So-called “mushroom” tourmaline, a botryoidal aggregate of pale pink tourmaline crystals, occurs in the Palelni mine, Khetchel village, Molo quarter, Momeik Township, northeast of Mogok, Mandalay Division, Shan State, Myanmar. It has been characterized by crystal-structure determination, electron-microprobe analysis (EMPA), secondary-ion mass spectrometry (SIMS), uranium extraction, and inductively coupled plasma – mass spectrometry (ICP–MS). This sample consists of sprays of radially arranged crystals up to ~9 mm in length and ~50–150 μm in diameter. Chemical data show that Al increases, whereas the Mn content decreases, from the center of the aggregate to the pyramidal faces. The composition at the center can be described as a Mn-bearing elbaite with ~1.6 wt% MnO. Near the pyramidal faces, the composition of these tourmaline crystallites is relatively Al-rich (up to 43.6 wt% Al2O3), and Mn-poor (0.1–0.3% MnO). A crystal-structure refinement of the Li-bearing olenite (near the pyramidal faces), in combination with analytical results, gives the optimized formula X(Na0.55Ca0.08Pb0.01□0.36) Y(Al2.13Li0.75Mn2+0.01□0.11) ZAl6T(Si5.34B0.66)O18 (BO3)3V(OH)3W[(OH)0.50O0.26F0.24], with a 15.7561(6), c 7.0703(5) Å. The structure refinement yields a relatively high B content (Si0.87(1)B0.13(1)), and a <T–O> distance of 1.604 Å, the shortest observed in natural tourmaline to date. Together with Al-rich tourmalines from other localities, a positive correlation between Al at Y site and B with r2 ≈ 0.998 was found. Furthermore, this tourmaline aggregate shows a relatively high Pb content (1640 ppm) and a significant Bi content (153 ppm). The REE pattern (∑REE: ~7.2 ppm) exhibits a weak negative Eu anomaly, a strong positive Tm anomaly, and a positive Tb anomaly. Such a pattern cannot be derived simply from a late-stage fractionated magma of a S-type granite or from differentiated crustal melts alone.
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- © 2007 Mineralogical Association of Canada