New Minerals from the Redmond Mine, North Carolina, USA: IX. Siligiite, a Lead-Zinc Sulfate with a Novel Heteropolyhedral Sheet

Abstract The new mineral siligiite, [Pb(H2O)5(SO4)][Zn9(OH)18] (IMA2023-117), was found in the Redmond mine, Haywood County, North Carolina, USA, where it occurs in a highly unusual secondary assemblage that comprises a variety of rare Pb-Zn-Cu sulfates, thiosulfates, and carbonates. Siligiite was found in vugs in massive galena-sphalerite-chalcopyrite-quartz in close association with anglesite, bechererite, cherokeeite, cuprocherokeeite, elyite, kennygayite, leadhillite, and muscovite. Siligiite crystals are blades or flattened needles, up to about 1 mm long, with lengthwise striations, occurring in radiating clusters. The mineral is colorless with vitreous to silky luster. The mineral is brittle with splintery fracture and two cleavages: perfect on {001} and good on {100}. The Mohs hardness is about 2½. The calculated density is 3.513 g/cm3. The mineral is biaxial (+) with α = 1.624(3), β = 1.650(3), and γ = 1.656(3) (white light). The 2V is 50(2)° with distinct r > v dispersion. The optical orientation is Y = b,X ≈ c*, Z ≈ a. Electron probe microanalyses provided the empirical formula [Pb0.99(H2O)5(SO4)][(Zn8.81Cu0.19)Σ9.00(O0.9989H)18.02]. Siligiite is monoclinic, P21/n, a = 13.658(3), b = 9.488(3), c = 19.209(4) Å, β = 102.577(6)°, V = 2429.4(1) Å3, and Z = 4. The crystal structure of siligiite (R1 = 0.0576 for 5448 I > 2σI), contains a novel sheet comprised of Zn(OH)6 octahedra and Zn(OH)4 tetrahedra. A Pb2+ cation, an SO42− group, and several H2O groups are located between successive sheets.