科研之友
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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 similar to 2.5% using graphite-monochromated Mo-K alpha X-radiation; the same crystals were analysed by electron microprobe. B-11 and Al-27 magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were collected on four regions of the wheatsheaf crystal, and show similar to 0.3 a.p.f.u. B-[4] and <0.1 a.p.f.u. Al-[4] in the structure. Fe-57 Mossbauer spectroscopy was done on the dark green rim at the termination of the crystal, showing all Fe in this region (similar to 0.6 a.p.f.u.) to be Fe2+. Detailed electron-microprobe traverses show that the principal compositional variation involves the substitutions B-[4] + Al-Y -> Si + Fe-Y*, where transition metals are present, and B-[4](2) + Al-Y -> Si-2 + Li-Y, 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.
- 出版日期2011-2
