What is Nepheline?
Nepheline is a silica-undersaturated feldspathoid that helps identify alkaline igneous rocks and nepheline-syenite complexes.
Mineralogy and crystal structure
Reference formula: (Na,K)AlSiO4. Nepheline is a hexagonal framework silicate with variable sodium and potassium. It is unstable in many surface environments and can alter to zeolites, cancrinite and other secondary minerals. Nepheline is classified with silicates and has a hexagonal crystal system. Typical hardness is 5.5–6 on the Mohs scale and specific gravity is approximately 2.5–2.7.
Colour, lustre and habit
Typical colours include colourless, white, grey, yellow, green. The usual prismatic, granular, massive habit and vitreous, greasy, pearly lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Nepheline forms in or is associated with alkaline nepheline syenites, volcanic lavas, pegmatites and contact zones. Important documented localities include Russia, Canada, Norway, Brazil, Greenland, Tanzania and the United States.
The feldspathoid that reveals unusual magma
Nepheline is a key geological clue: its presence signals a magma that did not contain enough silica to make ordinary quartz. A good specimen links pale crystals to an alkaline rock province.
Collecting and care
Protect crystals from acids and moisture; avoid harsh testing because alteration can weaken surfaces.
