Niobian calciotantite and plumboan-stannoan cesstibtantite from the islandof Uto, Stockholm Archipelago, Sweden

Niobian calciotantite and plumboan-stannoan cesstibtantite are associated w ith albite, K-feldspar, quartz and pink Ca-enriched tourmaline in the pollu cite-bearing Grundberg outcrop of the northern Nykopingsgruvan pegmatite on the island of Uto, Stockholm Archipelago, south-central Sweden. Cesstibtan tite forms an overgrowth on subhedral, compositionally zoned grains of niob ian calciotantite; both phases are locally intergrown with skeletal mangano columbite, cassiterite and an unknown Ta-rich oxide phase. The maximum Nb c ontent of the (Na,Pb)-poor calciotantite is as high as 19.90 wt.% Nb2O5 (1. 263 apfu Nb, 31.6% of the B-sire population). The (Pb,Sn,Nb)-enriched cesst ibtantite averages at (Pb0.35Sb0.25Sn0.10Ca0.12Na0.12) (Ta1.40Nb0.55W0.05) (O-5.70[OH](0.30)) ([OH](0.66)Cs-0.30). Low-temperature hydrothermal fluids triggered a unique two-stage alteration of cesstibtantite, leaving the oth er minerals intact. Diffuse cation-exchange spreads from microfractures in cesstibtantite. Gradual loss of Cs, Pb, Sb and Sn is compensated by progres sive introduction of Ca, Na and Sr, and minor F substitutes for OH, but B-s ite cations are not affected. The resulting compositions attain (Ca0.32Na0. 25Sb0.25Sn0.03Pb0.20Sr0.02) (Ta1.40Nb0.55W0.05) (O-5.85[OH](0.15)) ([OH](0. 55)O-0.10 F0.10Cs0.15). Extremely thin rims of the microfractures, 1 to 4 m u m across, display much enhanced cation-exchange of the above type, combin ed with substantial fluorination, which generates secondary cesian microlit e (Ca0.80Na0.40Sb0.15Sr0.05Pb0.05Sn0.02) (Ta1.40Nb0.55W0.05) (O-5.90[OH](0. 10)) ([OH](0.36)O0.15F0.30Cs0.10).