1. Introduction
During our long-term systematic mineralogical research focused on supergene arsenate minerals from various occurrences (Sejkora et al. 1999, 2009, 2018; Plumhoff et al. 2020; Steciuk et al. 2021), dobšináite, a new Ca dominant member of the roselite group, was discovered; in this paper, we report its description. Arsenate mem¬ bers of the roselite group comprise monoclinic minerals of generalized composition Ca,M(AsO,), 2H,O. The octahedral M site is occupied mainly by Mn 2+ (brandtite, Herwig and Hawthorne 2006), Co 2+ (roselite, Hawthorne and Ferguson 1977), Cu 2+ (rruffite, Yang et al. 2011), Mg²+ (wendwilsonite, Kolitsch and Fleck 2006) and Zn²+ (zincroselite, Keller et al. 2004). Dobšináite is named after its type locality Dobšiná, which represents the most important accumulation of Ni arsenides and sulfarsenides in the Western Carpathians and in the past, it was a significant producer of the high- grade Ni and Co ores in Europe. The new mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Associa- tion (IMA 2020-81). The holotype sample of dobšináite is deposited in the collections of the Moravian Museum in Brno, Department of Mineralogy and Petrography, Zelný trh 6, Brno, Czech Republic, under the catalog number B12257.
2. Occurrence
Dobšináite was found in several specimens collected in the short abandoned adit at the Dionýz mining field, Zemberg-Terézia vein system, which is located 2.2 km NE of the Dobšiná town, Spišsko-gemerské rudohorie ghtly rounded thin tabular to platy crystals; holotype sample, the field of view is 1.6 mm. Mts., Rožňava Co., Košice Region, Slovak Republic (48.837977°N, 20.383550°E). The Zemberg-Terézia vein system is represented by siderite-ankerite hydrothermal carbonate–quartz veins with Ni–Fe–Co–Cu ore mineralization hosted in the Early Paleozoic gneiss-amphibolite complex of the Klátov group belonging to the Gemeric tectonic unit (Rozložník 1959, 1965; Halahyjová-Andrusovová 1964a; Grecula et al. 1995). The vertical extent of the veins exceeds 1.5 km, generally with the SW-NE orientation and moderate to steep dip (40° to 70°) to the S or SE. The veins are usually 0.7 to 1.5 m thick. The two different ore miner- alization types were distinguished: carbonate (siderite, ankerite–dolomite)–quartz vein filling with baryte and abundant chalcopyrite and tetrahedrite is typical for the upper parts of the veins. In contrast, Ni–Co–As miner- als are prevailing ore minerals in the deeper parts of the veins (Halahyjová-Andrusovová 1959, 1964b; Grecula et al. 1995). The dominant gangue minerals are carbonates, mainly Fe-rich dolomite to ankerite and siderite with quartz and minor tourmaline. Gersdorffite and arsenopy- rite are the most frequent ore minerals, accompanied by minor pyrite, cobaltite, nickelskutterudite–skutterudite, nickeline, millerite, rammelsbergite, safflorite, ullman- nite, löllingite, chalcopyrite, tetrahedrite group minerals, Bi sulfosalts and hematite (Halahyjová-Andrusovová 1959; Kiefer et al. 2017, 2020; Števko and Sejkora 2020). The two adits in the Dionýz mining field (Dionýz adit and the short adit above, where the samples with dobšináite were collected) explored one of the short hydrother- mal ore veins developed at the southern margin of the Zemberg-Terézia vein system. Dobšináite was found in a relatively dry area of the mine in assemblage with other Ca and Co phases. This supergene post-mining assemblage was found growing on the surface of loose fragments of vein filling containing intergrown thin tabular to platy crystals associated with hemispherical aggregates of minerals from crythrite-hörnesite solid-solution; holotype sample, the field of view is 1.8 mm. calcite–dolomite–quartz matrix with rich impregnations of safflorite and Co-rich arsenopyrite. Dobšináite is closely associated with phaunouxite, picropharmacolite, erythrite–hörnesite, gypsum and aragonite. Rauenthalite is also present in this assemblage but never in direct association with dobšináite. The supergene assemblage with dobšináite was formed by oxidation of primary Co and As-bearing ore minerals (safflorite, arsenopyrite) in a Ca enriched environment (abundant calcite in the matrix) under the relatively dry conditions (Števko et al. unpublished data).
