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Eliopoulosite, V7S8, A New Sulfide from the Podiform Chromitite of the Othrys Ophiolite, Greece

The new mineral species, eliopoulosite, V7S8, was discovered in the abandoned chromium mine of Agios Stefanos of the Othrys ophiolite, located in central Greece. The investigated samples consist of massive chromitite hosted in a strongly altered mantle tectonite, and are associated with nickelphosphide, awaruite, tsikourasite, and grammatikopoulosite. Eliopoulosite is brittle and has a metallic luster. In plane-reflected polarized light, it is grayish-brown and shows no internal reflections, bireflectance, and pleochroism. It is weakly anisotropic, with colors varying from light to dark greenish. Reflectance values of mineral in air (Ro, Re’ in %) are: 34.8–35.7 at 470 nm, 38–39 at 546 nm, 40–41.3 at 589 nm, and 42.5–44.2 at 650 nm. Electron-microprobe analyses yielded a mean composition (wt.%) of: S 41.78, V 54.11, Ni 1.71, Fe 1.1, Co 0.67, and Mo 0.66, totali 100.03. On the basis of Σatoms = 15 apfu and taking into account the structural data, the empirical formula of eliopoulosite is (V6.55Ni0.19Fe0.12Co0.07Mo0.04)Σ = 6.97S8.03. The simplified formula is (V, Ni, Fe)7S8 and the ideal formula is V7S8, which corresponds to V 58.16%, S 41.84%, total 100 wt.%. The density, based on the empirical formula and unit-cell volume refined form single-crystal structure XRD data, is 4.545 g·cm−3. The mineral is trigonal, space group P3221, with a = 6.689(3) Å, c = 17.403(6) Å, V = 674.4(5) Å3, Z = 3, and exhibits a twelve-fold superstructure (2a × 2a × 3c) of the NiAs-type subcell with V-atoms octahedrally coordinated by S atoms. The distribution of vacancies is discussed in relation to other pyrrhotite-like compounds. The mineral name is for Dr. Demetrios Eliopoulos (1947–2019), a geoscientist at the Institute of Geology and Mineral Exploration (IGME) of Greece and his widow, Prof. Maria Eliopoulos (nee Economou, 1947), University of Athens, Greece, for their contributions to the knowledge of ore deposits of Greece and to the mineralogical, petrographic, and geochemical studies of ophiolites, including the Othrys complex. The mineral and its name have been approved by the Commission of New Minerals, Nomenclature, and Classification of the International Mineralogical Association (No. 2019-96).


Introduction
Only eight minerals containing V and S are in the list of valid species approved by the International Mineralogical Association (IMA). They include, in alphabetic order: colimaite, K 3 VS 4 [1]; colusite,

