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

Abstract

ABSTRACTThe new mineral markeyite (IMA2016-090), Ca9(UO2)4(CO3)13·28H2O, 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 isr<v(weak); the optical orientation isX=c,Y=b,Z=a; and pleochroism isX= light greenish yellow,YandZ= light yellow (X>Y≈Z). Electron microprobe analyses (energy-dispersive spectroscopy mode) yielded CaO 18.60, UO342.90, CO221.30 (calc.) and H2O 18.78 (calc.), total 101.58 wt.% and the empirical formula Ca8.91(U1.01O2)4(CO3)13·28H2O. The six strongest powder X-ray diffraction lines are [dobsÅ(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) Å3andZ= 2. The structure of markeyite (R1= 0.0435 for 3427Fo> 4σF) 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 CO3group linking the Ca–O polyhedra. The layers are linked to one another and to interlayer H2O groups only via hydrogen bonds. The structure bears some similarities to that of liebigite.