phonolite

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phonolite A fine-grained, porphyritic, extrusive, igneous rock consisting of essential alkali feldspar (sanidine or anorthoclase), nepheline, sodic pyroxene, and sodic amphibole (with or without iron-rich olivine), with accessory sphene, apatite, and zircon. Where present, phenocrysts can consist of alkali feldspar, sodic pyroxene, or sodic amphibole. Phonolites are the extrusive equivalent of nepheline syenites and are found on off-axis ocean islands and in continental regions subjected to anorogenic upwarping and rifting. The name, from the Greek phone meaning ‘sound’, refers to the fact that the rock rings like a bell when struck with a hammer.

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