New Minerals from the Redmond Mine, North Carolina, USA: XI. Sigogglinite, a New Sulfate with the Novel Pb6Zn(OH)8 Dimer

Abstract The new sulfate mineral sigogglinite, (IMA2024-069), Pb6Zn(OH)82(SO4)6·(H2O)8–x, where 0 ≤ x ≤ 1, was discovered in material collected underground at the Redmond mine, Haywood County, North Carolina, USA, where it occurs as a very scarce component of a unique secondary assemblage comprising rare Pb-Zn-Cu sulfates, thiosulfates, and carbonates. Sigogglinite occurs in small vugs in massive galena-sphalerite-chalcopyrite-quartz veins in association with cherokeeite, redmondite, steverustite, and sulfatoredmondite. Two monoclinic polytypoids are known (P2/n and P21/c) and exhibit distinct crystal habits and ideal hydration. Sigogglinite-P2/n forms small colorless pseudohexagonal crystals with characteristic 010 striations on its 001 faces, whereas the P21/c polytypoid is only known from one reticulated lathe-like crystal. The new mineral is brittle with good cleavage and a Mohs hardness of ca. 3. Raman spectroscopy and optics data are also provided. The empirical formula for the P2/n polytypoid determined by electron microprobe analysis is Pb6(Zn0.95Pb0.05)(OH)82(SO4)2.59(S2O3)0.412·7H2O. Sigogglinite-P2/n has cell parameters a = 14.8109(2), b = 9.1360(1), c = 17.3693(3) Å, β = 92.879(1)°, V = 2347.31(6) Å3, and Z = 2, whereas sigogglinite-P21/c has cell parameters a = 9.0260(2), b = 17.6478(3), c = 14.8685(3) Å, β = 95.164(1)°, V = 2358.78(8) Å3, and Z = 4. Both polytypoids have novel structures consisting of slabs of Pb6Zn(OH)8 dimers networked with SO4 groups and H2O through hydrogen bonds and weak Pb–O bonds. In the P21/c structure, slab rotation deconvolutes the disordered H2O site in P2/n sigogglinite into an ordered site with a multiplicity of two, leading to a marginally higher hydration for the P21/c structure (ideal formula Pb6Zn(OH)82(SO4)6·8H2O). Like many of the other new minerals discovered at the Redmond mine, sigogglinite incorporates structural elements derived from the Pb4(OH)4 “Pb-cubane” motif. The Pb6Zn(OH)8 clusters in sigogglinite represent the simplest case of the corner-sharing Pb-cubane motif and differ from topologically similar clusters in redmondite, hydroredmondite, and sulfatoredmondite, being fully hydroxylated and lacking apical Pb atoms.