Isselite, Cu6(SO4)(OH)10(H2O)4⋅H2O, a new mineral species from Eastern Liguria, Italy

Abstract The new mineral isselite, Cu6(SO4)(OH)10(H2O)4⋅H2O, has been discovered in the Lagoscuro mine, Monte Ramazzo mining complex, Genoa, Eastern Liguria, Italy. It occurs as sprays of blue acicular crystals, up to 0.1 mm long, associated with brochantite and posnjakite. Streak is light blue and the lustre is vitreous. Isselite is brittle, with irregular fracture and good cleavage on {001} and {100}. Measured density is 3.00(2) g/cm3. Isselite is optically biaxial (–), with α = 1.599(2), β = 1.633(2) and γ = 1.647(2) (determined in white light). The measured 2V is 63.6(5)°. Dispersion is moderate, with r > v. The optical orientation is X = b, Y = c and Z = a. Isselite is pleochroic, with X = light blue, Y = blue, Z = blue; X << Z < Y. Electron microprobe analyses give (wt.%): SO3 11.45(21), MgO 0.31(7), CoO 1.07(14), NiO 9.41(90), CuO 51.29(126), ZnO 1.10(20), H2Ocalc 24.21, total 98.84. The empirical formula of isselite, based on Σ(Mg,Co,Ni,Cu,Zn) = 6 atoms per formula unit, is (Cu4.80Ni0.94Co0.11Zn0.10Mg0.06)Σ6.00(S1.06O4.19)(OH)10⋅5H2O. Isselite is orthorhombic, space group Pmn21, with unit-cell parameters a = 6.8070(14), b = 5.8970(12), c = 20.653(4) Å, V = 829.0(3) Å3 and Z = 2. The crystal structure of isselite was refined from single-crystal X-ray diffraction data to R1 = 0.067 on the basis of 2964 reflections with Fo > 4σ(Fo). It shows a layered structure formed by zig-zag {001} layers of Cu-centred polyhedra. Sulfate groups occur in the interlayer along with one H2O group. Isselite is chemically related to redgillite and montetrisaite.


Introduction
The ore deposits of Eastern Liguria, Northern Apennines, Italy, are well-known among mineralogists for the occurrence of several rare species from the Mn ore deposits hosted in middle Jurassic metacherts of the Graveglia Valley, Genoa Province (Cabella et al., 1998;Bindi et al., 2013;Biagioni et al., 2019a) and the Cerchiara Mine, La Spezia Province Kolitsch et al., 2018). In addition, Cu-Fe-Ni-Co sulfide ore deposits are also hosted in ophiolitic rocks of the Northern Apennines and the Sestri-Voltaggio Zone, where several types of Volcanic Massive Sulfide (VMS) deposits occur in maficultramafic rocks at different stratigraphic positions within the ophiolitic sequences (Ferrario and Garuti, 1980;Zaccarini and Garuti, 2008;Schwarzenbach et al., 2012). Among them, stockwork-vein and seafloor stratiform orebodies are associated with serpentinised mantle peridotites and serpentinite breccias (e.g. Ferrario and Garuti, 1980), like in the Monte Ramazzo-Lagoscuro area, where a pyrrhotite-dominated mineralisation had been actively exploited since the beginning of the 19th Century (Rolandi, 1974;Pipino, 1977). The primary mineralisation underwent subsequent fluid-rock interaction, hydrothermal mobilisation, and multi-stage alteration processes which led, on the one hand, to sulfide reconcentration and recrystallisation along tectonic structures, on the other hand to metal reworking and the formation of secondary phases, such as (Ni,Co)-bearing oxy-hydroxides (asbolane and heterogenite), carbonates (kolwezite, Co-rich malachite and spherocobaltite) and silicates (pecoraite). This peculiar mineral assemblage occurring in the Monte Ramazzo-Lagoscuro ore deposit is quite different from other hydrothermal sulfide mineralisation described so far in ophiolites from the Eastern Liguria, where (Ni,Co)-enrichment occurs mostly in pyrite and/or accessory minerals like millerite, siegenite and pentlandite (e.g. Cortesogno et al., 1977;Schwarzenbach et al., 2012;Moroni et al., 2019).
During the examination of secondary minerals from the Monte Ramazzo mining complex, blue acicular crystals from the Lagoscuro mine were encountered. Crystallographic studies and chemical analyses showed this phase to be a new mineral species, which we describe herein and name isselite. The mineral and its name have been approved by the International Mineralogical Association (IMA) Commission on New Minerals, Nomenclature and Classification (CNMNC) (IMA2018-139, Biagioni et al., 2019b). Holotype material for isselite is deposited in the mineralogical collection of the Museo di Storia Naturale, Università di Pisa, Via Roma 79, Calci, Pisa, Italy, under catalogue number 19904, and of the Dipartimento di Scienze della Terra, dell'Ambiente e della Vita (DISTAV), Università di Genova, Corso Europa 26, Genova, Italy, under catalogue number MO484. Cotype material is preserved in the collections of the Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA, catalogue number 67195. The new mineral is dedicated to Arturo Issel (1842-1922 for his pioneering work on the geology, palaeontology and mineralogy of Italy, with a special reference to Liguria (Issel, 1892). He was professor of Geology and Mineralogy at the University of Genoa beginning in 1866 and Director of the Institute of Geology and the Geological Museum until 1917. Arturo Issel was the author of important scientific monographs on a variety of topics (not only geology and palaeontology, but also anthropology and ethnology) and contributed to several geological maps of Liguria.
Isselite was identified in only a few specimens as sprays of blue acicular crystals, striated parallel to [100] and tabular on {001}, up to 0.1 mm long (Fig. 1). Observed forms are {001}, {010}, {103} and {10 3}. The streak is light blue. Isselite is vitreous and transparent. The mineral is brittle, with an irregular fracture and good cleavage on {001} and {100}. Hardness was not measured, owing to the small crystal size. The density measured by sinkfloat in methylene iodinetoluene is 3.00(2) g/cm 3 , compared with a density of 2.946 g/cm 3 , calculated on the basis of the empirical formula and the unit-cell volume refined from singlecrystal X-ray diffraction data at room temperature.
Isselite is a late-stage, secondary mineral that crystallised from a low-temperature, aqueous solution. In holotype material, it is associated closely with brochantite and posnjakite. Other associated minerals are pyrrhotite, pentlandite, allophane, chrysocolla and langite. The source of the Cu in isselite may be valleriite, along with minor chalcopyrite, covellite and digenite. These sulfides are widespread in the Monte Ramazzo-Lagoscuro ore deposit, though not in close association with isselite in the samples studied.