Geological Background and Occurrence of Eliopoulosite
Eliopoulosite was discovered in a heavy-mineral concentrate obtained from massive chromitite collected in the mantle sequence of the Mesozoic Othrys ophiolite, located in central Greece ( Figure 1A). The geology, petrography, and geodynamic setting of the Otrhys ophiolite have been discussed by several authors [11][12][13][14][15][16][17][18][19][20][21][22][23][24]. The studied sample was collected in the abandoned chromium mine of Agios Stefanos, located a few km southwest of the Domokos village ( Figure 1B). In the studied area, a mantle tectonite composed of plagioclase lherzolite, harzburgite, and minor harzburgite-dunite occurs in contact with rocks of the crustal sequence ( Figure 1C). The investigated chromitite was collected from the harzburgite-dunite that is cut across by several dykes of gabbro ( Figure 1C).
The ophiolite of Othrys is a complete but dismembered suite (Mirna Group) and consists of three structural units: the uppermost succession with variably serpentinized peridotites, which is structurally bounded by an ophiolite mélange; the intermediate Kournovon dolerite, including cumulate gabbro and local rhyolite; and the lower Sipetorrema Pillow Lava unit also including basaltic flows, siltstones, and chert. The Mirna Group constitutes multiple inverted thrust sheets, which were eventually obducted onto the Pelagonian Zone, during the Late Jurassic-Early Cretaceous [11][12][13][14]. Three types of basalts with different geochemical signatures have been described: i) alkaline within-plate (WPB), occurring in the mélange, which is related to oceanic seamounts or to ocean-continent transition zones; ii) normal-type mid-ocean ridge (N-MORB); and iii) low-K tholeiitic (L-KT) rocks, which are erupted close to the rifted margin of an ocean-continent transition zone. Radiolarian data from interbedded cherts suggest Middle to Late Triassic ages [15]. The Othrys ophiolite is structurally divided into the west and east Othrys suites, which are thought to derive from different geotectonic environments. The former is related to an extension regime (back-arc basin or MORB [16][17][18]), while the latter formed in a supra-subduction zone (SSZ) setting [17][18][19]. This difference is reflected in the contrasting geochemical compositions of their ultramafic rocks, as well as in the diverse platinum group minerals (PGM) assemblages of their host chromitites [20,21], which suggest large mantle heterogeneities in this area. Conflicting views with regards to the evolution of the Othrys ophiolite suggest the involvement of Pindos [11,22,23] or Vardar (Axios) [15] oceanic domains.
The studied spinel-supergroup minerals that host eliopoulosite can been classified as magnesiochromite, although their composition is rather heterogeneous [9,10,25]   The studied spinel-supergroup minerals that host eliopoulosite can been classified as magnesiochromite, although their composition is rather heterogeneous [9,10,25]

Analytical Methods
The heavy minerals were concentrated at SGS Mineral Services, Canada, following the methodology described by several authors [9,10,21,25,26]. After the concentration process, the heavy minerals were embedded in epoxy blocks and then polished for mineralogical examination.
Quantitative chemical analyses and acquisition of back-scattered electron images of eliopoulosite were performed with a JEOL JXA-8200 electron-microprobe, installed in the E. F. Stumpfl laboratory, Leoben University, Austria, operating in wavelength dispersive spectrometry (WDS) mode. Major and minor elements were determined at 20 kV accelerating voltage and 10 nA beam current, with 20 s as the counting time for the peak and 10 s for the backgrounds. The beam diameter was about 1 µ m in size. For the WDS analyses, the following lines and diffracting crystals

Analytical Methods
The heavy minerals were concentrated at SGS Mineral Services, Canada, following the methodology described by several authors [9,10,21,25,26]. After the concentration process, the heavy minerals were embedded in epoxy blocks and then polished for mineralogical examination.
Quantitative chemical analyses and acquisition of back-scattered electron images of eliopoulosite were performed with a JEOL JXA-8200 electron-microprobe, installed in the E. F. Stumpfl laboratory, Leoben University, Austria, operating in wavelength dispersive spectrometry (WDS) mode. Major and minor elements were determined at 20 kV accelerating voltage and 10 nA beam current, with 20 s as the counting time for the peak and 10 s for the backgrounds. The beam diameter was about 1 µm in size. For the WDS analyses, the following lines and diffracting crystals were used: S = (Kα, PETJ), V, Ni, Fe, Co = (Kα, LIFH), and Mo = (Lα, PETJ). The following standards were selected: metallic vanadium for V, pyrite for S and Fe, millerite for Ni, molybdenite for Mo, and skutterudite for Co.
The ZAF correction method was applied. Automatic correction was performed for interference Mo-S. Representative analyses of eliopoulosite are listed in Table 1. A small grain of eliopoulosite was hand-picked from the polished section under a reflected light microscope. The crystal (about 80 µm in size) was carefully and repeatedly washed in acetone and mounted on a 5 µm-diameter carbon fiber, which was, in turn, attached to a glass rod in preparation of the single-crystal X-ray diffraction measurements.
Single-crystal X-ray diffraction data were collected at the University of Florence (Italy) using a Bruker D8 Venture equipped with a Photon II CCD detector, with graphite-monochromatized MoKα radiation (λ = 0.71073 Å). Intensity data were integrated and corrected for standard Lorentz-polarization factors with the software package Apex3 [27,28]. A total of 1289 unique reflections was collected up to 2θ = 62.24 • .
The reflectance measurements on eliopoulosite were carried out using a WTiC standard and a J&M TIDAS diode array spectrophotometer at the Natural History Museum of London, UK.

