Barium-Bearing Micas of the Muscovite–Ganterite Series from Alkaline Rocks of the Sredneziminsky Ijolite-Syenite-Carbonatite Massif (Eastern Siberia, Russia)

Barium-bearing micas have been found in hydrothermally altered ijolites and alkaline syenite from the Sredneziminsky ijolite-syenite-carbonatite massif (Eastern Siberia, Russia). The BaO content varies from 1.2 to 18.7 wt %. These micas occur in the products of low-temperature replacement of cancrinite, in association with natrolite, analcime, calcite, diaspore/bӧhmite, celsian, and strontianite. Ba-bearing micas are represented by flakes up to 1 mm that are heterogeneous in chemical composition. The Ba amount increases in the rim of grains. One can also observe enrichment of some mica layers with barium. The main isomorphic substitution in muscovite corresponds to the scheme K+ + Si4+ ↔ Ba2+ + IVAl3+. The empirical formula of barium-richest areas in one of the mica grains is (Вa0.54–0.56Sr0–0.09K0.46)Σ1.02–1.06Al1.98–2.01(Si2.37–2.40All.60–1.63)Σ4.00O10(OH1.70–2.00F0–0.30)2, which corresponds to ganterite. However, the maximum content of BaO in most muscovite grains from the Sredneziminsky massif is 14.0–14.9 wt %, which is equal to 0.41–0.44 apfu Ba. It is believed that orthoclase containing 0.5–0.9 wt % BaO that underwent albitization at the postmagmatic stage served as the source of barium in the hydrothermal solution. The frequent occurrence of sulfides in the rocks indicates low oxygen fugacity, which hinders the formation of barite but promotes the formation of Ba-bearing muscovite and celsian.