Nielsbohrite, a new potassium uranyl arsenate from the uranium deposit of Menzenschwand, southern Black Forest, Germany

Authors: Walenta, Kurt; Hatert, Frédéric; Theye, Thomas; Lissner, Falk; Röller, Klaus

Source: European Journal of Mineralogy, Volume 21, Number 2, March 2009 , pp. 515-520(6)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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Abstract:

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 (K_{0.430□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).

Keywords: NIELSBOHRITE; NEW MINERAL; OPTICAL PROPERTIES; INFRARED SPECTRUM; CRYSTAL STRUCTURE; POTASSIUM URANYL ARSENATE; BLACK FOREST; GERMANY

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/2009/0021-1913

Publication date: 2009-03-01

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