of previously overlaying epsomite

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Raman spectroscopy

The Raman spectrum (Fig. 3) of katsarosite was obtained by means of a LabRAM HR Evolution system. This dispersive spectrometer has a focal length of 800 mm and is equipped with a Si-based, Peltier-cooled charge-coupled device detector. A 50 × objective (numerical aperture NA = 0.50) was used to focus the laser light (473 nm; 17 mW) on the randomly oriented sample surface. Further analytical details are described elsewhere (Zeug et al. 2018).

Fig. 3
figure 3

Raman spectrum of katsarosite, compared with the spectra of four synthetic reference metal oxalate dihydrate compounds (Ni analog, extracted from Bickley et al. 1991; Mn and Fe analogs, extracted from Echigo and Kimata 2008; Mg analog, extracted from D’Antonio et al. 2010). The asterisk marks an analytical artefact (described by D’Antonio et al. 2010 as “instrumental noise”). The inset shows the O–H stretching range (same intensity scale as the main spectrum). Katsarosite spectra are means of five analyses of randomly oriented crystals, to account for the orientation dependence of band intensities

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The Raman spectrum of katsarosite is, as expected, broadly similar to those of other metal oxalate dihydrates (Fig. 3). The main band at 1473 cm–1, assigned to C–O stretching (Bickley et al. 1991), has a similar Raman shift compared to its counterparts in the spectra of synthetic Ni oxalate dihydrate (1482 cm–1; Bickley et al. 1991), Mn oxalate dihydrate (1469 cm–1; Echigo and Kimata 2008), Fe oxalate dihydrate (1463 cm–1; Echigo and Kimata 2008) and Mg oxalate dihydrate (1473 cm–1; D’Antonio et al. 2010), as well as in the spectra of natural moolooite, Cu(C2O4)·nH2O (1489 cm–1), and humboldtine (1468 cm–1; Frost and Weier 2003). The O–H stretching region of katsarosite, similar to that of synthetic FeC2O4⋅2H2O (Echigo and Kimata 2008), shows one major band (Fig. 3, inset). There are additional bands, detected as shoulders of very low intensity, at 3185 and 3282 cm–1. It is well known that O–H stretching vibrations of H2O molecules in minerals are controlled by nearest neighbouring cations (for instance, Huong et al. 2010). We may speculate that the main band is to be assigned to water molecules associated with Zn, whereas the shoulders are caused by water molecules having different cationic configurations. The low intensity of the latter seems to concur well with the clear dominance of Zn2+ and only minor contents of Fe2+, Mg2+, and Mn2+ in the sample.

X-ray crystallography

X-ray powder diffraction

X-ray powder diffraction data (Table 1) were collected on a “background-free” silicon holder with CuKα radiation using a Bruker D8 Advance Eco Diffractometer. Unit-cell parameters obtained from Rietveld refinement using TOPAS (Bruker 2017) are as follows: Space group: C2/c (no. 15), a = 11.8175(14), b = 5.40543(6), c = 9.9264(13) Å, β = 127.7063(7)º, V = 501.662(12) Å3 (Z = 4).

Table 1 X-ray powder diffraction data for katsarosite
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Single crystal X-ray work

A selected crystal fragment of katsarosite was studied on a Bruker APEXII diffractometer equipped with a CCD area detector and an Incoatec Microfocus Source IµS (30 W, multilayer mirror, MoKα). Several sets of phi scans and omega scans with 2° scan width were measured up to 70° 2ϴ (full Ewald sphere) at 200 K (Oxford Cryosystems Cryostream 800 Plus) at a crystal-detector distance of 40 mm. The studied crystal showed pseudomerohedral twinning with approximately equal volumes of both components. The twin matrix is

$$\left(\begin{array}{ccc}\overline{1 }& 0& 0\\ 0& \overline{1 }& 0\\ 1& 0& 1\end{array}\right).$$

For data handling including integration and multi-scan absorption correction the Bruker APEX3 software suite (Bruker 2020) was used. The refinement by full-matrix least-squares techniques on F2 was performed with the program SHELXL-2018/3 (Sheldrick 2015) within the ShelXle user interface (Hübschle et al. 2011). The atomic coordinates of synthetic Zn(C2O4)·2H2O given by Giester (1997) were taken as a starting parameter set. The refinement of the Zn site was done with site occupancies of cations fixed by the chemical analysis. Anisotropic displacement parameters for all non-hydrogen atoms were applied. The refinement converged at R(F) = 0.08. The relatively high value is attributed to the low quality of crystals available. A compilation of crystal data and details of measurement and refinement are contained in Table 2. Final structure parameters are listed in Table 3, and selected interatomic bond distances (including hydrogen bonds) and bond angles are provided in Table 4.

