Abstract
AbstractThe 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, withr>v. The optical orientation isX=b,Y=candZ=a. Isselite is pleochroic, withX= light blue,Y= blue,Z= blue;X<<Z<Y. Electron microprobe analyses give (wt.%): SO311.45(21), MgO 0.31(7), CoO 1.07(14), NiO 9.41(90), CuO 51.29(126), ZnO 1.10(20), H2Ocalc24.21, 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 groupPmn21, with unit-cell parametersa= 6.8070(14),b= 5.8970(12),c= 20.653(4) Å,V= 829.0(3) Å3andZ= 2. The crystal structure of isselite was refined from single-crystal X-ray diffraction data toR1= 0.067 on the basis of 2964 reflections withFo> 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.
Highlights
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 (Kampf et al, 2013; Kolitsch et al, 2018)
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, carbonates and silicates. 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)
Crystallographic studies and chemical analyses showed this phase to be a new mineral species, which we describe and name isselite
Summary
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 (Kampf et al, 2013; Kolitsch et al, 2018). Isselite occurs as a secondary phase in the Fe–Ni–Co sulfide mineralisation formerly exploited at the Lagoscuro mine (44°28′35’’N, 8°51′35′′E), Ceranesi, Genoa Province, Liguria, Italy This mine, which belongs to the Monte Ramazzo mining complex, operated from 1815 to 1825, mainly for magnesium sulfate (Pipino, 1977). 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. The short O⋅⋅⋅O distances are given, confirming the importance of H bonds in the crystal structure of isselite, in agreement with the undersaturation of all O atoms. 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
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