Polylithionite is a characteristic lithium mica of alkaline pegmatites, metasomatized peralkaline granites, and zones of alkaline metasomatism. Among lithium-bearing micas, polylithionite shows the highest lithium oxide content of 5.5–8.0%, which corresponds to 1.5–2.0 Li atoms per formula unit. The theoretical composition of polylithionite can be expressed as KLi2Al[Si4O10]F2.