3. Appearance and physical properties
Dobšináite occurs as clusters or polycrystalline ag¬ gregates up to 1–4 mm in size consisting of densely intergrown, slightly rounded thin platy crystals up to 0.1 mm in size (Figs 1 and 2), usually the thickness of individual crystals does not exceed 10 µm. It has a white to pink color depending on the Co content; the holotype is white, locally with a light pink/violet tint. The pink variety seems to be more coarse crystalline than the white variety. Its aggregates are opaque to translucent; individual crystals or tiny fragments are translucent to transparent. It has a white streak and a vitreous luster. It does not fluoresce under either short- or long-wave ultraviolet light. Cleavage on {010} is good, the Mohs hardness is ~3 (scratch tests), and the mineral is brittle with an uneven fracture. The density D cals = 3.395 g/cm³ was calculated on the basis of the empirical formula and unit-cell volume refined from powder data; for the ideal formula, we obtained 3.405 g cm 3. Dobšináite is optically biaxial negative, with α΄ = 1.601(2) and γ΄ = 1.629(2) (measured at 589 nm), 2V msas, = 60° (+/- 20°). Crystals and aggregates are formed of parallel intergrowths and bent fine elongated domains. Extinction has an undulatory character and is parallel to the elongation or with a minimum angle. The character of the sample hampers a more precise char- acterization. Dispersion is weak ( r > v ), and dobšináite exhibits no noticeable pleochroism. The Gladstone-Dale compatibility index 1-( K o /K c ) calculated from incom- plete optical data is 0.003 for the empirical formula and unit-cell parameters from X-ray powder diffraction pattern, indicating superior compatibility (Mandarino 1981).
4. Chemical composition
Initial chemical analyses of dobšináite were col¬ lected utilizing Cameca SX-100 electron micro¬ probe (Masaryk University, Brno) during a routine investigation of the secondary arsenate mineral as- semblage from Dobšiná. Subsequently, a more sys- tematic chemical characterization of dobšináite was performed by a Cameca SX-100 electron mi¬ croprobe (National Museum, Prague) operating in the wavelength-dispersion mode with an ac¬ celerating voltage of 15 kV, a specimen current of 4 nA, and a beam diameter of 20 µm. The follow¬ ing lines and standards were used: K : diopside (Mg), flu- orapatite (P), rhodonite (Mn), wollastonite (Ca), Co (Co), Ni (Ni), celestine (S), and L : clinoclase (As). Peak counting times (CT) were 20 s, CT for each background was one-half of the peak time. The raw in- tensities were converted to the concentrations automatically using PAP (Pouchou and Pi- choir 1985) matrix-correction software. Other elements, such 50 as Al, Cu, Fe, Si, V and Zn were found to be below the 60 detection limits (0.03–0.08 wt. %). Water content could not be 70 analyzed directly because of the minute amount of material 80 available and was calculated Fig. 3 Ternary graph of occupation of Co+Ni vs. Ca vs. Mg (atom. unit) in the octahedral M position of dobšináite from Dobšiná; data for holotype sample are compared with white and pink dob- šináite aggregates from other samples.
position of roselite group minerals on the basis of 2 H,O (from the ideal composition of roselite group minerals). Table 1 gives the chemical composition of the holo- type sample of dobšináite (mean of nineteen determi- nations). The empirical formula of dobšináite based on 10 O atoms per formula unit is Ca 2 (Ca 0.84 Mg 0.10C00.03Ni0.02)z0.99[(AsO 4 ) 1.99(SO 4 ) 0.01]z2.00·2H 2 O. The simplified formula is Ca 2 (Ca,Mg)(AsO 4 ) 2 ·2H 2 O, and the ideal end-member formula is Ca,Ca(AsO 4 ), 2H,O, which requires CaO 38.75, As,O, 52.94. H,O 8.30, total 100.00 wt. %. In other samples, besides white ones, pink ag- gregates with increased Co (up to 3.66 wt. % CoO,
H,O* – content calculated on the basis of 2 H,O from the ideal composition of roselite group minerals 0.21 apfu ) and Ni contents (up to 0.79 wt. % NiO, 0.05 apfu) were observed; this composition also cor¬ responds very well to dobšináite (Fig. 3). The repre¬ sentative analyses for all dobšináite samples are given in Tab. 2.
