Nephelinites

Nephelinites are a group of volcanic rocks that are undersaturated with silica. This means they contain minerals like nepheline or feldspathoids, which are rich in sodium and aluminum and crystallize in the absence of free silica. They are typically dark-colored, fine-grained rocks, often appearing as extrusive equivalents of intrusive rocks called essexites. The composition of nephelinites is characterized by a high alkali content, particularly sodium and potassium, and a relatively low calcium content. Common minerals found in nephelinites include nepheline, augite, olivine, plagioclase feldspar (often sodic), and sometimes leucite or melilite. Accessory minerals can include magnetite, ilmenite, and apatite.

Nephelinites are found in various geological settings, often associated with continental rifts, intraplate volcanic regions, and sometimes in oceanic islands. They can form lava flows, dikes, and sills. Their formation is thought to occur from the partial melting of mantle peridotite under conditions of low pressure and high water content, or from the fractional crystallization of more primitive magmas. The presence of feldspathoid minerals distinguishes them from more common volcanic rocks like basalts. Their occurrence is relatively rare compared to basalts and andesites.