Nielsbohrite, ideally K(UO2)3(AsO4)(OH)4·H2O, is a new mineral species belonging to the secondary mineral assemblage of the uranium deposit of Menzenschwand in the southern Black Forest, Baden-Württemberg, Germany. It is associated with hematite, pyrite, schoepite, metazeunerite and the gangue minerals quartz and barite. Nielsbohrite occurs mainly in euhedral rhombohedron-like crystals reaching 0.15 mm in size, often altered, and showing the forms {001}, {100} or {010}, and {210} or {120}. The mineral is yellow with a vitreous to dull lustre and with a yellowish streak. It is non-fluorescent in ultraviolet light, brittle, and shows an irregular fracture without any visible cleavage. The estimated Mohs hardness is about two, and the calculated density is 5.45–5.65 g/cm3. Nielsbohrite is biaxial negative, with nα = 1.756(2), nβ = 1.764(2), and nγ = 1.765(2). No pleochroism was observed. The measured 2V angle is 35(5)°, and the calculated 2V angle is 39°, orientation Y = c. A strong r > v dispersion has been observed, with anomalous blue-green and reddish brown interference colours. Electron-microprobe analysis gives K2O 1.90, UO3 83.34, As2O5 10.64, H2O* 5.07, total 100.95 wt% (*: calculated value). The resulting empirical formula, calculated on the basis of 10 O + 4 OH structural anions, is (K0.430□0.570)(AsO4)0.987(UO2)3.106(OH)4·1.00H2O. The single-crystal unit-cell parameters are a = 8.193(3), b = 11.430(4), c = 13.500(5) Å, and V = 1264.1(8) Å3, with Z = 4, space group Cccm. The strongest lines of the powder X-ray diffraction pattern [d(in Å)(I)(hkl)] are: 6.71(80)(110), 6.03(100)(111), 3.78(70)(113), 3.33(80)(220), 2.96(60)(024), 2.63(50)(204), 1.942(50)(244). The original crystal structure of nielsbohrite is related to those of minerals of the phosphuranylite group, and was refined to a R1 factor of 11.04 %. The mineral species and its name were approved by the Commission on New Minerals, Nomenclature and Classifications of the International Mineralogical Association (# IMA 2002-045b).

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