Ichnusaite, Th(MoO4)2{middle dot}3H2O, the first natural thorium molybdate: Occurrence, description, and crystal structure

Abstract

The new mineral species ichnusaite, Th(MoO 4 ) 2 ·3H 2 O, 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(MoO 4 ) 2 ·H 2 O. 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%): MoO 3 47.86(1.43), ThO 2 43.40(79), total 91.26(87). On the basis of 8 O atoms per formula unit and assuming 3 H 2 O groups, in agreement with the crystal structure data, the chemical formula of ichnusaite is Th 0.99 Mo 2.01 O 8 ·3H 2 O. 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 P2 1 /c, with a = 9.6797(12), b = 10.3771(13), c = 9.3782(12) Å, β = 90.00(1)°, V = 942.0(2) Å 3 , Z = 4. The crystal structure has been solved and refined to a final R 1 = 0.051 on the basis of 2008 observed reflections [with F o > 4σ(F o )]. It consists of electroneutral [Th(MoO 4 ) 2 (H 2 O) 2 ] 0 (100) sheets of polymerized ThO 7 (H 2 O) 2 and MoO 4 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.