Tonalite (from the Tonale Pass in Italy ) is an igneous plutonic rock of acidic composition , a normal series of alkalinity from the granodiorite family. Previously, this name was often used as a synonym for quartz diorite, however, the current classification of IUGS defines tonalite as a rock containing more than 20% quartz, and quartz diorite as containing from 5% to 20%. It can be defined as an essentially plagioclase variety of granodiorite. The main minerals of the tonalite, in order of decreasing the volume content - plagioclase , quartz , hornblende , biotite . Accessory minerals - apatite , zircon , garnet , titanite , etc.
By the amount of silica belongs to moderately acidic rocks. The average chemical composition of SiO 2 is 63-68%, TіO 2 is 0.2-1%, Al 2 O 3 is 13-18%, Fe 2 O 3 is 1-3.5%, FeO is 1-5%, MgO is 1-3.5%, CaO 3- 6%, Na 2 O 3-5%, K 2 O 0.8-2% [1] . Being one of the oldest rocks on Earth , tonalite is important for geological studies , since by analyzing the degree of decay of uranium-238 in a rock, one can determine the age of continental plates .
Also, tonalite is used in construction , as a building and facing stone .
Notes
- ↑ Petrographic Code of Russia. - SPb .: VSEGEI, 2008. - P. 144. - 200 p. - 1 500 copies - ISBN 978-5-93761-106-2