Chemical and spectroscopic analyses
Preliminary energy-dispersive spectroscopy (EDS) chemical analyses showed that isselite contains Cu, Ni, Co and S as the only elements with Z > 8. Quantitative chemical analyses were performed using a CAMECA SX50 electron microprobe (Istituto di Geologia Ambientale e Geoingegneria, CNR, Rome) operating in wave-length dispersive mode. Operating conditions were: accelerating voltage 15 kV, beam current 15 nA, and beam diameter 5 μm. Standards (element, emission line) were: periclase (MgKα), baryte (SKα), synthetic Co (CoKα), synthetic Ni (NiKα), synthetic Cu (CuKα) and synthetic Zn (ZnKα). Iron and Pb were sought but found to be below detection limits. The small amount of available material precluded the determination of H 2 O by direct measurement; consequently, the H 2 O content was inferred from the crystal structure analysis. Its presence is confirmed by Raman spectroscopy (see below).  Micro-Raman spectra were obtained on an unpolished sample of isselite in nearly back-scattered geometry with a Jobin-Yvon Horiba XploRA Plus apparatus, equipped with a motorised x-y stage and an Olympus BX41 microscope with a 10× objective (Dipartimento di Scienze della Terra, Università di Pisa). The 532 nm line of a solid-state laser was used. The minimum lateral and depth resolution was set to a few μm. The system was calibrated using the 520.6 cm -1 Raman band of silicon before each experimental session. Spectra were collected through multiple acquisitions with single counting times of 60 s. Back-scattered radiation was analysed with a 1200 gr/mm grating monochromator. The Raman spectrum of isselite is shown in Fig. 2. The strongest band occurs at 984 cm -1 and can be related to the symmetrical stretching ν 1 of the SO 4 group. The bands at 386, 440 and 495 cm -1 can be interpreted as due to the symmetrical bending ν 2 of the SO 4 group, whereas the weak band at 630 cm -1 could be related to the antisymmetrical bending ν 4 . Finally, the band at 250 cm -1 may be attributed to lattice modes. The broad band at 3484 cm -1 can be attributed to the O-H stretching modes. No evident bending of H-O-H bonds, in the typical range between 1595-1700 cm -1 (e.g. Kolesov,  2006), was observed. The interpretation of the spectral bands is in agreement with Frost et al. (2004).

