nephelinite

Nephelinite is a rare alkaline igneous rock, either volcanic or plutonic, distinguished by silica undersaturation and the presence of nepheline as an essential mineral. In nephelinite, quartz is absent or very limited, and the mineral assemblage typically includes nepheline, olivine, and pyroxene (often clinopyroxene), with feldspathoids other than nepheline sometimes present. The texture can range from fine-grained volcanic glass to coarser-grained plutonic equivalents.

Chemically, nephelinite is high in alkalis relative to silica, with low SiO2 content for an alkaline rock class. It often contains significant amounts of alkalis (sodium and potassium) and magnesium, and it may host accessory minerals such as magnetite, apatite, and various feldspathoid minerals (in addition to nepheline, leucite may appear in some varieties). The rock is part of the silica-undersaturated, alkaline end of the basalt–komatiite–nephelinite spectrum, and quartz is explicitly not a defining phase.

Geologically, nephelinite forms from mantle-derived magmas that have undergone low-degree partial melting or specific fractional crystallization paths favoring feldspathoid stability. It is commonly associated with intraplate volcanism, continental rift zones, or volcanic centers near oceanic settings, where alkalic magmas can pond and evolve. Its occurrence is relatively sporadic worldwide, making it a diagnostic marker for particular mantle source features and melting conditions.

In classification, nephelinite is contrasted with other alkaline rocks such as phonolite, tephrite, and basanite, primarily by the essential presence of nepheline and the overall silica-undersaturated character. Because of its distinctive mineralogy and chemistry, it provides insights into mantle processes and alkaline magmatism.