Bismutotantalite-stibiotantalite-stibiocolumbite assemblage from elbaite pegmatites at Molo near Momeik, northern Shan State, Myanmar
Novák, Milan; Sejkora, Jiří Škoda; Budina, Václav
published: Jul 22, 2008
ArtNo. ESP154018501003, Price: 29.00 €
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) Å and V = 326.63(2) Å3), up to about 5 mm in size, and their aggregates, up to 3 cm in diameter, forming ∼95 vol.% 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 μm 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 μm 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.% WO3). 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 μm 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 (W6+ ≤ 0.275 apfu in stibiocolumbite veinlets, Pb2+ ≤ 0.052 apfu, Sn4+ ≤ 0.010 apfu, As3+ ≤ 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 ABO4 stoichiometry from A/B ∼1 to ΣA-site cations up to 1.216 apfu with ΣA+B = 2.000. The simple exchange vectors Sb1 Bi−1 and Nb1 Ta−1 are dominant in all phases examined, but the minor substitution Pb2+1 W6+1 Sb3+−1 Nb5+−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.