Alkaline rocks—undersaturated

Abstract

Nepheline syenite is a pale-colored, coarse-grained rock composed essentially of alkali feldspar (ca. 70%) and nepheline (ca. 20%) with minor proportions of dark minerals such as sodic pyroxene, sodic hornblende, or biotite; its volcanic equivalent is phonolite. It was first described from Foya (hence the original name, foyaite ) in S Portugal by Blum (1861). Rosenbusch (1877) proposed the alternative name nepheline syenite. An historical review, and a systematic discussion of varieties and related rocks is given by Johannsen (1938). Use of the term foyaite has been confused by its adoption by some authors as a name for any nepheline syenite that shows fluidal textures. Common accessory minerals are sphene (titanite), apatite, and iron ore, but nepheline syenites may carry a large range of uncommon minerals containing elements such as Ti, Zr, Nb, REE, U, Th ( Vlasov et al., 1966). The feldspathoid nepheline (NaAlSiO 4) reflects an inadequate silica content in the rock...