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Magnesiovesuvianite (IMA 2015-104), ideally Ca19Mg(Al,Mg) 12Si18O69 (OH) 19, a new vesuvianite-group member, was found in an old museum specimen labelled as vesuvianite from the Tuydo combe, near Lojane, Republic of Macedonia. It occurs as radiating aggregates up to 2 cm across consisting of acicular tetragonal crystals (up to 7 mm long and 5-40 μm thick) with distinct fibre-optic effect. Associated minerals are calcite, grossular--andradite and clinochlore. Single crystals of magnesiovesuvianite are transparent, light pink with a silky lustre. Dominant crystal forms are {100}, {110}, {101} and {001}. The Mohs' hardness is 6. The measured and calculated densities are 3.30(3) and 3.35 g/cm³, respectively. Mineral is optically uniaxial, negative, ω = 1.725(2), ε = 1.721(2). The chemical composition (wt. % electron-microprobe data) is: SiO2 36.73, Al2O3 20.21, CaO 36.50, MgO 1.80, MnO 0.18, FeO 0.04, Na2O 0.01, H2O 3.10, total 98.37. The empirical formula based on 19 (Ca + Na) apfu is: (Ca18.99Na0.01)Σ19.00(Mg0.60Al0.40)Σ1.00(Al11.05Mg0.70Mn0.07Fe0.02)Σ11.84Si17.84O68.72(OH)9. Absorption bands in the IR spectrum are: 393, 412, 443, 492, 576, 606, 713, 802, 906, 968, 1024, 3200, 3450, 3630 Cm-1. The eight strongest lines of the powder X-ray diffraction pattern are (I-d(Å)-hkl): 23-10.96-110, 22-3.46-240, 33-3.038-510, 100-2.740-432, 21-2.583-522, 94-2.365-620, 19-2.192-710, 25-1.6165-672. Magnesiovesuvianite is tetragonal, space group P4/n, unit-cell parameters refined from the powder data are a 15.5026(3), c 11.7858(5), V 2832.4(2) Ų, Z = 2. The crystal structure has been refined to R1 = 0.027 for 3266 unique observed reflections with Fo ≥ 4σF. The structure refinements provided scattering factors of the Y1A,B sites close to 12 e- that supports predominant occupancy of these sites by the Mg2+ cations, in perfect agreement with the 27Al MAS NMR data. Magnesiovesuvianite is a member of the vesuvianite group with Mg2+ as a dominant cation at the Y1 site. The name magnesiovesuvianite is given to highlight the species-defining role of Mg. [ABSTRACT FROM AUTHOR]

Single crystals of two new uranyl selenates K3 (H3O)[(UO2)4 (SeO4)6 (H2O) 4]·5H2O (I) and K2.5(no)(NO3)0.5[(UO2)2(SeO4)3 (H2O)]·4H2O (II) have been prepared by room-temperature evaporation from aqueous solution of uranyl nitrate, selenic acid, potassium carbonate and (for the compound I) carbamide. The crystal structure of I has been solved by direct me-thods [monoclinic, P21/m, a = 12.001(3), b = 13.613(3), c = 13.753(3) Å, β = 109.187(4)°, V = 2122.0(8) ų= and Z = 2] and refined to R1 = 0.029 (wR2 = 0.084) for 4865 reflections with |Fo| ≥ 4σF using least-square methods. The crystal structure of II has been solved by direct methods [monoclinic, C2/c, a = 20.290(4), b = 10.380(2), c = 21.436(4) Å, β = 103.446(3)°, V= 4391.0( 13) ų and Z=4] and refined to R1 = 0.027 (wR2, = 0.066) for 7944 reflections with |Fo| ≥ 4σF using least-square techniques. The structures of I and II are based upon the [(UO2)2 (SeO4)3 (H2O) 2]2- and [(UO2)2 (SeO4)3 (H2O)]2-layers, respectively, consisting of UO7 pentagonal bipyramids sharing comers with SeO4 tetrahedra. Potassium cations induce curvature of the uranyl selenate layers, which is mediated by the interlayer water molecules, hydronium ions and nitrate groups. The topology of the 2D units in the structtire of I is novel for the structural chemistry of uranyl selenates. [ABSTRACT FROM AUTHOR]