Oldsite, K₂Fe²⁺[(UO₂)(SO₄)₂]₂(H₂O)₈, a new uranyl sulfate mineral from Utah, USA: its description and implications for the formation and occurrences of uranyl sulfate minerals

Abstract

Oldsite (IMA2021-075), ideally K₂Fe²⁺[(UO₂)(SO₄)₂]₂(H₂O)₈, is a new uranyl sulfate mineral found on specimens from the North Mesa Mine group, Temple Mountain, San Rafael district, Emery County, Utah, USA. It is a secondary mineral occurring with alum-(K), halotrichite, metavoltine, quartz, römerite, stanleyite, sulphur, szomolnokite and mathesiusite. It forms rectangular blades flattened on {010} and elongated on [001], reaching ~0.3 mm in length. Crystals are yellow in colour, transparent with a vitreous lustre; the streak is very pale yellow. The mineral is non-fluorescent. Cleavage is excellent on {100} and perfect on {010}; the Mohs hardness is ~2. Crystals are brittle with irregular, splintery fracture. The density measured by flotation in a mixture of methylene iodide and toluene is 3.31 g⋅cm⁻³; the calculated density is 3.298 g⋅cm⁻³ for the empirical formula and 3.330 g⋅cm⁻³ for the ideal formula. Oldsite is biaxial (+), with α = 1.552(2), β = 1.556(2) and γ = 1.588(2) (measured in white light). The 2V measured directly on a spindle stage is 37(1)°; the calculated 2V is 39.6°. Dispersion is r < v, moderate. The optical orientation is X = b, Y = a and Z = c. The mineral is non-pleochroic. The empirical formula of oldsite (on the basis of 28 O apfu) is K₁.₉₃(Fe²⁺₀.₅₃Zn₀.₃₁V³⁺₀.₀₉Mg₀.₀₈)_(Σ1.02)[(U₀.₉₈O₂)(S₁.₀₁O₄)₂]₂(H₂O)₈. The Raman spectrum is dominated by the vibrations of SO₄²⁻ and UO₂2²⁺ units. Oldsite is orthorhombic, Pmn2₁, a = 12.893(3), b = 8.276(2), c = 11.239(2) Å, V = 1199.2(5) ų and Z = 2. The five strongest powder X-ray diffraction lines are [d_(obs), Å (I, %) (hkl) ]: 8.29 (59) (010), 6.47 (82) (200), 5.10 (62) (210), 4.65 (100) (012, 211) and 3.332 (55) (022, 221). The crystal structure of oldsite was refined from single-crystal X-ray data to R = 0.0258 for 2676 independent observed reflections, with I_(obs) > 3σ(I). Oldsite is an Fe²⁺ analogue of svornostite; its crystal structure is based upon infinite chains of uranyl-sulfate polyhedra, which comprises pentagonal UO₇ bipyramids sharing four of their equatorial vertices with sulfate tetrahedra such that each tetrahedron is linked to two uranyl bipyramids to form an infinite chain (the free, non-linking equatorial vertex of the uranyl bipyramid is an H₂O group). The broader discussion on the origin and composition of uranyl sulfate minerals is made. The new mineral name honours American mineralogist, Dr. Travis A. Olds for his contribution to uranium mineralogy.

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