REEDMERGNERITE AND ASSOCIATED MINERALS FROM PERALKALINE PEGMATITE, DARA-I-PIOZ, SOUTHERN TIEN-SHAN, TAJIKISTAN

Reedmergnerite occurs in pegmatites associated with a Jurassic complex dominated by subalkaline granitoids. It forms aggregates up to severa l cm across in coarse quartz and microcline, and is extensively embaye d by microveinlets of fine-granular quartz containing minor microcline and accessory albite, aegirine, eudialyte, pyrochlore, leucosphenite, leucophanite and zektzerite. The coarse microcline is cut by microvei nlets of finer albite and microcline containing accessory hyalotekite and aegirine. Microcline is Or97-99 with 0.02 wt % B2O3; albite is An0 -1 with 0.005 wt % B2O3. Aegirine contains 83-98 mole % NaFeSi2O6, les s-than-or-equal-to 5 % NaAlSi2O6, less-than-or-equal-to 10% Ca(Mg,Mn)S i2O6 and less-than-or-equal-to 14% Na(Fe2+,Mn)0.5Ti0.5Si2O6. Polylithi onite composition is K2Li3.9Al1.6Ti0.24Fe0.1Si4O10F4. Reedmergnerite c ontains no impurities in amounts exceeding 0.05 wt% oxide. Average ele ctron microprobe analyses of reedmergnerite from Utah (type) and Tajik istan give nearly stoichiometric NaBSi3O8, Whereas individual analyses follow a trend whereby Na + B varies inversely with Si for Si = 2.91- 3.07 per formula unit. Reedmergnerite had nearly ideal B-Si order at t he 450-500-degrees-C temperatures and P less-than-or-equal-to 2 kbar p ressures estimated for the pegmatitic assemblage orthoclase + reedmerp erite + quartz. We suggest that this assemblage crystallized from a hi ghly alkaline, sodic residual melt that evolved under conditions where by B saturation was attained prior to vapor saturation.