Crystallography
The powder X-ray diffraction pattern of isselite was obtained using a 114.6 mm diameter Gandolfi camera, with Ni-filtered CuKα radiation. The observed powder X-ray diffraction pattern is compared with the calculated one (obtained using the software PowderCell; Kraus and Nolze, 1996) in Table 2. Unit-cell parameters, refined from the powder data on the basis of 22 unequivocally indexed reflections through the software UnitCell (Holland and Redfern, 1997), are a = 6.789(1), b = 5.923(1), c = 20.651(3) Å, V = 830.4(2) Å 3 and Z = 2. The single-crystal X-ray diffraction study was carried out at the XRD1 beamline, ELETTRA synchrotron facility (Lausi et al., 2015). A monochromatic wavelength of 0.59040 Å was used on a 50 μm × 50 μm beam size, using a Dectris Pilatus 2M hybrid pixel area detector at a distance of 85 mm. A total of 500 frames was collected using w scan mode in Δw = 0.5°slices, with an exposure time of 1 s per frame. The diffraction data, collected at room temperature, were indexed, integrated, scaled, and corrected for the Lorentz-polarisation factor using the software XDS (Kabsch, 2010). The refined unit-cell parameters are a = 6.8070(14), b = 5.8970(12), c = 20.653(4) Å and V = 829.0(3) Å 3 . The a:b:c ratio calculated from unit-cell parameters is 1.1543:1:3.5023.
The statistical tests on the distribution of |E| values (|E 2 -1| = 0.703) suggest the acentric nature of isselite. The examination of the systematic absences indicated the possible space groups P2 1 nm and Pmn2 1 . Only the choice of this latter space group allowed the solution of the crystal structure through direct methods using Shelxs-97 (Sheldrick, 1997). After having located the heavier elements, the structure was completed through successive difference-Fourier maps; owing to the quality of available material, H atoms were not located. After several cycles of anisotropic refinement, performed using Shelxl-2018 (Sheldrick, 2015) and using neutral scattering curves taken from the International Tables for Crystallography (Wilson, 1992), the refinement converged to R 1 = 0.0669 for 2964 unique reflections with F o > 4σ(F o ) and 140 refined parameters. The occurrence of high electron density residuals (up to 6.19 e -/Å 3 ) is related to the less-than-ideal quality of the available crystals of isselite. An attenuation of the residuals can be obtained by truncating the intensity data to low angular values. For instance, the truncation of the dataset to 30°in 2θ results in a highest residual of 2.3 e -/Å 3 . However, the ratio between observed reflections and least-square parameters significantly decreases (from ∼21 to ∼ 4.5). An attempt at truncating the dataset at 2θ = 50°resulted in a slightly better R value (0.0625 for 2269 reflections) and slightly lower residuals (up to 5.4 e -/Å 3 ). However, the estimated standard uncertainties on bond lengths slightly increased. For this reason, the full dataset is here presented, with the awareness of the non ideal quality of the structure refinement (in particular as regards the high residuals) but supported by the crystal-chemical soundness of the proposed structural model.
Details of the data collection and crystal-structure refinement are given in Table 3. Atom coordinates and equivalent isotropic   (6) 0.037 (2) Wyk. = Wyckoff position Table 5. Selected bond distances (in Å) for isselite.

General organisation
The crystal structure of isselite (Fig. 3) includes five independent Cu sites, one S site, and fourteen O positions. It can be described as formed by {001} zig-zag layers of CuΦ 4-6 polyhedra, where Φ = (OH or H 2 O). Sulfate groups occur in the interlayer, together with one H 2 O group. No H positions were located from the structure determination; the assignments of OH and H 2 O groups were based on the bond-valence analysis (see Table 6).

