Cesiokenopyrochlore, The First Natural Niobate With An Inverse Pyrochlore Structure

Cesiokenopyrochlore is a new mineral belonging to the pyrochlore group. It was discovered in a specimen of granitic pegmatite collected at Tetezantsio, Betafo region, Madagascar. The mineral forms rough equant crystals up to 0.05 mm in size intergrown with behierite and rynersonite. Cesiokenopyrochlore is light-brown, translucent, with resinous luster. D-calc. = 5.984 g/cm(3). In reflected light it is light gray, isotropic, with strong light-brown internal reflections. The crystal structure was refined to R-1 =0.0212. The mineral is cubic, Fd (3) over barm, a = 10.444(1) angstrom, V= 1139.5(2) angstrom(3). and Z= 8. The strongest lines of the powder X-ray diffraction pattern [d, angstrom, (I, %) (hkl)] are: 6.03 (37) (111), 3.70 (9) (220), 3.15 (100) (311), 3.02 (36) (222), 2.012 (17) (511, 333), 1.848 (19) (440), 1.576 (11) (622). The chemical composition is (wt.%; electron microprobe): Cs2O 22.66, Na2O 1.74, CaO 0.64, Nb2O5 20.87, Ta2O5 21.27, WO3 30.67, H2O (calc) 0.12, total 97.97. The empirical formula of the holotype specimen calculated on the basis of (Nb+Ta+W) = 2 apfu and (O+OH) = 6 apfu and written according to the pyrochlore-supergroup nomenclature is Na0.29Ca0.06Nb0.81W0.69Ta0.50)(Sigma 2)[O-5.93(OH)(0.07)](Sigma 6)Cs-0.83. The simplified formula of the holotype specimen is rectangle(2)(Nb,W,Ta)(Sigma 2)O6Cs. Cesiokenopyrochlore is the first natural niobate to adopt the inverse pyrochlore structure.