Wilancookite, ideally (Ba,K,Na)8(Ba,Li,□)6Be24P24O96·32H2O, is a new mineral species from the Lavra Ponte do Piauí complex granitic pegmatite, Minas Gerais, Brazil. It occurs as tiny dodecahedral {1 1 0} crystals, deposited on moraesite fibres. Associated primary minerals are albite, montebrasite, Li-bearing micas, cassiterite, elbaite and quartz, while the secondary phosphate association contains fluorapatite, childrenite, eosphorite, zanazziite, greifenstenite, guimarãesite, ushkovite, saléeite and moraesite. The mineral is transparent and colourless, with a vitreous lustre; it is non-fluorescent, brittle, and its streak is white. The estimated Mohs hardness is 4–5, and the calculated density is 3.05 g/cm3. Wilancookite is isotropic, colourless, non-pleochroic, with n = 1.560(2) (measured under λ = 590 nm). Electron- and ion-microprobe analyses give (in wt%): P2O5 36.19, SiO2 0.04, Al2O3 0.41, BaO 34.65, Na2O 0.09, K2O 0.32, BeO 12.86, Li2O 0.50, and H2Ocalc 12.31, total 97.37 wt%. The resulting empirical formula, calculated on the basis of 96 anhydrous oxygen atoms, is (Ba7.54K0.32Na0.14)Σ8.00(Ba3.04Li1.571.39)Σ6.00Be24.08(P23.88Al0.38Si0.03)Σ24.29O96·32H2O. The single-crystal unit-cell parameters are a = 13.5398(2) Å and V = 2482.21(7) Å, space group I23. The eight strongest lines in the powder X-ray diffraction pattern [d(in Å)(I)(hkl)] are: 6.90(60)(2 0 0), 5.54(80)(2 1 1), 3.630(60)(3 2 1, 3 1 2), 3.212(70)(3 3 0, 4 1 1), 3.043(100)(4 2 0, 4 0 2), 2.885(70)(3 3 2), 2.774(80)(4 2 2), and 2.398(60)(4 4 0). The crystal structure of wilancookite has been refined, based on single-crystal X-ray diffraction data, to R1 = 4.58%; the beryllophosphate framework is similar to that occurring in pahasapaite, and is based on zeolite-RHO cages. The mineral species and name were approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2015-034).

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