Cation coordination and site occupancies
In isselite, the Cu sites display different kinds of coordination (Fig. 4). The Cu(2) and Cu(3) sites show the typically distorted (4 + 2) octahedral coordination of Cu 2+ , related to the Jahn-Teller effect (e.g. Burns and Hawthorne, 1996). The average value of the four shorter distances is 2.013 Å for both sites, somewhat longer than the equatorial bond length of 1.97 Å reported by Eby and Hawthorne (1993). The two apical Cu-O distances have average values of 2.237 and 2.197 Å at the Cu(2) and Cu(3) sites, respectively, shorter than the average value of 2.44 Å reported by Eby and Hawthorne (1993). The smaller than typical octahedral distortions observed for the Cu(2) and Cu(3) sites suggest that minor Ni and Co detected in isselite are located at these two sites. Taking into account the nature of the ligands, Cu(2) and Cu (3)  A square-planar coordination is shown by the Cu(4) site, having four ligands at an average distance of 1.944 Å, in agreement with the ideal mean value of 1.94 Å for square-planar Cu 2+ (Eby and Hawthorne, 1993). Taking into account the two very long distances Cu(4)-Ow(12) and Cu(4)-Ow(13), ≈2.9 Å, a very distorted octahedron Cu(OH) 4 (H 2 O) 2 can be identified, instead of a square-planar group Cu(OH) 4 .
Bond-valence sums (BVS) at the Cu sites range between 1.98 and 2.13 valence units (vu), in agreement with the occurrence of Cu in the divalent state. The higher values occur at the Cu(2) and Cu(3) sites and can be related to the admixture of minor Ni 2+ and very small amounts of Co 2+ , Zn 2+ and Mg 2+ .
The SO 4 group is hosted in the interlayer and is bonded to the {001} Cu layers through hydrogen bonds, as revealed by the occurrence of several underbonded O in the bond-valence calculation (Table 6) and the occurrence of short O⋅⋅⋅O distances ( Table 7). The BVS at the S site is 6.48 vu, considerably greater than expected for S 6+ . In this regard, it should be noted that the position of O(11) seems to be affected by disorder, as indicated by a U eq value larger than those of other O atoms belonging to the SO 4 group.

Hydrogen bonds
The short O⋅⋅⋅O distances are given in Table 7, confirming the importance of H bonds in the crystal structure of isselite, in agreement with the undersaturation of all O atoms. Taking into account the results of BVS given in  hosted at O(9) is an acceptor of two H bonds from two symmetry-related (OH)groups hosted at OH(5); the oxygen atom at O(10) is an acceptor of H bonds from two symmetry related (OH)groups hosted at OH (6) and from an H 2 O group occurring at the Ow (14) site. Finally, O(11) is involved in a more complex H-bond system. Indeed, O(11) is an acceptor of H bonds from OH(4) and it is at H-bond distances with Ow(7), Ow(8), and Ow (14). In addition, Ow (14) is an acceptor of H bonds from two symmetry-related OH(3) groups, whereas a still more complex situation involves the H 2 O groups hosted at Ow (7) and Ow(8). These two H 2 O groups are bonded to the Cu atoms hosted at Cu(3) and Cu(2) sites, respectively. Both are at short O⋅⋅⋅O distances with O(11). In addition, the distance Ow(7)⋅⋅⋅Ow(8), 2.85 Å, suggests another H-bond contact. Owing to the lack of knowledge about the H positions and the occurrence of different possible configurations, the actual H-bond system involving these atoms has to be considered only speculative and it will not be detailed here. As a matter of fact, the possibility of different configurations involving the oxygen atom hosted at O(11) could be the reason for its relatively high U eq value. Its position could be an average position between different H-bond configurations. Moreover, this uncertainty could affect the BVS of S, as discussed briefly above.
Oxygen atoms hosted at OH(4) and OH(6) are both donor (in the H bonds described above) and acceptor from H 2 O groups hosted at Ow(12) and Ow(13), respectively. It is worth noting that Ow(12) and Ow(13) act as donor to two symmetry-related OH(4) and OH(6), respectively, whereas they are acceptors of H bonds from OH(2) and OH(1), respectively.

Isselite and relations with other Cu sulfates
Fifteen different mineral species are currently known in the system CuO-SO 3 -H 2 O (Table 8). From a chemical point of view, the most simple species are those having the chemical formula Cu(SO 4 )⋅nH 2 O (0 < n < 7): chalcocyanite (n = 0), poitevinite (n = 1), bonattite (n = 3), chalcanthite (n = 5) and boothite (n = 7). Their crystal structures are reviewed by Eby and Hawthorne (1993). Chalcocyanite typically occurs in fumarolic environments. Indeed, it was first described from Vesuvius, Italy, by Scacchi (1873), along with another anhydrous copper oxy-sulfate, dolerophanite, Cu 2 O(SO 4 ). The other hydrated copper sulfates listed above are usually related to the weathering of Cu ores, although in some case they can be found also as sublimates in fumaroles (e.g. Balassone et al., 2019).
The remaining species, commonly associated with weathered Cu ores (e.g. Zittlau et al., 2013), are represented by the two basic copper sulfates antlerite and brochantite and seven hydrated basic copper sulfates. Isselite belongs to this latter group of copper sulfates, showing the presence of (OH)and H 2 O groups. In particular, it is chemically related to redgillite, Cu 6 (SO 4 )   (11) 3.07(2) 0.12 *Calculated using the relationships of Ferraris and Ivaldi (1988).

