New Minerals from the Redmond Mine, North Carolina, USA: X. Blueridgeite, a New Example of the Lead Cubane Chain Topology

The new mineral blueridgeite, [Pb8Zn3Cu2+(OH)16](SO4)2(S2O3)2·2H2O (IMA2024-071), was found underground in the Redmond mine, Haywood County, North Carolina, USA, where it occurs in a highly unusual secondary assemblage consisting of rare Pb-Zn-Cu sulfates, thiosulfates, and carbonates. Blueridgeite occurs in small dissolution voids in massive galena-sphalerite-chalcopyrite-quartz veins and may be associated with a variety of other secondary minerals such as cuprocherokeeite, elyite, sidpietersite, steverustite, susannite, and a potentially new Pb-Cu sulfate (‘Redmond UK8’). Blueridgeite forms compact submillimetric spheres of pale to deep blue, translucent crystals with rhombohedral terminations. The empirical formula determined by electron microprobe analysis is [Pb8(Zn2.60Cu1.25Pb0.15)(OH)16](SO4)2.38(S2O3)1.62·2H2O. Blueridgeite is monoclinic, P21/c, with cell dimensions a = 12.1530(7), b = 14.4758(7), c = 11.0034(6) Å, β = 116.212(3)°, V = 1736.70(17) Å3, and Z = 2. Blueridgeite is closely related to cuprocherokeeite. These two minerals share the same [Pb8(Zn3Cu2+)(OH)16] chain motif wherein cubane-like [Pb2(Zn,Cu2+)2(OH)4] units share Zn and Cu2+ corners to form infinite chains along a. Chains are linked laterally through a complex network of hydrogen bonds between chain hydroxyl and inter-chain sulfate, thiosulfate, and water, as well as weak Pb–O and Pb–S bonds to sulfate and thiosulfate. Sulfate and thiosulfate are ordered into distinct crystallographic positions with thiosulfate groups ordered alongside Cu2+ chain links in the bc plane.