Línekite, K2Ca3[(UO2)(CO3)(3)](2) . 8H2O, a new uranyl carbonate mineral from Jáchymov, Czech Republic
Abstrakt
Linekite, K2Ca3[(UO2)(CO3)(3)](2) . 8H2O, is a new uranyl tricarbonate mineral from Jachymov, Western Bohemia, Czech Republic. It occurs in association with grimselite, andersonite, liebigite, cejkaite, schrockingerite, agricolaite, jezekite and braunerite.
Linekite forms from uranium-rich aqueous solutions and its origin is associated with post-mining processes. Linekite is orthorhombic, space group Pnnm, with a = 17.0069(5) A, b = 18.0273(5) A, c = 18.3374(5) A and V = 5622.1(2) A(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.
Linekite 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, alpha = 1.546(2), beta = 1.550, gamma = 1.562(2).
The 2V(obs) is moderate; the calculated 2V is + 60 degrees. Optical orientation: Y = a, X = b, Z = c.
The electron microprobe analyses (average of 28) provided: Na2O 0.06, K2O 6.89, CaO 14.11, CuO 0.12, UO3 48.76, CO2 * 22.51, H2O* 12.20 (similar to 12.9 from TG) (*calculated), total 104.65 wt%. The empirical formula (based on 30.22 O apfu) is: (K1.73Na0.02)(Sigma 1.75)(Ca2.97Cu0.02)(Sigma 2.99)[(UO2)(CO3)(3)](2.02)(H2O)(8.00).
The Raman and infrared spectra exhibit prominent features consistent with the mineral being a hydrated uranyl tricarbonate, with fundamental vibrations of H2O molecules, CO(3)(2-)anions and UO22+ ions. The seven strongest powder X-ray diffraction lines are [d(obs) in angstrom (hkl) I-rel]: 8.627 (200) 100, 6.436 (022) 60, 5.935 (212) 11, 5.153 (222) 43, 4.592 (004) 19, 4.505 (040) 12 and 4.053 (204) 15.
The structure of linekite was refined from single-crystal X-ray data to R = 0.034 for 4468 unique observed reflections (I-obs > 3 sigma I). The structure consists of prominent (Ca(H2O)(2)[(UO2)(CO3)(3)])(2-) layers parallel to (100), which define a square grid, leading to a strong tetragonal pseudosymmetry of linekite.
Between the layers, disordered K+ cations and H2O 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[(UO2)(CO3)(3)](4) Ca.