The crystal chemistry of ‘wheatsheaf’ tourmaline from Mogok, Myanmar

摘要

Tourmalines of unusual (mushroom) habit are common in granitic pegmatites of Momeik, northeast of Mogok, Myanmar. Here, we examine a sample of elbaite of significantly different habit, consisting of a series of diverging crystals, resembling a sheaf of wheat and ranging in colour from light purplish-red at the base to dark purplish-red at the tip with a thin green cap at the termination. The crystal structures of eight crystals are refined to R_1-indices of ～2.5% using graphite-monochromated Mo-K_α X-radiation;the same crystals were analysed by electron microprobe. ~(11)B and ~(27)Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were collected on four regions of the wheatsheaf crystal, and show ～0.3 a.p.f.u.~[4]B and <0.1 a.p.f.u.~[4]Alin the structure. ~(57)Fe Mossbauerspectr oscopy was done on the dark green rim at the termination of the crystal, showing all Fe in this region (～0.6 a.p.f.u.) to be Fe~(2+).Detailed electron-microprobe traverses show that the principal compositional variation involves the substitutions ~[4]B + ~YAl → Si + ~YFe~*, where transition metals are present, and ~[4]B_2 + ~YAl→Si_2+~YLi, where transition metals are not present, although several other minor substitutions also affect crystal composition. Successive microscopic bifurcation of crystallites causes divergence of growth directions along the c axis, imparting the overall ‘wheatsheaf’ shape to the crystal aggregate. We suggest that such bifurcation is common in pegmatitic elbaite crystals, resulting in their common divergent habit.