Boron-bearing 2M(1) polylithionite and 2M(1)+1M boromuscovite from an elbaite pegmatite at Recice, western Moravia, Czech Republic

An elbaite-subtype pegmatite from Recice, western Moravia, Czech Republic, carries abundant B-rich minerals: dominant tourmaline (schorl to elbaite an d rare liddicoatite), and subordinate tusionite, B-bearing polylithionite a nd boromuscovite. The pegmatitic unit contains numerous, randomly distribut ed pockets lined with crystals of orthoclase, quartz, two generations of re d to pink elbaite, subordinate albite, minor polylithionite and two generat ions of boromuscovite. Polylithionite, as a previously unreported 2M(1) pol ytype, contains similar to 80 mol.% polylithionite end-member K-2(Li4Al2)Si 8O20F4 and similar to 20 mol.% of K-2(LiAl3)Si8O20F4; the content of 0.44 w t.% B2O3 is the highest found so far in a lepidolite. Boromuscovite is comp ositionally variable from similar to 50 to almost 100 mol.% of end-member b oromuscovite. Dominant boromuscovite I consists of a mixture of 83 vol.% 2M (1) and 17 vol.% 1M polytypes. Boromuscovite II forms rare overgrowths on l arger flakes of boromuscovite I and is slightly Fe-, Mg-enriched. NMR MAS s pectra confirm the substitution of B-IV for Al-IV in all three micas. Cryst allization of B-bearing polylithionite after the abundant red elbaite I ind icates that highly evolved residual melt attained saturation of a K-,Li-,F- rich phase, while the high activity of B was maintained from early stages. The hydrothermal fluids which precipitated boromuscovite were relatively de pleted in Li, Rb, Cs and F, but were B-rich. In agreement with experimental studies, Li and F are concentrated in late magmatic minerals (elbaite, pol ylithionite), but their activities are negligible in hydrothermal fluids an d their precipitates. In contrast, high activity of B is maintained from th e magmatic to the hydrothermal stage, as indicated from abundant early scho rl to late Li-,F-poor boromuscovite. Crystallization of the massive pegmati te units and pockets including boromuscovite I seems to be a near-isobaric process in a closed system. Formation of late slightly Fe-, Mg-enriched bor omuscovite II overgrowths indicates mixing of residual and metamorphic flui ds during late pocket consolidation after thermal reequilibriation of the p egmatite with host rocks.