MONAZITE-(Sm), A NEW MEMBER OF THE MONAZITE GROUP FROM THE ANNIE CLAIM #3 GRANITIC PEGMATITE, SOUTHEASTERN MANITOBA

Abstract

Monazite-(Sm) was found in the Annie Claim #3 pod of lepidolite-subtype granitic pegmatite within the Greer Lake intrusion of pegmatitic leucogranite, in the Archean Bird River Subprovince of the Superior Province, in southeastern Manitoba. It occurs as tabular crystals ≤0.4 mm in diameter, associated with manganocolumbite, quartz, albite and lithian muscovite. Monazite-(Sm) is yellowish, translucent, with a white streak, vitreous to greasy luster, and no observed fluorescence. One good cleavage is present, tenacity is brittle, and the fracture is uneven. Monazite-(Sm) has no observable pleochroism; 1.768 (5), 1.771(3), > 1.808(3); 2Vmeas 29(8)°, X = b, Z c = 9 ° ( obtuse). It is monoclinic, space group P21/n; a refinement from single-crystal and powder (Gandolfi) X-ray-diffraction data gave a 6.725(1), 6.739(3), b 6.936(1), 6.951(3), c 6.448(1), 6.462(3) Å, 104.02(1)°, 104.03(4)°, V 291.8(1), 293.6(2) Å3, Z = 4; Dcalc derived from average chemical composition is 5.512 and 5.478 g/cm3. The strongest six lines of the (Gandolfi) X-ray-diffraction pattern [d in Å(I)(hkl)] are: 4.647(5)(011), 4.164(8)(¯111), 3.492(4)(111,020), 3.264(7)(200), 3.065(10)(120), and 2.857(9)(¯112,012). The monazite-(Sm) contains, in wt.%, up to 14.29 Sm2O3, 13.48 Gd2O3 and 6.28 Nd2O3, and moderate percentages of the huttonite and brabantite components. The most Sm-rich composition gives (Sm0.197Gd0.179Ce0.148Th0.125Ca0.107Nd0.090La0.030Y0.030Pr0.023Tb0.017 Zr0.017Dy0.016Pb0.016U0.002)0.997 (P0.963 Si0.044)1.007 O4. Adjustment of U and Th of the monazite-(Sm) to their original contents 2.64 Ga ago slightly improves the stoichiometry and proves that all the Pb present is radiogenic. The middle-REE-dominant signature of the Annie Claim #3 monazite-(Sm) is shared with the broadly associated Y(Ta,Nb)O4 mineral (formanite?) and dysprosian xenotime-(Y); this exotic pattern of REE abundances