Wilancookite, (Ba, K, Na)8 (Ba, Li, [])6 Be24 P24 O96·32H2O, a new beryllophosphate with a zeolite framework

Abstract

Wilancookite, ideally (Ba,K,Na) 8 (Ba,Li,□) 6 Be 24 P 24 O 96 ·32H 2 O, is a new mineral species from the Lavra Ponte do Piaui 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, guimaraesite, ushkovite, saleeite 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/cm 3 . Wilancookite is isotropic, colourless, non-pleochroic, with n = 1.560(2) (measured under λ = 590 nm). Electron- and ion-microprobe analyses give (in wt%): P 2 O 5 36.19, SiO 2 0.04, Al 2 O 3 0.41, BaO 34.65, Na 2 O 0.09, K 2 O 0.32, BeO 12.86, Li 2 O 0.50, and H 2 O calc 12.31, total 97.37 wt%. The resulting empirical formula, calculated on the basis of 96 anhydrous oxygen atoms, is (Ba 7.54 K 0.32 Na 0.14 ) Σ8.00 (Ba 3.04 Li 1.57 □ 1.39 ) Σ6.00 Be 24.08 (P 23.88 Al 0.38 Si 0.03 ) Σ24.29 O 96 ·32H 2 O. The single-crystal unit-cell parameters are a = 13.5398(2) A and V = 2482.21(7) A 3 , space group I 23. The eight strongest lines in the powder X-ray diffraction pattern [ d (in A)( 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 11), 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 R 1 = 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).