Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Anthony R. Kampf, Jiří Sejkora, Thomas Witzke, Jakub Plášil, Jiří Čejka, Barbara P. Nash, Joe Marty

Rietveldite, Fe(UO₂)(SO₄)₂(H₂O)₅, a new uranyl sulfate mineral from Giveaway-Simplot mine (Utah, USA), Willi Agatz mine (Saxony, Germany) and Jáchymov (Czech Republic)

Journal of Geosciences, volume 62 (2017), issue 2, 107 - 120

DOI: http://doi.org/10.3190/jgeosci.236

Rietveldite (IMA2016-081), Fe(UO₂)(SO₄)₂·5H₂O, is a new uranyl sulfate mineral described from three localities: Giveaway-Simplot mine (Utah, USA), Willi Agatz mine (Saxony, Germany) and Jáchymov (Western Bohemia, Czech Republic). The mineral rarely occurs in blades up to 0.5 mm long, in association with other post-mining supergene uranyl sulfates and U-free sulfates. Rietveldite is orthorhombic, space group Pmn2₁, a = 12.9577(9), b = 8.3183(3), c = 11.2971(5) Å, V = 1217.7(1) ų and Z = 4. Thin blades are elongated on [001] and flattened on {010}. Rietveldite is brownish yellow; powdery aggregates have yellowish beige color; and it has a white streak. It does not exhibit fluorescence under either long- or short-wave UV. It is transparent to translucent with a vitreous luster. Crystals are brittle, with curved fracture and Mohs hardness ~2. Cleavage is good on {010}, and fair on {100} and {001}. Rietveldite is easily soluble in room-temperature H₂O. The density is 3.31 g/cm³. Rietveldite is optically biaxial (+), with α = 1.570(1), β = 1.577(1) and γ = 1.586(1) (white light); 2V_calc. = 83.3°, 2V_meas. = 82(1)°. Dispersion is very strong (r > v). Rietveldite exhibits barely noticeable pleochroism in shades of light brownish yellow color, Y < X ≈ Z. The optical orientation is X = b, Y = a, Z = c. Chemical analyses for rietveldite from Giveaway-Simplot (WDS, 4 spots on 2 crystals) provided FeO 9.56, ZnO 1.06, MgO 0.14, MnO 0.10, SO₃ 26.99, UO₃ 47.32, H₂O (calc.) 15.39, total 100.56 wt. %, which yields the empirical formula (Fe_0.79Zn_0.08Mg_0.02Mn_0.01)_Σ0.90(UO₂)_0.99(SO₄)_2.01·5.10H₂O (based on 15 O apfu). Prominent features in the Raman and infrared spectra include the O-H stretching vibrations, symmetric and antisymmetric stretching vibrations of (UO₂)²⁺ ion, and stretching and bending vibrations of symmetrically non-equivalent (SO₄)^2- groups. The eight strongest powder X-ray diffraction lines are [d_obs Å (I_rel.) (hkl)]: 8.309(34)(010), 6.477(100)(200), 5.110(58)(210), 4.668(48)(012), 4.653(36)(211), 3.428(41)(013), 3.341(33)(221), 3.238(49)(400). The crystal structure of rietveldite (R₁ = 0.037 for 2396 reflections with I_obs > 2σ[I]) contains infinite uranyl sulfate chains of composition [(UO₂)(SO₄)₂(H₂O)]^2- along [001]. The adjacent chains are linked in the [100] direction by FeO₆ octahedra, which share vertices with SO₄ tetrahedra resulting in a heteropolyhedral sheet parallel to {010}; adjacent sheets are linked by hydrogen bonding only. The uranyl sulfate chains are the same as those in the structures of several other uranyl sulfate minerals. Rietveldite is named for Hugo M. Rietveld (1932-2016).