peralkaline

Peralkaline is a geochemical term applied to certain igneous rocks in which the combined molar abundances of sodium oxide (Na2O) and potassium oxide (K2O) exceed that of aluminum oxide (Al2O3). A commonly used measure is the peralkaline index (PI) = (Na2O + K2O) − Al2O3 in molar proportions; PI > 0 indicates a peralkaline composition. By contrast, peraluminous rocks have Al2O3 > (Na2O + K2O), and metaluminous rocks lie in between.

Peralkaline rocks are typically silica-undersaturated and often nepheline-bearing or alkali-rich, with mineral assemblages that can include nepheline, aegirine-augite, riebeckite, leucite, and cancrinite. Quartz is usually absent or rare in strictly peralkaline compositions, although some peralkaline granitoids may contain limited quartz.

Geochemically and petrogenetically, peralkaline rocks form when alkaline magmas accumulate an excess of alkalis relative to aluminum during differentiation. They are commonly associated with continental rift zones and intraplate volcanic provinces or other settings that promote alkaline magmatism and low silica activity. These rocks can display a range of textures from coarse-grained to fine-grained, and may occur as plutonic bodies or dike swarms.

Economic and petrological interest centers on their distinctive mineralogy and potential hosts for certain rare minerals and rare earth elements in some districts. Peralkaline rocks thus help illuminate late-stage differentiation processes in alkaline magmatic systems and the tectonic contexts that produce extreme alkali enrichment.