Barium-rich phlogopite and biotite from some Quaternary alkali mafic lavas, West Eifel, Germany

Barium-rich phlogopite and biotite occur in leucite nephelinites and nephelinites from several quarries northeast of Gerolstein in the West Eifel Quaternary province of Germany. The micas occur usually as groundmass minerals but also as microphenocrysts. The degree of Ba-enrichment in the micas is variable, and reaches a maximum of 14 wt.% BaO. With increasing BaO, there is a decrease in K 2 O and SiO 2 and increase in Al 2 O 3 for all micas whereas. TiO 2 and F tend to increase. Compared to Ba-rich micas from other localities, the West Eifel micas exhibit very comparable substitutions, with the Ba in the interlayer site and with a coupled substitution K+3(Mg, Fe)+3Si=Ba+2Ti+3Al accounting for the mica chemistry