Fehrite, Mgcu4(So4)(2)(Oh)Center Dot 6h(2)O, The Magnesium Analogue Of Ktenasite From The Casualidad Mine Near Banos De Alhamilla, Almeria, Spain

The new mineral fehrite (IMA 2018-125a), MgCu4(SO4)(2)(OH)(6)center dot 6H(2)O, is a member of the ktenasite group and the Mg-analogue of ktenasite, ZnCu4(SO4)(2)(OH)(6)center dot 6H(2)O. The mineral was found in the Casualidad mine near Banos de Alhamilla, Almeria, Spain, in association with clinoatacamite, kapellasite, gordaite, serpierite, connellite and gypsum. The transparent turquoise-coloured mineral has a vitreous lustre, exhibits a pale blue-green streak and shows distinct pleochroism. It forms radial aggregates of thin lath-like crystals of up to 200 gm in length. Fehrite is not fluorescent. The monoclinic crystals show a perfect cleavage parallel to (001). The mineral has a brittle tenacity and an uneven fracture. The calculated density is 2.73 g/cm(3), the calculated mean refractive index is 1.584. The strongest lines observed in the X-ray powder diffraction pattern are [d in angstrom/I-rel in %/(hkt)] 11.94/100/002, 5.92/31/004, 2.66/12/202, 4.85/11/013, 3.93/11/006 and 2.96/10/008. The chemical composition, measured by means of an electron probe micro-analyser, was determined at (wt.%): MgO 5.31, MnO 0.49, CuO 33.12, ZnO 11.48, SO3 26.01, H2Ocalc. 24.63, total 101.04. The empirical formula based on 20 O pfu., including 6(OH) and 6(H2O), is Mg0.87Cu2.74Zn0.93Mn0.05S2.14O8(OH)(6)center dot 6H(2)O. The simplified end member formula is MgCu4(SO4)(2)(OH)(6)center dot 6H(2)O which requires MgO 5.92, CuO 46.74, SO3 23.52, H2O 23.82, total 100.00 (wt.%). Fehrite is monoclinic with space group P21/c (#14). Unit cell parameters determined by X-ray single crystal diffraction are a = 5.6062(8), b= 6.1294(11), c = 23.834(3) angstrom, beta= 95.29(1)degrees, V= 815.5(2) angstrom(3), Z= 2. The mineral is isotypic with ktenasite with Mg in place of Zn. The name is for the late Karl Thomas Fehr (1954-2014), Professor of Mineralogy at the Depart-ment of Geo- and Environmental Sciences at the Ludwig-Maximilians-University Munich, Germany.