Hydrokenopyrochlore, (□,#)₂Nb₂O₆·H₂O, a new species of the pyrochlore supergroup from the Sahatany Pegmatite Field, Antananarivo Province, Madagascar

The new mineral species hydrokenopyrochlore, ideally ^A(□,#)₂^BNb₂^XO₆·^YH₂O, has been discovered in the Antandrokomby pegmatite, Sahatany Pegmatite Field, Antananarivo Province, Madagascar. It occurs as tan to beige subhedral crystals, up to 1 mm in size, with a resinous luster, associated with quartz, tourmaline, orthoclase, Li-bearing mica, hübnerite, stibiotantalite, and an undetermined heftetjernite-like mineral. Electron-microprobe analysis of hydrokenopyrochlore yielded (in wt%): WO₃ 8.14, Sb₂O_5(tot) 14.33, Nb₂O₅ 44.09, Ta₂O₅ 13.97, SiO₂ 0.51, SnO₂ 0.21, CaO 0.86, MnO 0.04, Na₂O 1.79, Cs₂O 14.47, H₂O_calc 2.23, total 100.64. On the basis of 2 B-site cations per formula unit and by recalculating the Sb₂O₃:Sb₂O₅ ratio in agreement with structural data, the chemical formula of hydrokenopyrochlore is (□_1.32Sb³⁺_0.35Na_0.26Ca_0.07)_Σ2(Nb_1.47Ta_0.28W_0.16Sb⁵⁺_0.05Si_0.04)_Σ2.00O₆[(H₂O)_0.55Cs_0.45]. The main diffraction lines, corresponding to multiple hkl indices, are [d in Å (visually estimated relative intensity) hkl]: 3.136 (s) 311, 3.006 (s) 222, 2.010 (ms) 333/511, 1.846 (s) 440, and 1.588 (ms) 622. The crystal structure study returned a cubic unit cell, space group Fd$3‾$m, with a = 10.4887(8) Å, V = 1153.9(3) ų, Z = 8. The crystal structure has been solved and refined to R₁ = 0.056 on the basis of 105 unique reflections with F_o > 4σ(F_o) and 14 refined parameters. The crystal structure is formed by a framework of BO₆ octahedra, as is typical of the members of the pyrochlore supergroup. The B site has a mixed (Nb, Ta, W) occupancy. The A site is mainly vacant, whereas the Y site is occupied by both H₂O groups and Cs. The high Cs content of hydrokenopyrochlore seems to be indicative of the central miarolitic and most evolved portions of the pegmatitic dikes in the Sahatany Pegmatite Field.