New Minerals from the Redmond Mine, North Carolina, USA: VIII. Dinilawiite, a Thiosulfate Mineral with an Open Framework Structure

Abstract The new mineral dinilawiite, [Pb4OAl(OH)6]2(S2O3)2·(S2O3)(H2O)5 (IMA2023-061), 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. Dinilawiite was found in vugs in massive galena-sphalerite-chalcopyrite-quartz in close association with hydroredmondite, steverustite, sulfatoredmondite, and susannite. Dinilawiite crystals are ubiquitously twinned tablets, up to about 0.4 mm across, with angled striations. Crystals always occur in tightly intergrown subparallel clusters. The mineral is colorless but sometimes appears light reddish brown due to inclusions or transmitted color of the matrix. The luster is vitreous. The mineral is brittle with curved fracture and no cleavage. The Mohs hardness is about 2. The calculated density is 4.727 g/cm3. The mineral is biaxial (+) with α = 1.805(5), β = 1.824 (calc), γ = 1.854 (calc) (white light). The 2V is 78(1)° with moderate r > v dispersion. The optical orientation is Z = b,Y^ c = 17° in the obtuse angle β and there is no pleochroism. Electron microprobe analyses provided the empirical formula [Pb4.00OAl1.00(OH)6]2(S2O3)2·(S2O3)0.60(SO4)0.28(OH)0.28(H2O)5.32. Dinilawiite is monoclinic, I2/a, a = 17.4100(5), b = 9.2191(2), c = 21.6980(15) Å, β = 107.276(8)°, V = 3325.5(3) Å3, and Z = 4. The crystal structure of dinilawiite (R1 = 0.0363 for 3190 I > 2σI), contains [Pb4OAl(OH)6]3+ chains formed by linkages of alternating OPb4 tetrahedra and Al(OH)6 octahedra. The chains are linked into an open framework by bonds to thiosulfate groups. Large channels within the framework contain H2O groups and a disordered mixed thiosulfate/sulfate group.