Peralkaline rock
Peralkaline rocks include those igneous rocks which have a deficiency of aluminium such that sodium and potassium are in excess of that needed for feldspar. The presence of aegerine and riebeckite are indicative of peralkaline conditions. Examples are the peralkaline rhyolites, comendite and pantellerite, with comendite being the more felsic rock. Another example is the peralkaline granite that forms the islet of Rockall in the North Atlantic Ocean.
Peralkaline rocks are indicative of continental rift-related volcanicity as well as continental and oceanic hotspot volcanicity. Peralkaline rocks related to subduction zone volcanicity have also been reported. Peralkaline magmas likely form when fractional crystallization removes a high proportion of plagioclase relative to mafic minerals.