Abstract
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, α 1.718(1), β 1.733(1), γcalc 1.750(1), 2 V meas = 87(2)°. It is triclinic, P 1, a 5.4303(9), b 11.924(2), c 11.747(2) A, α 112.927(3), β 94.750(3), γ 103.175(3)°, V 669.5(3) A3. 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)∑1.98Na0.95 (Mn5.40Zn0.73Fe0.53Na0.28Mg0.05)∑6.99(Nb1.33Zr0.40Ti0.24Ta0.04)∑2.01 (Si7.71Al0.32)∑8.03 O26(OH)4.00(O0.89F0.11)∑1.00 on the basis of 31 anions, or, ideally, K2Na(Mn,Zn,Fe)7(Nb,Zr,Ti)2Si8O26(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− ⇔ Nb5+ + O2−. Niobokupletskite is a postmagmatic phase and possibly the result of late-stage remobilization of Nb via late oxidized aqueous fluids.
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