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TETRAHEDRALLY COORDINATED BORON IN Li-BEARING OLENITE FROM "MUSHROOM" TOURMALINE FROM MOMEIK, MYANMAR

Andreas Ertl^1,, John M. Hughes^2,¶, Stefan Prowatke³, Thomas Ludwig³, Franz Brandstätter⁴, Wilfried Körner⁵ and M. Darby Dyar⁶

¹ Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Althanstrasse 14, A–1090 Vienna, Austria
² Department of Geology, Miami University, Oxford, Ohio 45056, U.S.A.
³ Mineralogisches Institut, Universität Heidelberg, Im Neuenheimer Feld 236, D–69120 Heidelberg, Germany
⁴ Mineralogisch-Petrographische Abteilung, Naturhistorisches Museum, Burgring 7, A–1010 Vienna, Austria
⁵ Institut für Geochemie, Geozentrum, Universität Wien, Althanstrasse 14, A–1090 Vienna, Austria
⁶ Department of Earth and Environment, Mount Holyoke College, South Hadley, Massachusetts 01075, U.S.A.

E-mail address: andreas.ertl{at}a1.net

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(Na_0.55Ca_0.08Pb_0.010.36) ^Y(Al_2.13Li_0.75Mn²⁺_0.010.11) ^ZAl₆ ^T(Si_5.34B_0.66)O₁₈ (BO₃)₃ ^V(OH)₃ ^W[(OH)_0.50O_0.26F_0.24], with a 15.7561(6), c 7.0703(5) Å. The structure refinement yields a relatively high ^[4]B content (Si_0.87(1)B_0.13(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 ^[4]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.

Keywords: olenite, tetrahedrally coordinated boron, crystal structure, chemical analyses, Momeik, Myanmar.