Physical and Optical Properties
In the polished section, eliopoulosite occurs as tiny crystals (from 5 µm up to about 80 µm) and is anhedral to subhedral in habit. It consists of polyphase grains associated with other minerals, such as tsikourasite, nickelphosphide, awaruite, and grammatikopoulosite ( Figure 2).
In plane-reflected polarized light, eliopoulosite is grayish-brown and shows no internal reflections, bireflectance, and pleochroism. It is weakly anisotropic, with colors varying from light to dark greenish. Reflectance values of the mineral in air (R in %) are listed in Table 2 and shown in Figure 3. In plane-reflected polarized light, eliopoulosite is grayish-brown and shows no internal reflections, bireflectance, and pleochroism. It is weakly anisotropic, with colors varying from light to dark greenish. Reflectance values of the mineral in air (R in %) are listed in Table 2 and shown in Figure 3.   Due to the small amount of available material, density was not measured. The calculated density was = 4.545 g·cm −3 , based on the empirical formula and the unit-cell volume refined from single-crystal XRD data.

X-Ray Crystallography and Chemical Composition
The mineral is trigonal, with a = 6.689 (3)  were determined by means of direct methods [29]. A least-squares refinement on F 2 using these heavy-atom positions and isotropic temperature factors produced an R factor of 0.123. Three-dimensional difference Fourier synthesis yielded the position of the remaining sulfur atoms. The program Shelxl-97 [30] was used for the refinement of the structure. The site occupancy factor (s.o.f.) at the cation sites was allowed to vary (V vs. structural vacancy) using scattering curves for neutral atoms taken from the International Tables for Crystallography [30]. Four V sites (i.e., V1, V2, V3, and V4) were found to be partially occupied by vanadium (75%), while V5 and V6 were found to be fully occupied by V and fixed accordingly. Sulfur atoms were found on four fully occupied general positions leading to an ideal formula V7S8. Given the almost identical partial occupancy of four V sites (i.e., 75%), we carefully checked either the possible presence of twinning or the acentricity of the model. The lack of the inversion center was confirmed using the Flack parameter in Shelxl (0.07 (1)) and the trigonal model was double-checked with the Platon addsymm routine. At the last stage, with anisotropic atomic displacement parameters for all atoms and no constraints, the residual value settled at R1 = 0.0363 for 398 observed reflections (Fo > 4σ(Fo)) and 79 parameters and at R1 = 0.0537 for all 1289 independent reflections. Refined atomic coordinates and isotropic displacement parameters are given in Table 3, whereas selected bond distances are reported in Table  4. Crystallographic Information File (CIF) is deposited.
The calculated X-ray powder diffraction pattern (Table 5) was computed on the basis of the unit-cell data above and with the atom coordinates and occupancies reported in Table 3. Due to the small amount of available material, density was not measured. The calculated density was = 4.545 g·cm −3 , based on the empirical formula and the unit-cell volume refined from single-crystal XRD data.

