Barium-bearing micas of the muscovite-ganterite series from alkaline rocks of the Sredneziminsky ijolite-syenite-carbonaite massif (Eastern Siberia, Russia)

Barium-bearing micas (BaO content from 1.2 to 18.7 wt %) were found in hydrothermally altered ijolites and alkaline syenite of the Sredneziminsky ijolite-syenite-carbonatite massif (Eastern Siberia, Russia). It occurs in the products of low-temperature replacement of cancrinite, in association with natrolite, analcime, calcite, diaspore/boehmite, celsian, and strontianite. Ba-bearing micas are represented by grains up to 1 mm, heterogeneous in chemical composition. The amount of Ba increases in the marginal parts of grains; enrichment of some layers in mica grains with barium is also observed. 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 (Ba0.54–0.56Sr0–0.09K0.46)∑1.02–1.06Al1.98–2.01(Si2.37–2.40All.60–1.63)∑4.00 O10(OH1.70–2.00F0–0.30)∑2.00, which corresponds to ganterite. The maximum content of BaO in the majority of muscovite grains of the Sredneziminsky massif is 14.0–14.9 wt % that is equal to 0.41‒0.44 apfu Ba. It is assumed that the source of barium in the hydrothermal solution was orthoclase containing 0.5–0.9 wt % BaO which underwent albitization at the post-magmatic stage. The widespread occurrence of sulfides in the rocks indicates low oxygen fugacity, which prevents the formation of barite and is favorable to the formation of Ba-bearing muscovite and celsian.