5. Crystallography
Attempts to obtain single-crystal X-ray data (Rigaku SuperNova single-crystal diffractometer with Atlas S2 CCD detector) were unsuccessful due to the nature of the studied material; it is formed by multiple intergrowths of sub-microscopic domains. X-ray powder diffraction data of dobšináite (Tab. 3) were recorded at room temperature using a Bruker D8 Advance diffractometer equipped with a solid-state Lynx- Eye detector and secondary monochromator producing CuKα radiation housed at the Department of Mineralogy and Petrology, National Museum, Prague. The instrument was operated at 40 kV and 40 mA. To minimize the back- ground, the powdered samples were placed (without any liquid) on the surface of a flat silicon wafer. The powder pattern was collected in the Bragg–Brentano geometry in the range 3–70° 2θ, step 0.01°, and a counting time of 20 s per step (total duration of the experiment was ca. 30 hours). The positions and intensities of diffractions were found and refined using the Pearson VII profile-shape function of the ZDS program package (Ondruš 1993). The unit-cell parameters were refined by the least-squares program of Burnham (1962) for monoclinic space group P2 , /c (#14) as follows: a = 5.990(2), b = 13.013(4), c =
/ 0 0 1 –1 –1 0 1 1 0 1 –1 –2 2 –1 0 1 –2 –1 2 0 2 –1 −2 –1 2 1 –1 3 0 2 2
6. Raman spectroscopy
The Raman spectrum of dobšináite (Fig. 5) was obtained employing a Horiba Labram HR Evolution spectrometer (Masaryk University, Brno). This dispersive, edge-filter- based system is equipped with an Olympus BX 41 optical microscope, a diffraction grating with 600 grooves per millimeter, and a Peltier-cooled, Si-based charge-coupled device (CCD) detector. The 633 nm He-Ne laser with the beam power of 5 mW at the sample surface was selected for spectra acquisition to minimize analytical artifacts. Raman signal was collected in the range of 50–4000 cm −1 with a 100× objective (NA 0.9) and the system being op- erated in the confocal mode; beam diameter was ~1 µm and the depth resolution ~2 µm. No visual damage to the analyzed surface was observed at these conditions after the excitation. For wavenumber calibration, the Rayleigh line and low-pressure Ne-discharge lamp emissions were used. The wavenumber accuracy was ~0.5 cm -1 , and the spectral resolution was ~2 cm −1 . Band fitting was done after appropriate background correction, assuming com¬ bined Lorentzian-Gaussian band shapes using the Voigt function (PeakFit; Jandel Scientific Software). The main bands observed are (in wavenumbers): 3358, 3170, 2960, 863, 828, 724, 474, 447, 416, 366, 333, 295, 196, 171, 125, 106, 81 and 53 cm -1 . Weak broad Raman bands with maxima at 3358, 3170 and 2960 cm −1 are as- signed to the ν OH stretching vibrations of structurally distinct differently hydrogen-bonded water molecules. The most prominent very strong Raman band at 828 cm −1 with a shoulder at 863 cm -1 is attributed to overlapping ν, (AsO a ) 3- symmetric stretching and ν, (AsO a ) 3- antisym- metric stretching vibrations. A weak shoulder band at 724 cm-' can be connected to the libration of H,O molecules. Raman bands 474, 447 and 416 cm -1 are attributed with the v , (AsO , ) 3 - bending vibrations. Raman bands at 366, 333 and 295 cm-' are related to the v, (AsO 4 ) 3- bend- ing vibrations and those below 200 cm -1 to the lattice modes (Vansant et al. 1973; Yang et al. 2011; Sejkora et al. 2020). In the area of v , and v , (AsO a ) 3- vibrations (1000–700 cm-'), the Raman spectrum of dobšináite is close to other As-dominant members of the roselite group and distinctly differs from those of As-dominant members of the fairfieldite group (Fig. 6).
