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Tonalite

tonalite

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The tonalite is an intrusive, igneous rock which is plutonic and has a phaneritic texture. It has a felsic composition meaning that it is highly composed of light elements which include aluminium, silicon, sodium, potassium and oxygen. It also has about 75% of plagioclase and orthoclase minerals with quartz occupying 20% of the rock.

The term “tonalite” was gotten from the locality of the Tonalites at the Tonales pass, Italian Alps. Although, the tonalite is widespread, it is still a rare plutonic rock referred to as a granitoid, which in turn is also referred to as a plagiogranite and trondjhemite. Granitoids are plutonic rocks that contain 20% to 60% quartz and are usually mistaken to be granite. They are usually a moderately equivalent mixture of alkali feldspar, quartz and the plagioclase feldspar. 

The alkali feldspar and the plagioclase are the two criteria to determine when a granitoid is a tonalite. Granitoids that have 90% of alkali feldspar are alkali-felspar granites. Granitoids with about 65% of alkali feldspar are syenogranites. Granitoids with the two feldspars in equal amounts are the monzogranites. Granitoids with 65% of plagioclase are granodiorites, while those containing 90% of plagioclase are the tonalities. When the alkali feldspar is extracted from the proper granite, it changes to a granitoid and afterwards, it becomes tonalite with quartz as the abundant mineral. 

Mostly, tonalities are composed of dark minerals such as pyroxene, but when it is white, it is a “leucocratic” and this categorizes it as a plagiogranite. On the other hand, the trondjhemite is a tonalite variety which is deficient in orthoclase. It is also a plagiogranite and its dark mineral is the biotite. The name “trondjhemite” was derived from “Trondheim”, one of the biggest cities in Norway. A variety of the tonalite is the dacite which is an extrusive rock.

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