nefelina

Nefelina, the Italian name for nepheline, is a feldspathoid mineral with the chemical composition NaAlSiO4. It is a primary mineral in silica-undersaturated, alkaline igneous rocks and is an important component of nepheline-bearing rocks such as nepheline syenite and related phonolites. The name derives from Greek nephos, cloud, in reference to its often cloudy appearance in some forms.

Crystallography and appearance: Nepheline crystallizes in the hexagonal system. It commonly forms colorless to pale gray or yellowish crystals and is transparent to translucent, with a vitreous luster. The mineral has a Mohs hardness of about 5.5 and a white to light-colored streak. It typically shows poor to indistinct cleavage and can exhibit a conchoidal fracture in hand specimens.

Geologic occurrence: Nepheline occurs in silica-undersaturated alkaline rocks, especially nepheline syenites, phonolites, and certain mantle-derived ultramafic to felsic rocks that have crystallized under low silica activity. It commonly coexists with alkali feldspars, leucite, cancrinite, melilite, and various oxides, and may appear as large crystals or as a constituent of granular rock masses.

Significance and uses: As a major constituent of nepheline syenites, nefeflina is of economic interest as a source of alumina and as a flux in the glass and ceramic industries through processing of nepheline-bearing rocks. In geology, nepheline is used as a diagnostic mineral for identifying silica-undersaturated igneous environments and for petrogenetic modeling of alkali-rich magmatism.

Taxonomy: Nepheline belongs to the feldspathoid group, distinguished from feldspars by its low silica activity and distinct crystal structure.