MUSHROOM TOURMALINE AND ASSOCIATED MINERALS FROM PEGMATITES NEAR MOMEIK, MOGOK STONE TRACT, MYANMAR (BURMA). A. U. Falster1, A. Peretti2, and W. B. Simmons1. 1Dept. of Earth and Environmental Sciences, University of New Orleans, New Orleans, LA 70148; 2GRS Gemresearch Swisslab AG, Hirschmattstr. 6, PO Box 4028, CH-6003 Lucerne, Switzerland.
Tourmaline in a fantastic mushroom-shaped morphology has been produced from Myanmar since the 1800s. Pegmatites in the vicinity of Momeik, north of Mogok, Myanmar, are the source of the pegmatites producing the peculiar mushroom-shaped tourmalines. These LCT-type (Li-Cs-Ta- enriched) pegmatites occur in a belt of evolved tin- tungsten S-type granites and their pegmatites. This 1,500-kilometer- long belt stretches north-south through Myanmar. The pegmatites are generally modest in size, 1-5 meters thick, and up to 300 meters long. Mining operations are mostly small.
The majority of these mushroom-shaped tourmalines are pale pink, almost colorless, to intensely red. Commonly, the core is dark, and occasionally thin bands of dark color occur in the pink parts of the crystals as well. Some of the tourmalines are of the typical prismatic morphology, whereas others show a diverging morphology, and still others form the most fantastic forms and intergrowths. The diverging crystals are only slightly offset from each other, but subsequent generations of offset subindividual crystals bring about more intense curvatures. More drastically offset crystallites would hinder their growth and would not be able to continue growth. The largest specimens of mushroom tourmaline weigh around 1 kilogram, but smaller sizes are more typical.
Electron microprobe analyses reveal compositions ranging from a dark and Ti-rich schorlitic core to a light-colored elbaitic overgrowth. A liddicoatite component is distinct in the pink portions but does not become dominant. Several samples consist of rossmanitic zones and, in some, an olenitic overgrowth as the final crystallization was observed.
Associated minerals in these pegmatites include the following species.
Quartz occurs in pale gray crystals.
Gray or white microcline in blocky crystals and white albitic plagioclase are rarely preserved, since feldspars have decomposed or are just not preserved for economic reasons.
High-fluorine topaz is abundant and forms fine crystals that may reach a mass of 1 kilogram.
Beryl (goshenite, aquamarine, and morganite) occurs. The morganite is alkali-element enriched.
Etched pollucite from miarolitic cavities occurs in centimeter- sized crystal remnants. The samples studied are true pollucite and not just cesian analcime, which is more common in the pocket stage in other districts.
Danburite occurs abundantly in some miarolitic cavities in crystals to several centimeters in length and has essentially ideal composition and very little chemical sustitution. Most crystals are colorless or yellowish in hue.
Columbite-group species are rare, and the only sample noted was a manganocolumbite. One small cluster of stibitantalite crystals to 3 mm in length has been noted as well.
Phenakite forms dominantly prismatic and mostly twinned crystals to several centimeters in length. Compositionally, phenakite shows no noticeable substitution.
Almandine-spessartine, biotite, magnetite, lepidolite, hubnerite- ferberite, and cassiterite have been reported, but no samples were available for this study.
Overall, the paragenesis bears considerable similarity to high-B and high-Cs pegmatites, such as the Malkhansk district in Siberia and the central Madagascar pegmatites. REFERENCE
Zaw, K. 1998. Geological evolution of selected granitic pegmatites in Myanmar (Burma): Constraints from regional setting, lithology, and fluid-inclusion studies. International Geology Review 40:647-62.
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