X-Ray Crystallography and Chemical Composition
The mineral is trigonal, with a = 6.689(3) Å, c = 17.403(6) Å, V = 674.4(5) Å 3 , and Z = 3. The reflection conditions (00l: l = 3n), together with the observed R int in the different Laue classes, point unequivocally to the choice of the space group P3 2 21. The structure solution was then carried out in this space group. The positions of most of the atoms (all the V positions and most of the S atoms) were determined by means of direct methods [29]. A least-squares refinement on F 2 using these heavy-atom positions and isotropic temperature factors produced an R factor of 0.123. Three-dimensional difference Fourier synthesis yielded the position of the remaining sulfur atoms. The program Shelxl-97 [30] was used for the refinement of the structure. The site occupancy factor (s.o.f.) at the cation sites was allowed to vary (V vs. structural vacancy) using scattering curves for neutral atoms taken from the International Tables for Crystallography [30]. Four V sites (i.e., V1, V2, V3, and V4) were found to be partially occupied by vanadium (75%), while V5 and V6 were found to be fully occupied by V and fixed accordingly. Sulfur atoms were found on four fully occupied general positions leading to an ideal formula V 7 S 8 . Given the almost identical partial occupancy of four V sites (i.e., 75%), we carefully checked either the possible presence of twinning or the acentricity of the model. The lack of the inversion center was confirmed using the Flack parameter in Shelxl (0.07 (1)) and the trigonal model was double-checked with the Platon addsymm routine. At the last stage, with anisotropic atomic displacement parameters for all atoms and no constraints, the residual value settled at R1 = 0.0363 for 398 observed reflections (Fo > 4σ(Fo)) and 79 parameters and at R1 = 0.0537 for all 1289 independent reflections. Refined atomic coordinates and isotropic displacement parameters are given in Table 3, whereas selected bond distances are reported in Table 4. Crystallographic Information File (CIF) is deposited.
The calculated X-ray powder diffraction pattern (Table 5) was computed on the basis of the unit-cell data above and with the atom coordinates and occupancies reported in Table 3. Table 3. Atoms, site occupancy, fractional atom coordinates (Å), and isotropic atomic displacement parameters (Å 2 ) for eliopoulosite.
The structure can be considered as a twelve-fold superstructure (2a × 2a × 3c) of the NiAs-type subcell with V atoms octahedrally coordinated by S atoms and sulfur located in a trigonal prism. There are six octahedral layers in the structure with rods of fully occupied V sites alternate to rods of partially occupied sites, so that every layer contains 3.5 vanadium atoms. In successive layers rods are directed along [110], [100], and [010] accordingly to the threefold screw axis. As shown in Figure 5, the distribution of fully (blue in color) and partially (pale blue) occupied sites determine the superstructure along the a 1 and a 2 axes. Differences in electron density between fully and partially occupied sites are indeed rather modest (22 vs. 16.5): Accordingly, hkl reflections with h,k = 2n + 1 are rather weak (<I/σ(I)> = 27.1 vs. 4890.3).
Since the octahedral vacancies' distribution within the layers at z = 0, z = 1/6, and z = 1/3 is perfectly replicated at layers at z = 1/2, z = 2/3, and z = 5/6 ( Figure 5), no contribution to hkl reflections with l = 2n + 1 is given by metal atoms. However, their intensities (<I/σ(I)> = 564.3 vs. 3349.2) are even higher than those leading to doubling of a-axis, due to the different position of sulfur atoms related to the different octahedral orientation in the layer at z = 0 with respect to z = 1/2 ( Figure 4).

