Abstract

The new mineral species ichnusaite, Th(MoO4)2·3H2O, has been discovered in the Mo-Bi mineralization of Su Seinargiu, Sarroch, Cagliari, Sardinia, Italy. It occurs as colorless thin {100} tabular crystals, up to 200 μm in length, associated with muscovite, xenotime-(Y), and nuragheite, Th(MoO4)2·H2O. Luster is pearly adamantine. Ichnusaite is brittle, with a perfect {100} cleavage. Owing to the very small quantity of available material and its intimate association with nuragheite, density and optical properties could not be measured. Electron microprobe analysis gave (mean of 4 spot analyses in wt%): MoO3 47.86(1.43), ThO2 43.40(79), total 91.26(87). On the basis of 8 O atoms per formula unit and assuming 3 H2O groups, in agreement with the crystal structure data, the chemical formula of ichnusaite is Th0.99Mo2.01O8·3H2O. Main diffraction lines, corresponding to multiple hkl indices, are [d(Å), relative visual intensity]: 5.66 (m), 3.930 (m), 3.479 (s), 3.257 (s), 3.074 (m). Ichnusaite is monoclinic, space group P21/c, with a = 9.6797(12), b = 10.3771(13), c = 9.3782(12) Å, β = 90.00(1)°, V = 942.0(2) Å3, Z = 4. The crystal structure has been solved and refined to a final R1 = 0.051 on the basis of 2008 observed reflections [with Fo > 4σ(Fo)]. It consists of electroneutral [Th(MoO4)2(H2O)2]0 (100) sheets of polymerized ThO7(H2O)2 and MoO4 polyhedra; successive sheets, stacked along [100], are connected through hydrogen bonds. Ichnusaite brings new understanding about the crystal chemistry of actinide molybdates, that may form during the alteration of spent nuclear fuel and influence the release of radionuclides under repository conditions.

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