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phonolite
Phonolite is an uncommon extrusive igneous rock, 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 "sounding stone" because of the metallic sound it produces if an unfractured plate is hit; hence the English name ''clinkstone''.
== Genesis ==
Unusually, phonolite 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 has no 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 usually occur alongside phonolites. Low-degree partial melting of underplates of granitic material in collisional orogenic belts may also produce phonolites.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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