English: Nepheline syenite, possibly from the Precambrian of Wisconsin, USA. (public display, Geology Department, Wittenberg University, Springfield, Ohio, USA)

Nepheline syenite is a crystalline-textured, alkaline, intrusive igneous rock. Unlike an ordinary syenite, this rock has an appreciable nepheline content. Nepheline is a feldspathoid mineral - a "feldspar-like mineral". Feldspathoids have more aluminum and less silicon than the feldspars. They are less stable than feldspars and usually decompose in strong acids - they don't effervesce, but become gels.

Nepheline is an aluminum-rich version of sodium feldspar - it has the chemical formula NaAlSiO4 (sodium aluminosilicate). The formula has also been cited as (Na,K)AlSiO4 (sodium potassium aluminosilicate).

Nepheline has a nonmetallic luster, is usually whitish to grayish in color, is moderately hard (H = 5.5 to 6 on the Mohs Hardness Scale), forms hexagonal crystals, and has poor cleavage. It is an important rock-forming mineral in many syenites. It is usually mixed with albite feldspar (= sodium feldspar, NaAlSi3O8).

Geologic unit & locality: unknown, but possibly from the Mesoproterozoic-aged Wausau Syenite Complex of Wisconsin, USA