Original paper
Hydrokenopyrochlore, (□,#)2Nb2O6·H2O, a new species of the pyrochlore supergroup from the Sahatany Pegmatite Field, Antananarivo Province, Madagascar
Biagioni, Cristian; Meisser, Nicolas; Nestola, Fabrizio; Pasero, Marco; Robyr, Martin; Roth, Philippe; Schnyder, Cédric; Gieré, Reto
European Journal of Mineralogy Volume 30 Number 4 (2018), p. 869 - 876
published: Oct 31, 2018
published online: May 9, 2018
manuscript accepted: Jan 31, 2018
manuscript revision received: Jan 30, 2018
manuscript received: Jan 16, 2018
Abstract
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 Fd 3‾ m, 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.
Keywords
hydrokenopyrochlore • new mineral species • pyrochlore supergroup • cesium; niobium • crystal structure • Sahatany Pegmatite Field • Madagascar