Paramarkeyite, a new calcium–uranyl–carbonate mineral from the Markey mine, San Juan County, Utah, USA

Abstract

The new mineral paramarkeyite (IMA2020-024), Ca 2 (UO 2 )(CO 3 ) 3 •5H 2 O, was found in the Markey mine, San Juan County, Utah, USA, where it occurs as a secondary phase on 2 gypsum-coated asphaltum in association with andersonite, calcite, gypsum and natromarkeyite. Paramarkeyite crystals are transparent, pale green-yellow, striated tablets, up to 0.11 mm across. The mineral has white streak and vitreous lustre. It exhibits moderate bluish white fluorescence (405 nm laser). It is very brittle with irregular, curved fracture and a Mohs hardness of 2½. It has an excellent {100} cleavage and probably two good cleavages on {010} and {001}. The measured density is 2.91(2) g cm -3 . Optically, the mineral is biaxial (-) with α = 1.550(2), β = 1.556(2), γ = 1.558(2) (white light); 2V = 60(2)°; strong r > v dispersion; orientation: Y = b; nonpleochroic. The Raman spectrum exhibits bands consistent with UO 2 2+ , CO 3 2and O-H. Electron microprobe analysis provided the empirical formula (Ca 1.83 Na 0.20 Sr 0.03 ) ∑2.05 (UO 2 )(CO 3 ) 3 •5H 2 O (+0.07 H). Paramarkeyite is monoclinic, P2 1 /n, a = 17.9507(7), b = 18.1030(8), c = 18.3688(13) Å, β = 108.029(8)°, V = 5676.1(6) Å 3 and Z = 16. The structure of paramarkeyite (R 1 = 0.0647 for 6657 I > 2I) contains uranyl tricarbonate clusters that are linked by Ca-O polyhedra to form heteropolyhedral layers. The structure of paramarkeyite is very similar to those of markeyite, natromarkeyite and pseudomarkeyite.