Introducing the akrochordite mineral group, with the new mineral vargite from the Långban Mn-Fe deposit, Filipstad, Värmland, Sweden

Abstract

Vargite, ideally MnCu2Mn2(OH)4(H2O)4(AsO4)2–named after the Swedish miner Erik Gustaf Varg (1886–1970), who collected the type specimen–was found in the Långban Fe-Mn deposit. It occurs in open cavities in a brecciated and later hydrothermally leached carbonate groundmass, in association with hausmannite, calcite, rhodochrosite, baryte, a serpentine-group mineral, and galena. Additional minor phases are hedyphane, phlogopite and yarrowite. Paragenetically, it is a late-stage mineral, formed as a result of the interaction between an As-rich hydrothermal fluid and Mn-oxide(s) and Cu-sulphide, under low P- and T-conditions. Vargite forms bright green, semi-spherical aggregates up to 0.5 mm across, consisting of numerous thin, lath-shaped crystals, elongated along [100] and with a maximum length of 200 µm. Mohs hardness is ≈ 3 and Dcalc= 3.49(1) g·cm−3. The empirical chemical formula obtained from electron probe micro-analyses analyses and based on 16 anions is (Cu1.77Mg0.33)Σ2.10(Mn2.94Ca0.04Pb0.01)Σ2.99(As1.95Si0.02)Σ1.97O8(OH)4.03·3.98H2O. The crystal structures of vargite and the isotypic mineral akrochordite [MnMn2Mn2(OH)4(H2O)4(AsO4)2] have been refined in the space group P21/c from single-crystal X-ray diffraction data to R1 = 3.07% and 2.46%, respectively, giving the following sets of unit-cell parameters: a = 5.6251(14), 5.6832(11) Å, b = 17.452(5), 17.631(5) Å, c = 6.905(2), 6.8417(19) Å, β = 100.21(5)°, 99.51(4)°, and V = 667.2(3), 676.1(3) Å3, with Z = 2. A Raman spectrum of vargite, with major bands at 3510, 1610, 850, 780, 476, 428, 389, and 308 cm−1, strongly resembles that of isotypic guanacoite, [MgCu2Mg2(OH)4(H2O)4(AsO4)2]. Vargite, akrochordite, and guanacoite constitute the newly established akrochordite group.