a magmatic rock that is relatively rich in the alkali metals sodium and potassium. The mineral composition of alkali rocks is typified by nepheline and other feldspathoids (so-dalite, cancrinite, and leucite), as well as by alkali pyroxenes and by amphiboles, such as aegerine and arfvedsonite.
Alkali rocks are divided into three groups according to silicon content: ultrabasic rocks—ijolites, melteigites, and unites (40–45 percent SiO2); gabbroid rocks—theralites and essexites (45–50 percent); and syenitic rocks—alkali syenites and nepheline syenites (more than 50 percent SiO2).
In the earth’s crust, alkali rocks are found chiefly in small (to 50–100 sq km) intrusive bodies of nepheline syenites and alkali syenites or their effusive varieties (nephelinites, phonolites, leucitites, and tephrites) within volcanic alkali-basaltic associations on the continents and oceanic islands. Ijolites and melteigites are typical of complex alkali-ultrabasic massifs, where pyroxenites and olivinites predominate. Major carbonatite deposits have been identified in these massifs. The nepheline syenites are divided into miascite, in which potassium and sodium together are less than aluminum, and the rarer agpaite, in which potassium and sodium together are more than aluminum. Various minerals rich in Zr, Ti, Nb, and Sr are typical of agpaitic rocks. The apatite and nepheline ores in the USSR (the Khibiny on the Kola Peninsula) and in Greenland, Canada, Brazil, and South Africa are also associated with alkali rocks.
REFERENCESGlavneishie provintsii i formatsii shchelochnykh porod. Moscow, 1974.
Shchelochnye proody: Sb. st. Moscow, 1976. (Translated from English.)
L. S. BORODIN