Imperial College Rock Library

Glossary: Phonolite

    A phonolite is a fine-grained alkaline igneous rock dominated by essential alkali feldspar, usually sanidine or anorthoclase, with alkali pyroxenes, usually aegirine-augite or aegirine, and/or alkali amphiboles, and feldspathoid. Titanite is a common accessory mineral, olivine occurs but is rare. Tephritic phonolite may contain plagioclase feldspars. Alkali feldspar, and feldspathoids, such as nepheline or leucite, are the common as phenocrysts found in phonolites. They often also contain sodalite group minerals. Some phonolites exhibit tracytic textures similar to the trachytes. Phonolites are the fine-grained equivalents of feldspathoid syenite.
 
Phonolites are most commonly associated with continental rift magmatism and evolve as a crystal fractionation product of silica undersaturated basanite magmas.

Related Terms

alkaline rock, syenite

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