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GJERDINGENITE-Na AND GJERDINGENITE-Ca, TWO NEW MINERAL SPECIES OF THE LABUNTSOVITE GROUP

Igor V. Pekov^1,, Nikita V. Chukanov², Natalia A. Yamnova³, Aleksandr E. Zadov⁴ and Peter Tarassoff⁵

¹ Department of Mineralogy, Faculty of Geology, Moscow State University, Vorobyovy Gory, 119899 Moscow, Russia
² Institute of Problems of Chemical Physics, 142432 Chernogolovka, Moscow Oblast, Russia
³ Department of Crystallography, Faculty of Geology, Moscow State University, Vorobyovy Gory, 119899 Moscow, Russia
⁴ NPO Regenerator, 3^rd Passage of Mar’ina Roshcha 40, 127018 Moscow, Russia
⁵ Redpath Museum, McGill University, 859 Sherbrooke Street West, Montreal, Québec H3A 2K6, Canada

E-mail address: igorpekov{at}mail.ru

Gjerdingenite-Na, (K,Na)₂Na(Nb,Ti)₄(Si₄O₁₂)₂(OH,O)₄·5H₂O, and gjerdingenite-Ca, K₂Ca(Nb,Ti)₄(Si₄O₁₂)₂(O,OH)₄·6H₂O, are two new mineral species of the labuntsovite group, kuzmenkoite subgroup, and analogs of gjerdingenite-Fe and gjerdingenite-Mn with dominant Na and Ca at the D site, respectively. They are alteration products of vuonnemite formed in hydrothermal assemblages of peralkaline pegmatites. Gjerdingenite-Na occurs at Mont Saint-Hilaire, Quebec, Canada, with microcline, albite, aegirine, analcime, a eudialyte-group mineral, natrolite, epistolite and polylithionite as a pseudomorph (up to 12 cm) after vuonnemite crystals, and as equant, prismatic or tabular crystals up to 0.2 x 0.3 x 0.5 mm. Gjerdingenite-Ca occurs at Mount Karnasurt, Lovozero massif, Kola Peninsula, Russia, as a fine-grained pseudomorph (up to 0.5 x 6 x 10 mm) after lamellar crystals of vuonnemite and as divergent crystals, elongate along [010], up to 0.2 x 0.3 x 2 mm, with microcline, natrolite, albite, aegirine, organovaite-Mn, organovaite-Zn, beryllite, epididymite and yofortierite. Both new minerals are transparent in tiny grains and translucent to opaque in aggregates, with a vitreous luster and a white streak. Gjerdingenite-Na is colorless to pale pink, whitish pink or cream colored. Gjerdingenite-Ca is white or pale brown to pinkish brown. Both minerals are brittle, with no observed cleavage and an uneven fracture. The Mohs hardness is 5. For gjerdingenite-Na, D_meas = 2.71(1), D_calc = 2.69; for gjerdingenite-Ca, D_meas = 2.79(1), D_calc = 2.775 g/cm³. The IR spectra are similar to those of other members of the kuzmenkoite subgroup. Both minerals are optically biaxial positive. For gjerdingenite-Na: 1.647(2), ß 1.653(2), 1.755(3), 2V_meas = 25(10)°, 2V_calc = 28.5°; for gjerdingenite-Ca: 1.680(1), ß 1.682(2), 1.762(3), 2V_meas = 25(10)°, 2V_calc = 19°. For both minerals, the optical orientation is Y = b. The chemical composition (electron microprobe, H₂O by TGA) of gjerdingenite-Na is Na₂O 4.04, K₂O 3.97, CaO 1.95, BaO 0.92, MnO 0.27, ZnO 0.17, Fe₂O₃ 0.61, Al₂O₃ 0.20, SiO₂ 41.02, TiO₂ 10.20, Nb₂O₅ 27.78, H₂O 9.85, sum 100.98 wt.%; that of gjerdingenite-Ca is Na₂O 1.14, K₂O 3.61, CaO 3.56, SrO 3.47, BaO 1.04, MnO 0.84, ZnO 0.05, Fe₂O₃ 0.19, Al₂O₃ 0.13, SiO₂ 39.29, TiO₂ 9.96, Nb₂O₅ 27.34, H₂O 9.23, sum 99.85 wt.%. The empirical formulae, based on [(Si,Al)₈O₂₄](O,OH)₄, are: for gjerdingenite-Na: (K_0.98Na_0.62Ca_0.37Ba_0.07)_2.04 (Na_0.90Ca_0.04Mn_0.04Zn_0.02)_1.00 (Nb_2.43Ti_1.49Fe_{3+ 0.09})_4.01 (Si_7.95Al_0.05)₈O₂₄[(OH)_2.09O_1.91]₄·5.32H₂O; for gjerdingenite-Ca: (K_0.93Na_0.45Sr_0.41Ca_0.15Ba_0.08)_{2.0 2} (Ca_0.62Mn_0.14Fe_0.03Zn_0.01)_0.80 (Nb_2.51Ti_1.52)_4.03 (Si_7.97Al_0.03)₈O₂₄[O_2.86(OH)_1.14]₄·5.67H₂O. Both minerals are monoclinic, C2/m, Z = 2. Unit-cell dimensions for gjerdingenite-Na are a 14.626(2), b 14.160(1), c 7.910(1) Å, ß 117.43(2)°, V 1454(1) ų, and for gjerdingenite-Ca: a 14.6365(6), b 14.2049(5), c 7.8919(4) Å, ß 117.467(5)°, V 1455.9(2) ų. The strongest reflections in the X-ray powder-diffraction pattern for gjerdingenite-Na [d(in Å)(I)(hkl)] are: 7.102(29)(020), 7.044(54)(001), 6.510(42)(200), 4.995(44)(02,021), 3.252(51)(42), 3.249(100)(400), 3.148(28)(02,022), and for gjerdingenite-Ca: 7.100(100)(020), 6.999(88)(001), 6.476(38)(20), 4.985(78)(02,021), 3.252(42)(42), 3.246(43)(400), 3.167(46)(04,041), 3.140(36)(02,022). The crystal structures of both minerals were refined by the Rietveld method using X-ray powder-diffraction data, yielding R_p = 3.41, R_B = 1.38, R_F = 0.96 for gjerdingenite-Na, and R_p = 2.74, R_B = 3.88, R_F = 2.29 for gjerdingenite-Ca. Both minerals are isotypic with other members of the kuzmenkoite subgroup.

Keywords: gjerdingenite-Na, gjerdingenite-Ca, new mineral species, labuntsovite group, kuzmenkoite subgroup, infrared spectroscopy, peralkaline pegmatites, Mont Saint-Hilaire, Quebec, Canada, Lovozero massif, Kola Peninsula, Russia.