Tourmaline from Malkhanskiy pegmatite district
by Simmons, William B, Webber, Karen L, Falster, Alexander U
The Transbaikal region of southern central Siberia is famous for its gem tourmaline. Pegmatites of the Malkhanskiy district are currently being mined, principally for gem-quality polychrome tourmaline. The pegmatites appear to be related to Jurassic granitic plutons that have intruded Proterozoic metamorphic and igneous rocks.
At least 200 pegmatites are located within the Malkhanskiy district. Of these, seven are currently productive. These include the Mokhovaya, Svetlaya, Orieshnaya, Sosedka, Oktyabrskaya, Zapadnaya and Karkadilovaya pegmatites. The most extensive and productive of these is the Mokhovaya pegmatite, which is granitic in composition and contains abundant miarolitic pockets. The pocket mineralogy consists of gem rubellite, bicolor tourmaline, albite and smoky quartz, with minor pink beryl, lepidolite, danburite, cookeite and pollucite. In some pockets tourmaline is coated with a crust of fine-grained danburite. A few pockets contain large (up to 5 cm), gemmy orange danburite crystals. Additional accessory minerals identified in the Malkhanskiy district include manganocolumbite, monazite, ixiolite, stroverite, bismuthinite, bismutite, topaz, spessartine, cesian beryl, biotite, amazonite and fluorite. Other minerals reported include xenotime, euxenite, bismutocolumbite, bismutomicrolite, microlite, hambergite, petalite, stilbite and apatite.
A suite of polychrome tourmaline, collected from throughout the Malkhanskiy district, ranges in color from dark pink to yellow to green to brown. Electron microprobe analyses reveal that the tourmalines are principally elbaitic in composition (Fig. 1) with fluorine contents of approximately 0.5 apfu.* Color is found to correlate strongly with Y-site chemistry. Pink tourmaline approaches nearly end-member elbaite composition. Green caps and overgrowths on pink tourmaline contain higher total iron and manganese. The unusual yellow-orange to yellow tourmaline has the highest concentration of Mn (up to 8.1 weight % MnO), but contains virtually no iron. Interestingly, yellow tourmaline contains less liddicoatite (Ca) component than do the rims of some elbaite crystals, which contain up to 42% Ca in the X-site. Overall, X-site vacancies are the highest in pink tourmaline, ranging up to 0.3 apfu. Calculated Li concentrations show a strong negative correlation with Mn content in all tourmalines. Although there is variability in the amount of Mn and Ti, there is a general trend of low Mn and Ti in pink tourmalines and high Mn and Ti in most yellow tourmalines. This suggests that the yellow color of Mn-rich, Fe-poor tourmaline may be the result of the Mn"-Ti" charge transfer.
The district is characterized by elbaite as the principal lithium mineral (which is more abundant than lepidolite), and by the low abundance of late-stage phosphate minerals. Tourmalines are Mnrich and F-rich with low X-site vacancies. Elbaite is associated with late-stage danburite and hambergite. Based on these characteristics, the Malkhanskiy pegmatites can be classified as belonging to the elbaite subtype of the rare-element class of granitic pegmatites.
The Malkhanskiy pegmatites have an unusual late-stage chemistry very rich in calcium and boron, as evidenced by the very late crystallization of danburite and hambergite as pocket minerals and overgrowths on miarolitic tourmaline. Tourmaline composition, especially that of the latest tourmaline, is influenced by this enrichment of calcium, which produces elbaite with significant calcium substitution for sodium in the X-site.
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