Nepheline Syenite

a magmatic alkali rock composed of nepheline, alkali feldspars (sanidine, orthoclase, or microcline), biotite, and alkali amphiboles and pyroxenes. It also contains zirconium-bearing silicates (zircon, eudialyte) and titanium-bearing silicates (sphene, lamprophyllite), as well as minerals containing F, P₂O₅, CO₂ (cancrinite, calcite, apatite, fluorite) and rare earths (rinkolite, loparite). Nepheline syenite contains 50–56 percent SiO₂, 19–24 percent Al₂O₃, a small quantity of Ca and Mg, and 15–17 percent K and Na.

There are numerous varieties of nepheline syenites, including (1) chibinites, greenish gray coarse-grained pyroxene nepheline syenites, with pegmatic texture; (2) rischorites, yellowish gray or greenish gray rocks with poikilitic texture; (3) lujauvrites, greenish black rocks with marked trachytic texture; (4) miascites, micaceous lamellar nepheline syenites found in the Urals; (5) mariupolites, distinctive albite-rich nepheline syenites; and (6) saibarites, melanocratic nepheline syenites found in Siberia and elsewhere. In addition to the USSR, nepheline-syenite deposits are also found in the vicinity of Oslo, Norway, and in southern Greenland, in Canada, in southern Africa, and on Madagascar. Nepheline syenites occur in association with apatites (Kola Peninsula), graphite (Saian Mountains), cryolite (Greenland), and a number of rare elements.