Gysinite-(Ce), PbCe(CO3)2(OH)(H2O), a New Member of the Ancylite Group

Abstract The new mineral gysinite-(Ce), PbCe(CO3)2(OH)(H2O) (IMA2023-035), was found at the Abendröthe Mine, St. Andreasberg, Braunlage, Goslar district, Lower Saxony, Germany, where it occurs in a single vug in association with albite, calcite, and mimetite. It is a secondary oxidation-zone mineral presumably crystallized from hydrothermal solutions. Gysinite-(Ce) occurs in intergrowths of equant pseudo-octahedral crystals, up to about 0.7 mm in maximum dimension. The color is brownish yellow, the streak is pale yellow, and the luster is vitreous to subadamantine. The mineral is brittle with curved, irregular fracture and no cleavage. The Mohs hardness is about 3½. The calculated density is 5.136 g/cm3. The mineral is biaxial (−) with α = 1.750(5), β = 1.825(5), γ = 1.860(5). The 2V is 65.1(4)° with no observable dispersion. The optical orientation is X = c; Y = a; Z = b by analogy with gysinite-(Nd) and there is no pleochroism. Electron probe microanalyses provided the empirical formula (Pb0.930Ce0.434La0.349Nd0.196Pr0.048Gd0.037Sm0.007)Σ2.000(CO3)2(OH)1.074(H2O)0.926. Gysinite-(Ce) is orthorhombic, Pmcn, a = 5.0780(4), b = 8.6689(6), c = 7.3255(6) Å, V = 322.47(4) Å3, and Z = 2. Gysinite-(Ce) is isostructural with gysinite-(La), gysinite-(Nd), and other members of the ancylite group. The crystal structure (R1 = 0.0268 for 396 I > 2σI) is a framework consisting of edge-sharing (PbCe)O8(OH)(H2O) polyhedra and CO3 triangles.