Katerinopoulosite, a new picromerite-group mineral, was discovered in the Esperanza mine, Lavrion District, Attikí Prefecture, Greece. The new mineral occurs in the oxidation zone of a sphalerite-rich orebody in association with chalcanthite, nickelboussingaultite, ammoniojarosite, aurichalcite, and goethite. Katerinopoulosite forms white, pale blue or pale green vermiform polycrystalline aggregates, in fact, anthodite crusts with separate anthodites up to 3-cm long and up to 5-mm thick. The lustre is vitreous, and the streak is white. Katerinopoulosite is brittle, has Mohs hardness of 2½ and an uneven fracture. Neither cleavage nor parting is observed. Dmeas = 1.97(2) g/cm3, Dcalc = 1.986 g/cm3. The new mineral is optically biaxial (+), with α = 1.492(2), β = 1.496(2), γ = 1.502(2) (589 nm). 2V (meas.) = 80(5)°, 2V (calc.) = 79°. The infrared spectrum shows the presence of ammonium cations, sulfate anions, and water molecules. The chemical composition is (EDS-mode electron microprobe for Ni, Cu, and Zn; gas chromatography of ignition products for H, N and S, wt%): SO3 38.33, (NH4)2O 11.9, NiO 1.82, CuO 0.37, ZnO 16.83, H2O 29.4, total 98.65. The empirical formula based on 2 S atoms per formula unit (apfu) is H0.13(NH4)1.91(Zn0.86Ni0.10Cu0.02)(SO4)2.00·6.75H2O. Katerinopoulosite is monoclinic, P21/a, a = 9.230(6), b = 12.476(4), c = 6.249(4) Å, β = 106.79(5)°, V = 688.9(9) Å3, and Z = 2. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 5.400 (37) (011), 4.411 (19) (200), 4.314 (19) (021), 4.229 (24) (12–1), 4.161 (100) (20–1, 210, 111), 3.749 (53) (130), 3.034 (29) (211, 11–2). The new mineral is named in honour of the prominent Greek geologist and mineralogist Prof. Dr. Athanassios Katerinopoulos. The type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia.

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