Abstract
Nioboaeschynite-(Y), with ideal chemical formula [(Y, Ln), Ca, Th](Nb, Ta, Ti, Fe)2O6, is a new member of the aeschynite group. It was discovered at the Bear Lake Diggings, Lot 9, conc. 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 grayish-brown streak, with a vitreous luster. It is brittle with a Mohs hardness of 5-6, and its measured micro-hardness VHN100 is 922. It shows no discernable cleavage and has conchoidal fracture. Reflectance values for the COM wavelengths [%, Roil, Rair]: 470 nm (3.4, 14.6), 546 nm (3.3, 14.1), 589 nm (3.2, 13.8), and 650 nm (3.2, 13.7). Microscopic color is dark brown. Pleochroism, bireflectance and anisotropy are absent. It is naturally metamict, and after heating at 1000°C it re-crystallizes orthorhombically, space group Pbnm, with a=5.279(3) A , b=10.966 (5) A , c=7.443(3) A , V=430.9(3) A 3, Z=4. The eight strongest X-ray powder diffraction lines for nioboaeschynite-(Y) [d in A (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). The 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 %. Water content was determined by TGA. The empirical formula of nioboaeschynite-(Y), based on 6 oxygen atoms, (and without H2O) is [(Y0.19Ln0.34)Ca0.31Th0.18U0.009Mn0.006] 1.04(Nb0.94Ti0.92Ta0.07Fe3+0.11) 2.04O6. Dmeas. = 5.34 g/cm3, Dcalc. = 5.33 g/cm3. Crystal structure is not refined, but it is the same type as for the other minerals of this group. The new mineral is named after the dominant cations. Nioboaeschynite-(Y) was approved by the IMA Commission on New Minerals and Mineral Names (2003-038a).
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