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

Abstract

The new mineral markeyite (IMA2016-090), Ca 9 (UO 2 ) 4 (CO 3 ) 13 ·28H 2 O, was found in the Markey mine, San Juan County, Utah, USA, where it occurs as a secondary phase on asphaltum in association with calcite, gypsum and natrozippeite. The mineral is pale yellowish-green with white streak and fluoresces bright bluish white under a 405 nm laser. Crystals are transparent and have vitreous to pearly lustre. It is brittle, with Mohs hardness 1½ to 2, irregular fracture and three cleavages: perfect on {001}; good on {100} and {010}. The measured density is 2.68 g cm –3 . Crystals are blades, flattened on {001} and elongate on [010], exhibiting the forms {100}, {010}, {001}, {110}, {101}, {011} and {111}. Markeyite is optically biaxial (–) with α = 1.538(2), β = 1.542(2) and γ = 1.545(2) (white light); the measured 2V is 81(2)°; the dispersion is r Y ≈ Z ). Electron microprobe analyses (energy-dispersive spectroscopy mode) yielded CaO 18.60, UO 3 42.90, CO 2 21.30 (calc.) and H 2 O 18.78 (calc.), total 101.58 wt.% and the empirical formula Ca 8.91 (U 1.01 O 2 ) 4 (CO 3 ) 13 ·28H 2 O. The six strongest powder X-ray diffraction lines are [ d obs Å( I )( hkl )]: 10.12(69)(001), 6.41(91)(220,121), 5.43(100)(221), 5.07(33)(301,002,131), 4.104(37)(401,141) and 3.984(34)(222). Markeyite is orthorhombic, Pmmn , a = 17.9688(13), b = 18.4705(6), c = 10.1136(4) Å, V = 3356.6(3) Å 3 and Z = 2. The structure of markeyite ( R 1 = 0.0435 for 3427 F o > 4σ F ) contains uranyl tricarbonate clusters (UTC) that are linked by Ca–O polyhedra forming thick corrugated heteropolyhedral layers. Included within the layers is an additional disordered CO 3 group linking the Ca–O polyhedra. The layers are linked to one another and to interlayer H 2 O groups only via hydrogen bonds. The structure bears some similarities to that of liebigite.