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NIOBOAESCHYNITE-(Y), A NEW MEMBER OF THE AESCHYNITE GROUP FROM THE BEAR LAKE DIGGINGS, HALIBURTON COUNTY, ONTARIO, CANADA

Vladimir Bermanec¹, Nenad Tomai^1,, Goran Kniewald², Malcolm E. Back³ and Georg Zagler⁴

¹ Institute of Mineralogy and Petrography, Faculty of Science, University of Zagreb, Horvatovac bb, HR–10000 Zagreb, Croatia
² Rudjer Bokovi Institute, Center for Marine and Environmental Research, POB 180, HR–10002 Zagreb, Croatia
³ Royal Ontario Museum, Department of Natural History, 100 Queen’s Park, Toronto, Ontario, M5S 2C6, Canada
⁴ Institute of Mineralogy, University of Salzburg, Hellbrunnerstr. 34, A–5020 Salzburg, Austria

E-mail address: vberman{at}public.carnet.hr

Nioboaeschynite-(Y), with ideal chemical formula [(Y,Ln),Ca,Th](Nb,Ta,Ti,Fe)₂(O,OH)₆, is a new member of the aeschynite group. It was discovered at the Bear Lake Diggings, Lot 9, concession X, Monmouth Township, Haliburton County, near Gooderham, Ontario, Canada. It occurs as subhedral crystals up to 1 cm in size, in association with apatite, amphibole, feldspar, biotite, calcite, quartz, monazite, pyrite, and uranian thorite. The mineral is translucent, deep brownish red with a grayish brown streak, with a vitreous luster. It is brittle, with a Mohs hardness of 5–6, and its measured microhardness VHN₁₀₀ is 922. It shows no discernable cleavage and has a conchoidal fracture. The reflectance values for the COM wavelengths [%, R_oil, R_air] are 3.4, 14.6 (470 nm), 3.3, 14.1 (546 nm), 3.2, 13.8 (589 nm), and 3.2, 13.7 (650 nm). It is dark brown in plane-polarized light. Pleochroism, bireflectance and anisotropy are absent. It is naturally metamict; after heating at 1000°C, it recrystallizes to an orthorhombic structure, space group Pbnm, with a 5.279(3), b 10.966 (5), c 7.443(3) Å, V 430.9(3) ų, Z = 4. The strongest eight X-ray powder-diffraction lines [d in Å(I)(hkl)] are: 3.009(100)(130), 2.931(69)(112), 3.079(20)(022), 1.580(16)(134), 1.863(14) (004), 2.783(12)(131), 2.636(12)(200) and 2.006(11)(222). An electron-microprobe analysis gave CaO 4.34, MnO 0.11, Fe₂O₃ 2.16, Y₂O₃ 5.34, La₂O₃ 0.84, Ce₂O₃ 4.50, Pr₂O₃ 0.65, Nd₂O₃ 4.47, Sm₂O₃ 1.21, Eu₂O₃ 0.10, Gd₂O₃ 0.91, Dy₂O₃ 0.60, Er₂O₃ 0.42, Tm₂O₃ 0.05, Yb₂O₃ 0.57, ThO₂ 12.10, UO₂ 0.59, TiO₂ 18.41, Nb₂O₅ 31.46, Ta₂O₅ 3.97, H₂O 2.61, total 95.41 wt%. The H₂O content was determined by TGA. The empirical formula of nioboaeschynite-(Y), based on six atoms of oxygen (without H₂O) is [(Y_0.19Ln_0.34)Ca_0.31Th_0.18U_0.009Mn_0.006]_1.04(Nb_0.94Ti_0.92Ta_0.07Fe³⁺_0.11)_2.04O₆. D_meas. = 5.34 g/cm_3, D_calc. = 5.33 g/cm³. The crystal structure was not refined, but it is of the same type as for the other minerals of this group. The name of the new mineral species (IMA 2003–038a) recalls the dominant B-site cation present in this aeschynite-group mineral.

Keywords: nioboaeschynite-(Y), aeschynite group, new mineral species, Bear Lake diggings, Ontario, Canada.