An elbaite-subtype pegmatite from Recice, western Moravia, Czech Republic, carries abundant B-rich minerals: dominant tourmaline (schorl to elbaite and rare liddicoatite), and subordinate tusionite, B-bearing polylithionite and boromuscovite. The pegmatitic unit contains numerous, randomly distributed pockets lined with crystals of orthoclase, quartz, two generations of red to pink elbaite, subordinate albite, minor polylithionite and two generations of boromuscovite. Polylithionite, as a previously unreported 2M(1) polytype, contains similar to 80 mol.% polylithionite end-member K-2(Li4Al2)Si8O20F4 and similar to 20 mol.% of K-2(LiAl3)Si8O20F4; the content of 0.44 wt.% B2O3 is the highest found so far in a lepidolite. Boromuscovite is compositionally variable from similar to 50 to almost 100 mol.% of end-member boromuscovite. Dominant boromuscovite I consists of a mixture of 83 vol.% 2M(1) and 17 vol.% 1M polytypes. Boromuscovite II forms rare overgrowths on larger flakes of boromuscovite I and is slightly Fe-, Mg-enriched. NMR MAS spectra confirm the substitution of B-IV for Al-IV in all three micas. Crystallization of B-bearing polylithionite after the abundant red elbaite I indicates 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 depleted 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, polylithionite), but their activities are negligible in hydrothermal fluids and their precipitates. In contrast, high activity of B is maintained from the magmatic to the hydrothermal stage, as indicated from abundant early schorl to late Li-,F-poor boromuscovite. Crystallization of the massive pegmatite units and pockets including boromuscovite I seems to be a near-isobaric process in a closed system. Formation of late slightly Fe-, Mg-enriched boromuscovite II overgrowths indicates mixing of residual and metamorphic fluids during late pocket consolidation after thermal reequilibriation of the pegmatite with host rocks.
Boron-bearing 2M(1) polylithionite and 2M(1)+1M boromuscovite from an elbaite pegmatite at Recice, western Moravia, Czech Republic
Ottolini L;
1999
Abstract
An elbaite-subtype pegmatite from Recice, western Moravia, Czech Republic, carries abundant B-rich minerals: dominant tourmaline (schorl to elbaite and rare liddicoatite), and subordinate tusionite, B-bearing polylithionite and boromuscovite. The pegmatitic unit contains numerous, randomly distributed pockets lined with crystals of orthoclase, quartz, two generations of red to pink elbaite, subordinate albite, minor polylithionite and two generations of boromuscovite. Polylithionite, as a previously unreported 2M(1) polytype, contains similar to 80 mol.% polylithionite end-member K-2(Li4Al2)Si8O20F4 and similar to 20 mol.% of K-2(LiAl3)Si8O20F4; the content of 0.44 wt.% B2O3 is the highest found so far in a lepidolite. Boromuscovite is compositionally variable from similar to 50 to almost 100 mol.% of end-member boromuscovite. Dominant boromuscovite I consists of a mixture of 83 vol.% 2M(1) and 17 vol.% 1M polytypes. Boromuscovite II forms rare overgrowths on larger flakes of boromuscovite I and is slightly Fe-, Mg-enriched. NMR MAS spectra confirm the substitution of B-IV for Al-IV in all three micas. Crystallization of B-bearing polylithionite after the abundant red elbaite I indicates 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 depleted 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, polylithionite), but their activities are negligible in hydrothermal fluids and their precipitates. In contrast, high activity of B is maintained from the magmatic to the hydrothermal stage, as indicated from abundant early schorl to late Li-,F-poor boromuscovite. Crystallization of the massive pegmatite units and pockets including boromuscovite I seems to be a near-isobaric process in a closed system. Formation of late slightly Fe-, Mg-enriched boromuscovite II overgrowths indicates mixing of residual and metamorphic fluids during late pocket consolidation after thermal reequilibriation of the pegmatite with host rocks.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
