Amphibolite

Amphibolite is defined as rock, which by the metamorphic transformation of basalt, gabbro or other equivalent whose deep meta- Basiten arose under pressure and temperature conditions of the amphibolite facies (T ≈ 550-700 ° C, P ≈ 200-1200 MPa).

• 4.1 Notes and references

Definition

Amphibolite in the proper sense is up to 50% by volume of representatives of the amphibole (eg hornblende, pargasite or tschermakite ), plagioclase (15-40 %), garnet, epidote, biotite, quartz, or olivine and ores such as magnetite and pyrite. The relative and absolute mineral proportions depend on both the chemical composition of the parent rock and the metamorphic grade. Thus occurs in the lower amphibolite facies ( ie at temperatures at the lower end of the spectrum) epidote, while forming in the upper amphibolite facies garnet and clinopyroxene. The typical Ti phase for rocks of the amphibolite is the mineral ilmenite. From marls and tuffites suitable composition may result in a similar to the amphibolite rock, which is in contrast to the igneous rocks formed from ortho- amphibolite referred to as para- amphibolite.

The use of the term " amphibolite " for amphibolreiche ( with a share of up to 30% by volume of amphibole ) rocks that have not arisen from basalt is controversial. For this purpose, alternative names, such as amphibole gneiss, are used. After Wimmenauer (1985 ), the term Amphibolgneis and Amphibolgehalten about 80 percent the name Amphibolschiefer is recommended for Feldspatgehalten of over 50 percent.

History

The rock name goes back to Alexandre Brongniart, the amphibolite in the Journal des Mines (Vol. XXXIV) first described in 1827. Bernhard von Cotta carries it in his work The rock doctrine of 1862 and Franz Loewinson - Lessing explained in Amphibolitgesteine ​​in Petrographical Encyclopedia (1893 ) as " general blanket term for rocks with substantial Amphibolgehalt (regardless of the structure and mode of origin ) ".

Properties

The color of amphibolite varies with the mineral constituents. Often, however, tones of black and gray to dark green or black-and- white patterned at high Plagioklasanteil. It is mainly used for flooring and wall cladding. It also Dechselklingen were produced in the Neolithic period ( shoe lasts wedges ).

Occurrence

In Canada, amphibolites have been dated from the Nuvvuagittuq greenstone belt known as an age of about 4.28 billion years - at the time the oldest known rocks on earth. In Germany amphibolite occurs in the Saxon Erzgebirge, in the Fichtel Mountains and the Black Forest. The occurrence of dark rock is eponymous for the Dunkelsteinerwald in Austria.