Na-Li-Be-rich cordierite from the Murzinka pegmatite field, Middle Urals, Russia

Abstract

Na-Li-Be-rich cordierite (0.16 Be, 0.05 Li, and 0.21–0.31 Na atom p.f.u.) enriched in Rb (64 ppm), Cs (217 ppm), Ce (26 ppm) and U (56 ppm) has been discovered in an Mg-enriched vein of granite pegmatite in the Murzinka field of beryllian granitic pegmatites in the Middle Ural Mts of Russia. The crystal-chemical formula of the cordierite, based on the refined structure, is |Na 0.24 Ch0 (H 2 O) 0.64 Ch1/4 |(Mg 1.66 Fe 0.26 2+ Li 0.06 Mn 0.02 ) ∑2.00 M [(Al 1.80 Be 0.16 Fe 0.04 2+ Si 1.00 ) ∑3.00 T1 (Si 4.00 Al 2.00 ) ∑6.00 T2 O 18 ]. The molecules in the channels are dominated by class II and class I H 2 O (2.21 wt%). According to the paragenesis, chemical composition and structural features of this corderite, the occurrence is typical for peraluminous granitic pegmatite. Cordierite within the Murzinka pegmatite field is generally restricted to pegmatite veins contaminated by interaction with peraluminous or silica-undersaturated country rocks. Cordierite, together with or instead of beryl, appears to have crystallized from aqueous weakly acidic chloride solutions rich in Na. The preservation of cordierite from subsequent replacement is achieved by low fluid-to-rock ratio.