Structural and compositional characterization of fehrite from Les Ferreres Mine (Ripollès, Catalonia)

Fehrite (MgCu₄(SO₄)₂(OH)₆·6H₂O) from the Casualidad mine (Sierra de Alhamilla, Almería, Spain) was described in 2021 as a new member of the ktenasite group. This secondary sulphate had been previously found in the Les Ferreres mine (Rocabruna, Ripollès, Catalonia, Spain) in 2012 and was reported by amateur collectors as a potential new mineral. The available samples, however, still lack a formal characterization. The aim of the present work is to investigate the fehrite from Les Farreres, which contains light green to bluish-green aggregates of very elongated tabular crystals of the mineral, typically forming parallel to radial acicular clusters. It occurs in association with brochantite, devilline, gypsum, and minor connellite, among others. SEM–EDS, Raman spectroscopy, and X-ray diffraction (XRD) data confirm that these crystals correspond to fehrite with the empirical formula Mg₀.₇₇Cu₃.₃₉Zn₀.₆₁S₂.₀₈O₈(OH)₆·nH₂O on the basis of 20 O atoms per formula unit, consistent with Mg dominance at one of the cation sites, Cu at the two other sites, and Zn in substitution of both Mg and Cu cations. The refined monoclinic unit-cell parameters (s.g. P2₁/c) are a = 5.626 Å, b = 6.137 Å, c = 23.861 Å, β = 95.21°, and V = 820.5 ų, which closely match the published data for the type material. The paragenesis of fehrite from Les Ferreres is also discussed and compared to that of the type locality.