English

Noun

phonolite

1. a light-coloured rock of volcanic origin composed mostly of alkali feldspars

Phonolite is an evolved lava which is considered as forming in shallow magma chambers. Phonolite is an igneous, volcanic (extrusive) rock, of intermediate (between felsic and mafic) composition, with aphanitic to porphyritic texture. Mineral assemblage is usually abundant feldspathoids (nepheline, sodalite, hauyne, leucite and analcite) and alkali feldspar (sanidine, anorthoclase or orthoclase), and rare sodic plagioclase. Biotite, sodium rich amphiboles and pyroxenes along with iron rich olivine are common minor minerals. Accessory phases include titanite, apatite, corundum. zircon, magnetite and ilmenite. Phonolites are silica under-saturated, as illustrated by the position of phonolite in the TAS classification and QAPF diagrams.

Phonolite is a fine-grained equivalent of nepheline syenite, and the genesis of such magmas is discussed in the treatment of that rock type.

Phonolite specimens may make a flat musical sound when struck - hence the common name clinkstone.

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