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Phonolite is a rare extrusive volcanic rock of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grain) to porphyritic (mixed fine- and coarse-grain).
The name phonolite comes from the Greek meaning (more or less) "sounding stone" because of the metallic sound it produces if an unfractured plate is hit; hence the English name clinkstone.
Phonolite is unusual in that it forms from magma with a relatively low silica content, generated by low degrees of partial melting (less than 10%) of highly aluminous rocks of the lower crust such as tonalite, monzonite and metamorphic rocks. Melting of such rocks to a very low degree promotes the liberation of aluminium, potassium, sodium and calcium via melting of feldspar, with some involvement of mafic minerals. Because the rock is silica undersaturated, it is free of quartz or other silica crystals, and is dominated by low-silica feldspathoid minerals more than feldspar minerals.
A few geological processes and tectonic events can melt the necessary precursor rocks to form phonolite. These include intracontinental hotspot volcanism, such as may form above mantle plumes covered by thick continental crust. A-type granites and alkaline igneous provinces are usually found alongside phonolites. Phonolites may also be produced by low degree partial melting of underplates of granitic material in collisional orogenic belts.
Phonolites, as they are products of low degree partial melts, are silica undersaturated, and have feldspathoids in their normative mineralogy.
Mineral assemblages in phonolite occurrences are 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.
Nepheline syenites and phonolites occur widely distributed throughout the world in Canada, Norway, Greenland, Sweden, the Ural Mountains, the Pyrenees, Italy, Eifel in Germany, Brazil, the Transvaal region, the Magnet Cove igneous complex of Arkansas, the Beemerville Complex of New Jersey, as well as on oceanic islands such as the Canary Islands.
- Devils Tower, a striking example of columnar jointed phonolite
- Dunedin, New Zealand
- Bořeň, northwestern Czech Republic
- Hoodoo Mountain, northwestern British Columbia, Canada
- Jebel Nefusa, Libya
- Teide, a stratovolcano on the island of Tenerife
- Mont Gerbier de Jonc South East France
Phonolites can be of interest as dimension stone or as aggregate for gravels.
Rarely, economically mineralised phonolite-nepheline syenite alkaline complexes can be associated with rare earth mineralisation, uranium mineralisation and phosphates, such as at Phalaborwa, South Africa.
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