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 several cm across in coarse quartz and microcline, and is extensively embayed 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 microveinlets 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, less-than-or-equal-to 5 % NaAlSi2O6, less-than-or-equal-to 10% Ca(Mg,Mn)Si2O6 and less-than-or-equal-to 14% Na(Fe2+,Mn)0.5Ti0.5Si2O6. Polylithionite composition is K2Li3.9Al1.6Ti0.24Fe0.1Si4O10F4. Reedmergnerite contains no impurities in amounts exceeding 0.05 wt% oxide. Average electron microprobe analyses of reedmergnerite from Utah (type) and Tajikistan 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 the 450-500-degrees-C temperatures and P less-than-or-equal-to 2 kbar pressures estimated for the pegmatitic assemblage orthoclase + reedmerperite + quartz. We suggest that this assemblage crystallized from a highly alkaline, sodic residual melt that evolved under conditions whereby B saturation was attained prior to vapor saturation.