SOKOLOVAITE AND EVOLVED LITHIAN MICAS FROM THE EASTERN MOBLAN GRANITIC PEGMATITE, OPATICA SUBPROVINCE, QUEBEC, CANADA

A new occurrence of sokolovaite, the Cs analogue of polylithionite, is recorded from the main pegmatite body in the Eastern block of the Moblan pegmatite cluster, located within the Frotet–Evans greenstone belt of the Opatica Subprovince in Quebec. The main pegmatite body of the Eastern block comprises a near-vertical dyke that is simply zoned, with an aplitic border surrounding an extensive zone of quartz–spodumene pegmatite. Within this extensive unit, irregular quartz- and lepidolite-rich pods are developed that contain a rim of sokolovaite on compositionally zoned grains of Rb-bearing lepidolite. Although not as common, sokolovaite is also observed as discrete, zoned grains inside fractures cutting through granular masses of lepidolite. Minerals associated with sokolovaite include: rubidian lepidolite, albite, quartz, and pollucite. In the zoned mica crystals, compositional zoning from core to rim consists of increasing Rb and Cs contents with decreasing K and Al contents, indicating a replacement of K by Rb and Cs, whereas Al contents reflect the solid-solution change from trilithionite to polylithionite. This compositional change from trilithionite to polylithionite is also demonstrated on a plot of the IV Al versus Cs/(Cs + K). The mantles of sokolovaite typically contain 17–22 wt.% Cs 2 O, ≤ 4.5 wt.% Rb 2 O and ≤ 1.25 wt.% K 2 O. Texturally, the lepidolite grains are associated with secondary albite and appear to have formed at the expense of K-feldspar and spodumene in quartz-rich pods in the core of the pegmatite, during the final stages of crystallization from subsolidus fluids; the enrichment of Rb and Cs in the fluids reflects the protracted magmatic differentiation of the felsic magma. The fracture-controlled lepidolite grains likely reflect late-stage propagation of these final metasomatic fluids along fractures into crystallized pegmatitic phases.