Tetrahedrally coordinated boron in Li-bearing olenite from "mushroom" tourmaline from Momeik, Myanmar
Tetrahedrally coordinated boron in Li-bearing olenite from "mushroom" tourmaline from Momeik, Myanmar
The Canadian Mineralogist (August 2007) 45 (4): 891-899
- alkali metals
- aluminum
- Asia
- boron
- Burma
- crystal chemistry
- crystal structure
- electron probe data
- Far East
- granites
- ICP mass spectra
- igneous rocks
- ion probe data
- lithium
- manganese
- mass spectra
- metals
- pegmatite
- plutonic rocks
- rare earths
- refinement
- ring silicates
- silicates
- spectra
- tourmaline group
- olenite
- Khetchel Burma
- Palelni Mine
- Momeik Myanmar
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 approximately 9 mm in length and approximately 50-150 mu m 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 approximately 1.6 wt% MnO. Near the pyramidal faces, the composition of these tourmaline crystallites is relatively Al-rich (up to 43.6 wt% Al (sub 2) O (sub 3) ), 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 (super X) (Na (sub 0.55) Ca (sub 0.08) Pb (sub 0.01) [] (sub 0.36) ) (super Y) (Al (sub 2.13) Li (sub 0.75) Mn (super 2+) (sub 0.01) [] (sub 0.11) ) (super Z) Al (sub 6) (super T) (Si (sub 5.34) B (sub 0.66) )O (sub 18) (BO (sub 3) ) (sub 3) (super V) (OH) (sub 3) (super W) [(OH) (sub 0.50) O (sub 0.26) F (sub 0.24) ], with a 15.7561(6), c 7.0703(5) Aa. The structure refinement yields a relatively high (super [4]) B content (Si (sub 0.87(1)) B (sub 0.13(1)) ), and a {T-O} distance of 1.604 Aa, 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 (super [4]) B with r (super 2) nearly equal 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 (Sigma REE: approximately 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.