Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Anthony R. Kampf, Anatoly V. Kasatkin, Jiří Čejka, Joe Marty

Plášilite, Na(UO₂)(SO₄)(OH)•2H₂O, a new uranyl sulfate mineral from the Blue Lizard mine, San Juan County, Utah, USA

Journal of Geosciences, volume 60 (2015), issue 1, 1 - 10

DOI: http://doi.org/10.3190/jgeosci.184

Plášilite (IMA 2014-021), Na(UO₂)(SO₄)(OH)•2H₂O, is a new uranyl sulfate mineral from the Blue Lizard mine, San Juan County, Utah, USA. The new mineral occurs in and on sandstone matrix in close association with atacamite, blödite, brochantite, calcite, chalcanthite, dickite, gerhardtite, gypsum, hexahydrite, johannite, manganoblödite, natrozippeite and tamarugite. It is a low-temperature, secondary mineral formed by the post-mining weathering of uraninite. Plášilite is monoclinic, with the space group P2₁/c, and unit cell parameters a = 8.7122(6), b = 13.8368(4), c = 7.0465(2) Å, β = 112.126(8)°, V = 786.89(7) ų and Z = 4. Crystals are long, thin blades, elongated on [001] and flattened on {100}; rarely as prisms, also elongated on [001]. Crystals exhibit the forms {100}, {010} and {011}, and are commonly twinned on {100}. Plášilite is greenish yellow, has a white streak and fluoresces bluish white under both long-wave and short-wave UV. It is transparent with vitreous luster. The mineral has a Mohs hardness probably between 2 and 3, brittle tenacity, even fracture and two perfect cleavages, {010} and {001}. The calculated density based on the empirical formula is 3.726 g/cm³. The mineral is optically biaxial (+), with α = 1.556(1), β = 1.581(1) and γ = 1.608(1) (measured with white light). The measured 2V is 88(1)° and the calculated 2V is 89°. Dispersion is moderate, r < v. The mineral is pleochroic with X = nearly colourless, Y = very pale yellow, Z = pale yellow; X < Y < Z. The optical orientation is X = b, Y ^ c = 4° in obtuse β. The empirical formula of plášilite is Na_0.94(UO₂)(S_1.01O₄)(OH)(H₂O)₂ (based on 9 O apfu). Prominent features in the Raman spectrum include the symmetric stretching vibrations of the uranyl (UO₂²⁺) group and sulfate tetrahedra and the O-H stretching and bending vibrations of the H₂O molecules. The eight strongest powder X-ray diffraction lines are [d_obs Å(I )(hkl) ]: 6.90(100)(020), 5.85(99)(011,111), 4.024(57)(200,130), 3.492(82)(102,220,040), 3.136(40)(122), 2.690(25)(141,102,241,032), 2.618(34)(240,150,302), 1.9212(30)(mult.). The crystal structure of plášilite (R₁ = 0.019 for 1603 reflections with F_obs > 4σF) contains uranyl sulfate sheets of composition [(UO₂)₂(SO₄)₂(OH)₂]^2- parallel to (010). Between the sheets and linking them to one another are chains of edge-sharing NaO₂(H₂O)₄ octahedra parallel to [001]. The uranyl sulfate sheet is based on the phosphuranylite anion topology. The sheets in plášilite and deliensite are geometrical isomers.