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Tonalite is an igneous, plutonic (intrusive) rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with 10% or less alkali feldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes are common accessory minerals.
In older references tonalite is sometimes used as a synonym for quartz diorite, however the current IUGS classification defines tonalite as having greater than 20% quartz and quartz diorite with from 5 to 20% quartz.
Trondhjemite is an orthoclase-deficient variety of tonalite with minor biotite as the only mafic mineral, named after Norway's third largest city, Trondheim.
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- This page was last modified on 1 October 2007, at 20:14.
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