Xenotime-(Yb), YbPO4, a new mineral species from the Shatford Lake pegmatite group, southeastern Manitoba, Canada

Xenotime-(Yb), ideally YbPO4, occurs as a primary phase in the saccharoidal albite unit of a granitic pegmatite located near the margin of the Lac du Bonnet batholith in the Shatford Lake pegmatite group, southeastern Manitoba, Canada. Xenotime-(Yb) occurs as granular aggregates less than or equal to 50 mu m across and as isolated small (similar to 20 mu m) grains encased in clots of ferrian muscovite, which are scattered throughout the saccharoidal albite unit. It is also associated with ferrocolumbite, albite, microcline and quartz. Xenotime-(ro) is transparent, colorless to slightly yellowish or brownish with a white streak, vitreous luster and no observed fluorescence Cleavage and parting were not observed, tenacity is brittle, and the fracture is uneven. Xenotime-(Yb) has no observable pleochroism and is uniaxial positive with omega = 1,715(3), epsilon = 1.802(5). It is tetragonal, space group I4(1)/amd, with a 6.866(2) Angstrom, c 6.004(3) Angstrom, V 283.0(2) Angstrom(3) and Z = 4. The strongest lines of the (Gandolfi) X-ray-diffraction pattern [d in Angstrom(I)(hkl)] are: 4.52(7)(011), 3.44(10)(020), 2.73(3)(121), 2.56(8)(112), 2.14(3)(031) and 1.76(5)(132). Electron-microprobe analysis of one of the grains characterized by X-ray diffraction gave an average composition of (Yb0.48Y0.20Er0.11Lu0.09Dy0.05Tm0.04 Ho0.01Gd0.004Tb0.003)(Sigma 0.99) P-1.00 O-4. The ratio Y/Yb (at.) varies between 0.39 and 0.68; this range is roughly comparable to the variation defined by xenotime-(Y) and xenotime-(Yb) in granitic pegmatites from the Kola Peninsula, Russia.