Remarks on the Origin of Eliopoulosite
Eliopoulosite was found in the same sample in which tsikourasite and grammatikopoulosite were discovered, and the three minerals occur in the same mineralogical assemblage [9,10]. Therefore, we can argue that they formed under the same chemical-physical condition (i.e., in a The eliopoulosite structure shows strong analogies with the superstructures observed in the pyrrhotite-group of minerals. It is identical to the structure inferred for synthetic VS 1.125 [31]. The mean <V-S> bond distances in eliopoulosite (in the range 2.413-2.418 Å) are in excellent agreement with those found for synthetic vanadium sulfides. To achieve the charge balance, one should hypothesize the presence of both divalent and trivalent vanadium in eliopoulosite (i.e., V 2+ 5 V 3+ 2 S 8 ). The octahedral sites, however, do not show any significant difference symptomatic of a V 2+ -V 3+ ordering. Likewise, we did not find any evidence of ordering of the minor substituents (i.e., Ni and Fe) at a particular structural site. Furthermore, eliopoulosite shows, for the analogy of stoichiometry, unit cell and space group, close relationships with the metastable, trigonal form of 3C-Fe 7 S 8 (a = 6.852 (6), c = 17.046(2), P3 1 21) [32]. However, the distribution of vacancies is quite different (Figure 5b): As in other types of pyrrhotites [33], in 3C-Fe 7 S 8 octahedral vacancies are completely ordered on one position alone so that the layers containing a void (i.e., three metal atoms) are alternating with fully occupied layers (i.e., four metal atoms).  . Blue and pale blue circles refer to metal positions with full and partial (75%) occupancy, respectively. White circles represent completely empty sites in synthetic 3C-Fe7S8 pyrrhotite [32]. The unit cell and the orientation of the structure are indicated.
Eliopoulosite does not correspond to any valid or invalid unnamed mineral [34]. According to literature data [1][2][3][4][5][6][7][8] eliopoulosite is the ninth mineral containing V and S that has been accepted by IMA. Noteworthy, eliopoulosite is the second V 2+ -bearing mineral, the first being dellagiustaite, Al2VO4 [35]. The presence of divalent V requires extremely reducing conditions (well below the iron-wüstite buffer). Furthermore, based on its chemical composition, eliopoulosite is the second sulfide that contains only V as a major component discovered so far. Recently, Ivanova et al. [36] reported on the presence of a V,Cr,Fe-sulfide in the extremely reduced assemblage of a rare CB Eliopoulosite does not correspond to any valid or invalid unnamed mineral [34]. According to literature data [1][2][3][4][5][6][7][8] eliopoulosite is the ninth mineral containing V and S that has been accepted by IMA. Noteworthy, eliopoulosite is the second V 2+ -bearing mineral, the first being dellagiustaite, Al 2 VO 4 [35]. The presence of divalent V requires extremely reducing conditions (well below the iron-wüstite buffer). Furthermore, based on its chemical composition, eliopoulosite is the second sulfide that contains only V as a major component discovered so far. Recently, Ivanova et al. [36] reported on the presence of a V,Cr,Fe-sulfide in the extremely reduced assemblage of a rare CB chondrite. Due to its small size (one micron) the grain was studied only by EBSD (Electron BackScattered Diffraction) and no crystallographic or optical data were provided. However, its ideal composition (V,Cr,Fe) 4 S 5 seems different compared to that of eliopoulosite in terms of S content and V/Fe ratio and the presence of abundant Cr.
Interestingly, Selezneva et al. [37] recently showed that synthetic V 7 S 8 shows differences in the magnetic behavior with respect to trigonal Fe 7 S 8 . Indeed, V 7 S 8 is observed to exhibit a Pauli-paramagnetic behavior opposite to the classic ferrimagnetic ordering usually observed in pyrrhotite. Unfortunately, the small amount of the natural material precludes any possible measurement of both the magnetic susceptibility and magnetization. Such an experiment would have allowed us to verify if the amount of structural vacancies observed for eliopoulosite are enough to compensate the magnetic moments to avoid the ferromagnetic order.

Remarks on the Origin of Eliopoulosite
Eliopoulosite was found in the same sample in which tsikourasite and grammatikopoulosite were discovered, and the three minerals occur in the same mineralogical assemblage [9,10]. Therefore, we can argue that they formed under the same chemical-physical condition (i.e., in a strongly reducing environment). This assumption is fully supported by the chemical composition of eliopoulosite, that points to a low valence state for V. However, as already proposed for the origin of tsikourasite and grammatikopoulosite, it is still not possible to provide a conclusive model to explain exhaustively the genesis of eliopoulosite. The following hypothesis can be formulated, since all of them imply the presence of a reducing environment [9,10,38-43]: i) low-temperature alteration of chromitite during serpentinization process; ii) high-temperature reaction of the chromitites with reducing fluids, at mantle depth; iii) post-orogenic surface lightning strike; or iv) meteorite impact.
However, the probability of intercepting a fragment of a meteorite or a fulgurite in the Otrhys ophiolite during the sampling of the studied chromitite seems very unlikely, although a V-bearing sulfide has been reported in a meteorite [36].