ABSTRACT

     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: α 1.647(2), β 1.653(2), γ 1.755(3), 2V_meas = 25(10)°, 2V_calc = 28.5°; for gjerdingenite-Ca: α 1.680(1), β 1.682(2), γ 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:

sum 100.98 wt.%

     that of gjerdingenite-Ca is:

     The empirical formulae, based on [(Si,Al)₈O₂₄](O,OH)₄, are: for


gjerdingenite-Na: (K_0.98Na_0.62Ca_0.37Ba_0.07)_Σ2.04


(Na_0.90Ca_0.04Mn_0.04Zn_0.02)_Σ1.00 (Nb_2.43Ti_1.49Fe³⁺_0.09)_Σ4.01


(Si_7.95Al_0.05)_Σ80₂₄[(OH)_2.09O_1.91]_Σ4 · 5.32H₂O;

     for gjerdingenite-Ca: (K_0.93Na_0.45Sr_0.41Ca_0.15Ba_0.08)_Σ2.02


(Ca_0.62Mn_0.14Fe_0.03Zn_0.01)_Σ0.80 (Nb_2.51Ti_1.52)_Σ4.03 (Si_7.97Al_0.03)_Σ8O₂₄


[O_2.86(OH)_1.14]_Σ4 · 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, β 117.43(2)°, V 1454(1) ų, and for gjerdingenite-Ca: a 14.6365(6), b 14.2049(5), c 7.8919(4) Å, β 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: