What is a nepheline syenite in geology ?

Nepheline syenite : definition

Streckeisen diagrams (or QAPF diagrams) - Classification of plutonic and volcanic rocks
It is a highly undersaturated grainy igneous rock, consisting mainly of alkali feldspars, usually perthitic microcline (on average 70%), and feldspathoids (mainly nepheline, more rarely sodalite, analcime or hauyne (on average 20 %), biotite, amphibole (riebeckite) and pyroxene  like aegirine, sometimes cancrinite. Apatite, titanitezirconfluorite and sometimes calcite, corundum and eudialyte are abundant, defining many varieties and allowing the exploitation of these rocks as ore of zirconium, rare earth elements or phosphorus.

Nepheline syenites draw small intrusive massifs associated with syenites, carbonatites or peridotites. Pegmatitic facies with large crystals, called pegmatitoids, frequently line the edges. Their volcanic equivalent is the phonolite.

The most famous deposits are those of the Kola Peninsula (Russia), Illimaussaq (Greenland) and the Arendal region (Norway).
Larvikite from Norway
Larvikite from Norway