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; α 1.768 (5), β 1.771(3), γ > 1.808(3); 2 V meas 29(8)°, X = b , Z ∧ c = 9° (β obtuse). It is monoclinic, space group P 2 1 / 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 ) A, β 104.02(1)°, 104.03 ( 4 )°, V 291.8(1), 293.6 ( 2 ) A 3 , Z = 4; D calc derived from average chemical composition is 5.512 and 5.478 g/cm 3 . The strongest six lines of the (Gandolfi) X-ray-diffraction pattern [ d in A( 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 Sm 2 O 3 , 13.48 Gd 2 O 3 and 6.28 Nd 2 O 3 , and moderate percentages of the huttonite and brabantite components. The most Sm-rich composition gives (Sm 0.197 Gd 0.179 Ce 0.148 Th 0.125 Ca 0.107 Nd 0.090 La 0.030 Y 0.030 Pr 0.023 Tb 0.017 Zr 0.017 Dy 0.016 Pb 0.016 U 0.002 ) ∑0.997 (P 0.963 Si 0.044 ) ∑1.007 O 4 . 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)O 4 mineral (formanite?) and dysprosian xenotime-(Y); this exotic pattern of REE abundances is possibly generated by selective and differential complexing of REE in the granite-to-pegmatite sequence of solidification.