Table 2 Crystal data and details of the intensity measurement and structure refinement for katsarosite
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Table 3 Atomic coordinates and displacement parameters for katsarosite
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Table 4 Selected interatomic bond distances and H2O bond angle for katsarosite
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Additionally, a crystal-structure analysis of a chemically pure Zn(C2O4)·2H2O crystal, obtained as a by-product of a hydrothermal synthesis at 175 °C to prepare ZnP-analogs of paracelsian and analcime, was performed. Unit-cell parameters are listed in Table 5. The refinement, using SHELXL-97 (Sheldrick 2015), gave a structure model in good agreement with that of synthetic Zn(C2O4)·2H2O (Giester 1997). The results (files katsarosite_nat.cif and katsarosite_syn.cif) are provided in the ESM (electronic supplementary material).

Table 5 Compilation of single-crystal unit-cell parameters for katsarosite and synthetic Zn(C2O4)·2H2O
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Discussion

Single-crystal unit-cell dimensions of katsarosite are somewhat smaller than those derived from the Rietveld refinement, most probably reflecting the different temperatures applied for the respective data collections. The largest shrinkage is found for the c-axis which might be explained by the specific arrangement of the hydrogen-bonding system. Unit-cell parameters reported for the synthetic analogue (Giester 1997) as well as for the hydrothermally synthesized by-product mentioned in the previous section, both obtained at ambient conditions, closely resemble those refined from the powder X-ray diffraction data. A compilation of the respective unit cells and settings is presented in Table 5.

Comparison of our data with published unit-cell parameters of isotypic humboldtine-type metal oxalate dihydrates is a bit challenging, as different unit-cell settings are used. Besides the most commonly used C2/c setting and a non-standard I2/a setting, a further alternative setting (also in C2/c) with shorter c-axis and slightly smaller monoclinic angle β might be used. The respective transformation of the unit cell commonly chosen for humboltine-group compounds (Deyrieux et al. 1973; in Table 5 marked “H”) to this alternative setting (in Table 5 marked “R”) is

$$\left(\begin{array}{ccc}\overline{1 }& 0& 0\\ 0& \overline{1 }& 0\\ 1& 0& 1\end{array}\right)$$

and hence corresponds to the twin matrix, and the transformation of the traditional setting (“H”) to the I-centred cell it is

$$\left(\begin{array}{ccc}\overline{1 }& 0& \overline{1 }\\ 0& \overline{1 }& 0\\ 0& 0& 1\end{array}\right).$$

Probably, the remarkable similarity of the two C-centred unit cell choices facilitates observed pseudomerohedral twinning.

The crystal structure of Zn(C2O4)·2H2O and isotypic humboldtine-group minerals and synthetic compounds is well described and published (e.g. Echigo and Kimata 2008; Giester 1997). The atomic arrangement in katsarosite (Fig. 4) features chains formed by isolated ZnO4Ow2 (Ow = O of H2O molecule) octahedra which are alternately linked (Fig. 5) along [010] via the oxalate anions. These chains are interconnected only through hydrogen bonds with Ow···O donor–acceptor distances of ~ 2.8 Å.

Fig. 4
figure 4

Crystal structure of katsarosite projected on (010). ZnO6 polyhedra are visualised green, H2O molecules are blue, O atoms of oxalate groups are red, and C atoms are grey. Hydrogen bonds are shown by dashed lines. The unit cell is outlined. The crystal structure drawings in Figs. 4 and 5 were done with Atoms (Dowty 2016)

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Fig. 5
figure 5

Structure detail of katsarosite illustrating the chain along [010], formed by ZnO6 octahedra connected via oxalate groups. Atom and molecule colors as in Fig. 4

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The divalent cations (i.e. dominant Zn2+ with very minor amounts of Fe2+, Mg2+ and Mn2+), located on Wyckoff site 4e with symmetry 2, are coordinated to four oxygen atoms of two oxalate groups and further to two H2O molecules. The coordination polyhedron exhibits moderate bond-length distortions with Zn−O distances of 2.07 – 2.08 Å and Zn−Ow distances of 2.11 Å. Bond-angle distortions are clearly pronounced; smallest angles (81.1° and 80.7°) are found for O1−Zn−O1 and O2−Zn−O2, respectively, which span common edges with the rigid oxalate unit.