The new mineral species hydrokenopyrochlore, ideally A(□,#)2BNb2XO6·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, Sb2O5(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, H2Ocalc 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+0.35Na0.26Ca0.07)Σ2(Nb1.47Ta0.28W0.16Sb5+0.05Si0.04)Σ2.00O6[(H2O)0.55Cs0.45]. 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 Fd3m, with a = 10.4887(8) Å, V = 1153.9(3) Å3, Z = 8. The crystal structure has been solved and refined to R1 = 0.056 on the basis of 105 unique reflections with Fo > 4σ(Fo) 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.

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