What is a phonolite in geology ?

Phonolite : definition

Streckeisen diagrams (or QAPF diagrams) - Classification of plutonic and volcanic rocks
A phonolite is a volcanic rock undersaturated in silica with a sound slab flow (hence its name), commonly covered with a clear patina. It belongs to the family of feldspathoid syenites, and consists of microlites of alkaline feldspars (sanidine, anorthose, orthoclase), feldspathoids (nepheline, sometimes leucite, haüyne or noséane), pyroxene (aegirine or aegirinic augite), sometimes sodium amphibole, in a scanty glassy matrix. The varieties are named according to the nature of the feldspathoid: hauÿne phonolite, sodalite, nepheline...

The plutonic equivalent of phonolite is nepheline syenite.

Phonolites form viscous lavas, common in the volcanism of the French Massif Central. They were used as roof coverings (lauzes) in the traditional rural habitat of Auvergne.


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