Línekite, K2Ca3[(UO2)(CO3)3]2.8H2O, a new uranyl carbonate mineral from Jáchymov, Czech Republic

Abstract

Línekite, K 2 Ca 3 [(UO 2 )(CO 3 ) 3 ] 2 .8H 2 O, is a new uranyl tricarbonate mineral from Jáchymov, Western Bohemia, Czech Republic. It occurs in association with grimselite, andersonite, liebigite, čejkaite, schröckingerite, agricolaite, ježekite and braunerite. Línekite forms from uranium-rich aqueous solutions and its origin is associated with post-mining processes. Línekite is orthorhombic, space group Pnnm, with a = 17.0069(5) Å, b = 18.0273(5) Å, c = 18.3374(5) Å and V = 5622.1(2) Å 3 , and Z = 8. It forms tabular, mostly isometric crystals, up to c. 0.5 mm across, typically in multiple intergrowths. The color is pale olive to khaki green and it has a greenish white to yellowish white streak. Crystals are transparent and have vitreous luster. The Mohs hardness is estimated to be between 2 and 3. Línekite is brittle with an uneven fracture and perfect cleavage on {100} and very good cleavage on {010}. It exhibits intense greenish yellow luminescence under both short-(254 nm) and long-wave (366 nm) UV radiation. The calculated density is 2.922 g/cm 3 . The mineral is biaxial (+) with indices of refraction, α = 1.546(2), β = 1.550, γ = 1.562(2). The 2V obs is moderate; the calculated 2V is +60°. Optical orientation: 12 and 4.053 (204) 15. The structure of línekite was refined from single-crystal X-ray data to R = 0.034 for 4468 unique observed reflections (I obs > 3σI). The structure consists of prominent (Ca(H 2 O) 2 [(UO 2 )(CO 3 ) 3 ]) 2-layers parallel to (100), which define a square grid, leading to a strong tetragonal pseudosymmetry of línekite. Between the layers, disordered K + cations and H 2 O molecules are localized. The structure is closely related to other uranyl tricarbonate minerals, e.g., albrechtschraufite and andersonite, due to the presence of a very characteristic paddle-wheel motif, Ca[(UO 2 )(CO 3 ) 3 ] 4 Ca.

Structure Refinement and Thermal Stability Studies of the Uranyl Carbonate Mineral Andersonite, Na2Ca[(UO2)(CO3)3]·(5+x)H2O

“…Uranyl carbonates are important phases in the geo-ecology of uranium deposits and nuclear facilities, including radioactive waste disposal sites. Uranyl carbonate minerals readily form during alteration of primary uranium minerals (e.g., pitchblende or uraninite) under the influence of water enriched with CO 2 , which may be derived from the dissolution of carbonate rocks or from the atmosphere [1][2][3][4][5][6]. Uranyl carbonates may play an important role in the transport of uranium in ground waters.…”

Paddlewheelite, a New Uranyl Carbonate from the Jáchymov District, Bohemia, Czech Republic

“…Paddlewheelite contains major Mg, Ca, Cu, and U, with minor Si and Fe. No other elements were detected, and the presence of H 2 O and CO 3 2− was confirmed by infrared spectroscopy.…”

“…This close similarity likely results in the strong Two paddles (two UTC units) from each wheel form the planarity of the sheet, and 7-coordinated Ca3, Ca4, Ca5, and Ca6 atoms connect paddlewheels together into an open-topology type sheet. These same Ca atoms bond to three H 2 O groups, and share one edge with a CO 3 2− triangle of one UTC unit, and one edge with a second UTC unit. Atoms Ca7, Ca8, Ca9, and Ca10 are seven-or eight-coordinated and each share one edge of a paddle that extends between sheets, and also bonds to five or six H 2 O groups.…”

“…Recent uranyl carbonate mineral discoveries have uncovered remarkably complex topological arrangements that formed under restrictive conditions of Ca-U-CO 3 equilibria [1][2][3]. In part, these new minerals lend their complexity to hydrated calcium polyhedra found in their structures, which can share numerous geometrical elements with uranyl tricarbonate units (UTC), (UO 2 )(CO 3 ) 3 4− , to form multidimensional structures.…”

“…Recent uranyl carbonate mineral discoveries have uncovered remarkably complex topological arrangements that formed under restrictive conditions of Ca-U-CO 3 equilibria [1][2][3]. In part, these new minerals lend their complexity to hydrated calcium polyhedra found in their structures, which can share numerous geometrical elements with uranyl tricarbonate units (UTC), (UO 2 )(CO 3 ) 3 4− , to form multidimensional structures. For example, UTC units in andersonite and liebigite form finite cluster topologies of "zero" dimensionality, as defined by the structural hierarchy for uranyl minerals [4,5], where one or more uranyl polyhedra link to oxyanions and share no other connections to high valence cations.…”