Bismutotantalite-stibiotantalite-stibiocolumbite assemblage from elbaite pegmatites at Molo near Momeik, northern Shan State, Myanmar

摘要

Bismutotantalite-stibiotantalite-stibiocolumbite (BSS) aggregates occur exclusively in pockets of the elbaite subtype pegmatites cutting serpentinized peridotite at Khetchel village, Molo quarter near Momeik Township, northeast of Mogok, Shan State, Myanmar (Burma). The pegmatites exhibit simple zoning with common pockets lined with K-feldspar, "mushroom-like" pink to red elbaite, beryl (aquamarine, morganite), petalite, phenakite, quartz, saccharoidal albite, adularia, hambergite and cookeite(?). The BSS aggregates consist of prismatic crystals of bismutotantalite-stibiotantalite (Bi/(Bi+Sb) = 0.47-0.51; Ta/ (Ta+Nb) = 0.64-0.67; a = 5.6017(3), b = 11.7802(3), c = 4.9497(3) A and V = 326.63(2) A~3), up to about 5 mm in size, and their aggregates, up to 3 cm in diameter, forming approx 95 vol. percent of the overall BSS aggregate. Oval to irregular grains blebs of stibiotantalite (Bi/(Bi+Sb) = 0.04-0.08; Ta/(Ta+Nb) = 0.62-0.68), up to 100 mu m in diameter, scarcely occur in bismutotantalite-stibiotantalite. Thin veinlets of stibiocolumbite (Bi/(Bi+Sb) = 0.01-0.05; Ta/(Ta+Nb) = 0.40-0.49) with brecciated textures, up to 30 mu m thick, cut bismutotantalite-stibiotantalite and rarely also stibiotantalite blebs. Two distinct compositions were found in the stibiocolumbite veinlets, W-poor and rare W-rich (up to 19.34 wt. percent WO_3). Late stibiocolumbite overgrowths (Bi/(Bi+Sb) = 0.01-0.03; Ta/(Ta+Nb) = 0.17-0.33) on crystals of bismutotantalite-stibiotantalite reach up to 20 mu m in size. Bismutotantalite-stibiotantalite + stibiotantalite blebs -> stibiocolumbite veinlets -> stibiocolumbite overgrowths - is the succession of crystallization. All phases contain along with dominant Bi, Sb, Nb and Ta also low to moderate concentrations of some cations (W~(6+) <= 0.275 apfu in stibiocolumbite veinlets, Pb~(2+) <= 0.052 apfu, Sn~(4+) <= 0.010 apfu, As~(3+)<= 0.032 apfu; Ti < 0.045 apfu and U < 0.005 apfu, the latter two only in the stibiocolumbite overgrowths). High amounts of W in the stibiocolumbite veinlets change slightly ABO_4 stoichiometry from A/B approx 1 to SIGMA A-site cations up to 1.216 apfu with SIGMA A+B = 2.000. The simple exchange vectors Sb, Bi_(-1) and Nb, Ta_(-1) are dominant in all phases examined, but the minor substitution (Pb~(2+))_1(W~(6+))_1 (Sb~(3+))_(-1)(Nb~(5+))_(-1) also was encountered in W-poor compositions. Fractionation in the BSS assemblage expressed by the Bi/(Bi+Sb) and Ta/(Ta+Nb) is asynchronous; the Bi/(Bi+Sb) value significantly drops down in the stibiotantalite blebs and it is very low in stibiocolumbite. Ta/(Ta+Nb) is constant in the bismutotantalite-stibiotantalite and stibiotantalite blebs, and then it decreases in the stibiocolumbite veinlets and particularly in the stibiocolumbite overgrowths.