Katsarosite Zn(C2O4)·2H2O, a new humboldtine-group mineral from the Lavrion Mining District, Greece

Katsarosite, ideally Zn(C2O4)·2H2O, named for Īraklīs Katsaros, is a new mineral found at the Esperanza Mine in the Kaminiza area of the Lavrion Mining District, Greece. Katsarosite usually occurs directly on sphalerite or embedded in jarosite and/or hydrozincite, often intimately intergrown with gypsum and overgrown by goslarite and/or epsomite. Crystal aggregates are mostly fine granular to earthy, with individual crystals being usually rounded with an average diameter of 30 µm, sometimes prismatic along [001] or platy, exhibiting the indistinct forms {100}, {001}, {110}, and {101}. Katsarosite is malleable with a Mohs hardness of 1½ – 2 and exhibits a perfect cleavage on {110}; the fracture is uneven in all other directions. The colour depends on the iron (Fe2+) content, ranging from pure white in almost Fe-free samples to yellow in Fe-rich specimens. It has a resinous luster and a white streak; no luminescence has been observed under either short- or long-wave ultraviolet radiation. Katsarosite is optically biaxial (+). Refractive indices measured at a wavelength of 589 nm are nα = 1.488(2), nβ = 1.550(2), nγ = 1.684(2), with 2Vobs = 71(3)°. Chemical analysis gave on average C2O3 38.32 wt%, ZnO 38.99 wt%, FeO 1.92 wt%, and H2O 19.04 wt% (the latter was deduced based on the crystal-structure refinement), with traces of MgO and MnO. The new mineral is readily soluble in dilute acids. Katsarosite is monoclinic, space group C2/c, with unit-cell parameters a = 11.768(3), b = 5.3882(12), c = 9.804(2) Å, β = 127.045(8)°, V = 496.2(2) Å3 (Z = 4). The strongest lines in the Gandolfi X-ray powder pattern [dobs in Å, Iobs/I100, (hkl)] are: 4.6745, 100, (200); 4.7678, 94, (202¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline{2 }$$\end{document}); 2.9533, 51, (402¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline{2 }$$\end{document}); 4.7030, 37, (1 1¯1¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline{1 }\,\overline{1 }$$\end{document}); 3.9266, 33, (002); 3.5686, 27, (111); 2.6574, 22, (1 1¯3¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline{1 }\,\overline{3 }$$\end{document}); 3.5992, 8, (1 1¯2¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline{1 }\,\overline{2 }$$\end{document}); 2.7032, 4, (020). The crystal structure was refined based on single-crystal X-ray diffraction data to R(F) = 0.08. The observed mass density of 2.50(2) g cm−3 compares well with the calculated value (2.508 g cm−3). Katsarosite belongs to the humboldtine group, whose crystal-structure type is well described for both isotypic minerals and synthetic compounds in the literature. The atomic arrangement in Zn(C2O4)·2H2O is characterized by chains consisting of isolated ZnO6 octahedra which are alternately linked along [010] via oxalate anions. These chains are interconnected through hydrogen bonds only, with Ow···O (with Ow denoting the O atom of the H2O molecule) donor–acceptor distances of ~ 2.8 Å.


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 [K 2 Fe 2+ (SO 4 ) 2 ·4H 2 O; Giester and Rieck 1995], niedermayrite [Cu 4 Cd(SO 4 ) 2 (OH) 6 ·4H 2 O; Giester et al. 1998 Rieck et al. 2020], and fabritzite, Zn 9 (SO 4 ) 2 (OH) 12 C l2 ·6H 2 O (Kolitsch andGiester 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 22 nd , 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).

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.

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

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 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 (CHBr 3 -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 n α = 1.488(2), n β = 1.550(2), n γ = 1.684(2), with 2V obs = 71(3)°, 2V calc = 73°. The orientation is X = a and Y = b. Calculating K c 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 -(K p /K c ) = 0.031, which is rated as excellent (Mandarino 1981). There is no discernible pleochroism, and the optical dispersion is very weak, with n red < n violet .

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). 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 (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 H 2 O 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 Zn 2+ and only minor contents of Fe 2+ , Mg 2+ , and Mn 2+ in the sample.

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) Å 3 (Z = 4). Table 2 Crystal data and details of the intensity measurement and structure refinement for katsarosite 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 F 2 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(C 2 O 4 )·2H 2 O 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.

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 and hence corresponds to the twin matrix, and the transformation of the traditional setting ("H") to the I-centred cell it is Probably, the remarkable similarity of the two C-centred unit cell choices facilitates observed pseudomerohedral twinning.
The crystal structure of Zn(C 2 O 4 )·2H 2 O 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 Fig. 4 of two oxalate groups and further to two H 2 O molecules. The coordination polyhedron exhibits moderate bondlength 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.
Acknowledgements Maja Mrak conducted the hydrothermal synthesis (done in 2001) that resulted in crystallization of Zn(C 2 O 4 )·2H 2 O as a by-product. Constructive reviews by two anonymous experts and editorial handling by Reinhard X. Fischer are gratefully acknowledged.
Funding Open access funding provided by University of Vienna.
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