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

Abstract

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) Å3 and Z = 4. Crystals are long, thin blades, elongated on [001] and flattened on {100}; rarely occur 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 4σ;F) 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.