Niobokupletskite, a new astrophyllite-group mineral from Mont Saint-Hilaire, Quebec, Canada: Description and crystal structure

Niobokupletskite is a new member of the astrophyllite group found in nepheline syenite pegmatites at Mont Saint-Hilaire, Quebec, in association with aegirine, albite, analcime, calcio-ancylite-(Ce), calcite, catapleiite, epididymite, fluorite, genthelvite, microcline, natrolite, pyrochlore, rhodochrosite and wurtzite. It is transparent, with a vitreous luster, brittle with a perfect {001} cleavage, and has an uneven to splintery fracture. Niobokupletskite is biaxial positive, alpha 1.718(1), beta 1.733(1), gamma(calc) 1.750(1), 2V(meas) = 87(2)degrees. It is triclinic, P (1) over bar, a 5.4303(9), b 11.924(2), c 11.747(2) Angstrom, alpha 112.927(3), beta 94.750(3), gamma 103.175(3)degrees, V 669.5(3) Angstrom(3). The average result of three electron-microprobe analyses is: Na2O 2.62, K2O 5.97, Rb2O 0.82, Cs2O 0.12, MgO 0.15, MnO 26.37, FeO 2.64, ZnO 4.08, Al2O3 1.14, TiO2 1.34, ZrO2 3.43, Nb2O5 12.13, Ta2O5 0.63, SiO2 31.85, F 0.14, H2O(calc.) 2.48, total 95.85 wt.%, corresponding to (K1.84Rb0.13Cs0.01)(Sigma 1.98)Na-0.95 (Mn5.40Zn0.73Fe0.53Na0.28Mg0.05)(Sigma 6.99) (Nb1.33Zr0.40Ti0.24Ta0.04)(Sigma 2.01) (Si7.71Al0.32)(Sigma 8.03) O-26(OH)(4.00)(O0.89F0.11)(Sigma 1.00) on the basis of 31 anions, or, ideally, K2Na(Mn,Zn,Fe)(7)(Nb,Zr,Ti)(2)Si8O26(OH)(4)(O,F). Niobokupletskite is a heterophyllosilicate with a HOH-structure consisting of a sheet of close-packed octahedra (O-sheet) sandwiched between two heterogeneous sheets (H-sheets) with a Si:Nb ratio of 4:1. It forms a solid-solution series with kupletskite via the coupled substitution Ti4+ + F- or OH- double left right arrow Nb5+ + O2-. Niobokupletskite is a postmagmatic phase and possibly the result of late-stage remobilization of Nb via late oxidized aqueous fluids.