Sundiusite, Pb10(SO4Cl2O8, is a new mineral from Långban, Sweden. It is monoclinic, C2, Cm, or C2/m, with a = 24.67(1), b = 3.781(1), c = 11.881(5)Å, β = 100.07(4)°, and Z = 2. The strongest lines in the X-ray powder pattern are (Å,I, hkl) 2.981 10 510; 2.737 8 113; 3.101 6 602,603; 3.044 6 800,403; 6.10 3 400; 3.744 3 110. Sundiusite occurs as plumose aggregates of white to colorless crystals with an adamantine luster. The Mohs hardness is about 3, and there is a perfect {100} cleavage. Optically, it appears to be biaxial (+) with all indices greater than 2.10; lath-shaped fragments are length-slow. The observed and calculated densities are 7.0 and 7.20 g/cm3, respectively. The mineral does not fluoresce in ultraviolet radiation.

The composition, as determined by electron microprobe, is PbO 93.1, FeO 0.5, SO3 3.5, Cl 3.0, less O ≡ Cl 0.7, total 99.4 weight percent, which yields the ideal formula Pb10(SO4)Cl2O8. The composition and cell geometry suggest a structural relationship to the nadorite group. Sundiusite is known only from Làngban and is identical with Flink unknown #284. The name is for the late Nils Sundius.

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