PHONOLITE, in petrology, a group of alkaline lavas contain ing much nepheline and sanidine felspar (Gr. cbcovi?, sound. and Small rounded enclosures of glass are often numerous in them. The pyroxenes may be pale green diopside, dark green aegirine augite, or blackish green aegirine (soda iron pyroxene), and in many cases are complex, the outer portions being aegirine while the centre is diopside. Fine needles of aegirine are often found in the ground-mass.
The chemical analyses of phonolites given herewith show that they are very rich in alkalis and alumina with only a moderate amount of silica, while lime, magnesia and iron oxides are present only in small quantity. They have a close resemblance in these respects to the nepheline-syenites of which they provide the effusive types. Most of these rocks are of Tertiary or Recent age, but in Scotland Carboniferous phonolites occur in several localities, e.g., Traprain in Haddingtonshire, in the Eildon Hills and in Renfrewshire; in Brazil phonolites belonging to the same epoch are also known and there are several districts in Europe where Tertiary or Recent phonolites occur in considerable num bers, as in Auvergne (Mont Dore), the Eifel and Bohemia. The
Wolf Rock, off the S. of Cornwall and the site of a well-known lighthouse, is the only mass of phonolite in England; it is sup posed to be the remains of a Tertiary lava or intrusion. In the United States phonolites occur in Colorado (at Cripple Creek) and in the Black Hills of South Dakota.
Leucite occurs in place of nepheline in a small group of phono lites (the leucite-phonolites), known principally from Rocca Mon fina and other places near Naples. When sanidine, nepheline and leucite all occur together in a volcanic rock it is classed among the leucitophyres (see PETROLOGY).