Nickel partitioning in isselite
After the first identification of isselite on the type specimen from the Lagoscuro mine, this mineral was identified, through powder X-ray diffraction, also on a sample from the Monte Ramazzo mine, as thin flattened acicular crystals associated with brochantite. Only EDS chemical data are available for this new occurrence; however, these data clearly indicated the occurrence of Ni, whereas Co and other elements are below the detection   5. Comparison between the crystal structure of isselite (a) and those of montetrisaite (b) and redgillite (c). The colour scheme is the same as in Fig. 3. limit. The Ni/(Ni + Cu) atomic ratio is close to 0.19, similar to the value observed in the type material, where the Ni/(Mg + Co + Ni + Cu + Zn) atomic ratio is 0.16. In both cases, the ideal composition is close to NiCu 5 (SO 4 )(OH) 10 ⋅5H 2 O. The similar scattering factors of Ni (Z = 28) and Cu (Z = 29) precluded the reliable description of the distribution of Ni and Cu atoms among different cation sites. Nickel and Cu are ordered in some minerals. Gillardite and the related mineral paratacamite-(Ni), both ideally Cu 3 Ni(OH) 6 Cl 2 , are atacamite-group minerals where Ni is partitioned in the octahedrally coordinated sites not showing the Jahn-Teller effect, whereas Cu 2+ is hosted at the distorted octahedrally coordinated sites (Clissold et al., 2007;Sciberras et al., 2013). Similarly, Ni 2+ is preferentially partitioned in the less distorted octahedrally-coordinated Me2 site of the crystal structure of glaukosphaerite, (Cu,Ni) 2 (CO 3 )(OH) 2 , whereas the Me1 site shows the typical 4 + 2 distorted octahedral coordination of Cu 2+ (Perchiazzi and Merlino, 2006). Another example is represented by hloušekite. This mineral, having the ideal formula NiCu 4 (AsO 4 ) 2 (AsO 3 OH) 2 (H 2 O) 9 , is a member of the lindackerite supergroup and Ni is hosted at a mixed (Ni,Co) site not showing the typical distortion of Cu 2+ -centred polyhedra (Plášil et al., 2014).
In isselite, the Cu(2) and Cu (3) sites have octahedral coordinations, showing the typical (4 + 2) distortion due to the Jahn-Teller effect for Cu 2+ . As discussed above, these sites have slightly longer equatorial bonds and shorter apical distances with respect to those reported by Eby and Hawthorne (1993) for a typical Cu 2+ -centred octahedron. This could suggest the substitution of Cu 2+ by Ni 2+ at these two positions. However, as there is no certainty about the quantification of Cu-Ni site distribution, for classification purposes the Cu(2) and Cu(3) sites could be merged, as occurs in the nomenclature of other minerals (e.g. M1 + M2 + M3 in amphibole; Hawthorne et al., 2012). In accord with the IMA-CNMNC guidelines , the end-member formula of isselite can be considered Ni-free. However, the actual role of Ni in stabilising this phase deserves further study.

Conclusion
Isselite is a new secondary Cu mineral showing a novel crystal structure and a complex hydrogen-bond system. Its Ni content, observed at both of its occurrences, opens the question about the role of this element in favouring the crystallisation of this new mineral species. It is likely that isselite could be more common than thought as an alteration product of Cu-Ni ores.
Along with ramazzoite (Kampf et al., 2018), isselite is the second new mineral discovered in the assemblages from the Monte Ramazzo mining complex, stressing the necessity for further investigation of this locality. In particular, a deep understanding of the geochemical relations between the Cu-Fe-Ni-Co primary mineralisation and their secondary alteration products would shed light on the possible role of hydrothermal fluids in Ni vs. Co selective mobilisation and enrichment as well as on the degree of hydrothermal alteration of the Monte-Ramazzo-Lagoscuro ore deposit.