7. Comparison with related minerals
Dobšináite is a Ca- and As-dominant member of the roselite group, Strunz class 8.CG.10, Dana class 40.2.3. A comparison of selected data for As-dominant members of this group is given in Tab. 4. An unambiguous deter- mination of dobšináite as a member of the roselite group
* – intensities were calculated using the software PowderCell 2.3 (Kraus and Nolze 1996) on the basis of the structural model (brandtite – Herwig and Hawthorne 2006 – with Ca replacing for Mg). 5.726(2) Å, β = 108.47(3)°, V = 423.3(3) Å³ and Z = 2. The a:b:c ratio calculated from unit-cell parameters is 0.4603:1:0.4400. The well-known crystal structure of As-dominant members of the roselite group (Ca,M 2 + (AsO 2 ), 2H,O) is characterized by isolated MO k (H,O), octahedra that are linked by corner-sharing with AsO 4 tetrahedra to form the kröhnkite-type chains parallel to the c axis. These chains are linked together by the large Ca cat¬ ions and hydrogen bonding (Hawthorne and Ferguson 1977; Keller et al. 2004; Herwig and Hawthorne 2006; Kolitsch and Fleck 2006; Yang et al. 2011). The values of unit-cell parameters (especially a and V ) positively correlate with the octahedral M cation's average ionic radii (Fig. 4). 0 Fig. 5 Raman spectrum of dobšináite (full-range). is based on its chemical composition (stoichiometry), X-ray powder diffraction data and Raman spectroscopy. A mineral phase corresponding to dobšináite was de- scribed in 1953 by Evgeniy I. Nefedov from Tuvinskaya deposit, Tuva Autonomous Republic, Russia. Nefedov provided an empirical formula Ca,(Ca,Mg)(AsO,), 2H,O, some structural, optical and physical data; however, a chemical analysis is missing. Nefedov (1953) named this mineral belovite and assigned it to the roselite group mainly based on similarities of general formula and in- accurate structural information. Borodin and Kazakova (1954) described a Sr, REE, Na-bearing member of the apatite supergroup and named it belovite, too. To reduce the confusion, belovite of Nefedov was then used to be call arsenate-belovite. Subsequent reinvestigation of belovite original material of Nefedov (Yakhontova and
et al. (2011); [1] this proposal; [2] Herwig and Hawthorne (2006); [3] Kolitsch and Fleck (2006); [4] Hawthorne and Ferguson (1977); [5] Keller et al. (2004); [6] Yang et al. (2011) Sidorenko 1956; Pierrot 1964) revealed that its chemi- cal composition is richer in Mg and corresponds to Ca,Mg(AsO,), 2H,O, and it is a triclinic member of a beta-roselite group. Hence, this material corresponds to mineral talmessite, described in 1960 by Bariand and Herpin (1960) from the Talmessi deposit, Iran. Synthetic Ca,(AsO,), 2H,O has been prepared experimentally by Nelson and Haring (1937). The au- thors determined its relatively narrow stability field in the CaO-As,O,-H,O system and performed an X-ray study; however, it is difficult to acquire more structural details from the photograph of the diffraction pattern they provided in the publication. A synthetic analog of dobšináite was synthesized by Guérin (Pierrot 1964). The X-ray pow- der diffraction data of this ma- terial, published by Pierrot (1964), are similar to those talmessite of dobšináite form Dobšiná, but also contain reflections at 11.7 Å and 6.1 Å, which were probably caused by the pres¬ nickeltalmessite ence of impurities in Guérin's phase, e.g., a synthetic analog of phaunouxite. The solubility of Ca,(AsO,), 2H,O in water was studied by Chukhlantsev roselite-beta (1956) and Mahapatra et al. (1984). Unfortunately, both an intensit studies lack any structural in- formation. There are many experimental works focused on the system CaO−As¸O¸−H¸O at rruffite various T-X conditions (Pearce and Avens 1937; Nishimura and Robins 1998; Bothe and Brown 1999; Zhu et al. 2006; roselite, Raičević et al. 2007; Hernan- dez-Barcenas et al. 2017, etc.), however Ca,(AsO,), 2H,O is usually missing among the zincroselite precipitated solid phases which indicates specific conditions wendwilsonite are necessary for dobšináite formation. Fig. 6 Raman spectra of dobšináite and those of As-dominant members of roselite (monoclinic) and fairfieldite (triclinic) group from the RRUFF pro- ject for comparison. The spectra of the figure are shown with a vertical offset for more clarity. Acknowledgments. The reviewers, Juraj Majzlan and Nicolas Meisser, same as handling editor Jiří Zachariáš, are thanked for a lot of their constructive comments. This work was supported financially by the Ministry of Culture of the Czech Republic (long-term project DKRVO 2019-2023/1.II.c National Museum, 00023272) for JS, MŠ, ZD and its institutional sup- port of long-term conceptual development of research institutions (ref. MK000094862) for JT and EV and the MUNI/A/1594/2020 research project of Masaryk University for RŠ and SH.