Charnockite are rocks with granite similar composition and magmatic structure. The name Charnockite refers to the name of the founder of Calcutta, Job Charnock, of whose mausoleum is built of a Charnockite. The first scientific description in 1893 (Journal of the Asiatic Society of Bengal ) and 1900 ( Memoirs of the Geological Society of India) by the authorities responsible for India country geologist Thomas Henry Holland, named after the rock from the deposits used for the burial at St coined. Thomas Mount.
Mineral composition and occurrence
Perform in their mineral composition K-feldspar, orthopyroxene, plagioclase and quartz. Hydrous silicates such as hornblende and mica are missing. Especially typical are the honey- yellow to green color of the potassium feldspars and the unusual oiliness of the feldspars. Charnockite come mainly in deeply eroded Precambrian basic mountains of Africa, of Antarctica, India and the north-eastern South America. In Central Europe there is no such rock deposits, only in Scandinavia, there are smaller deposits.
In petrology, there was until recent times the view that the Charnockite among the granites, ie are of magmatic origin. Meanwhile, it was demonstrated that at least a part of the Charnockite due to metamorphic processes. Charnockite arise in the middle of the earth's crust at a high temperature near 650 to 900 ° C and a pressure of 3-6 kbar. They are surrounded by metamorphic rocks from which they were melted at high temperatures. This led to a separation of hydrous magma with low viscosity that rose from the place of higher crustal melting in areas of a water-poor, tough residual melt that crystallized to a Charnockite.
The metamorphic Charnockite occur when flow containing CO2 fluids along shear zones or slate surfaces. This will give the thus partially converted output rocks a spotted or close engined appearance; in the English technical literature, this is referred to as incipient charnockite. By reducing the CO2 fluids graphite and other microscopic minerals are precipitated at the boundaries of mineral grains and microcracks. With these inclusions, follow the most specific crystallographic directions, the feldspars get a greasy luster, orthopyroxene gets a metallic sheen and quartz shows by trapped finest Rutilnädelchen a blue color. The isotope study of fluid inclusions in crystals and graphite, is a source of CO2 from the upper mantle.
Occurrence of charnockites are mined in India, Brazil and Nigeria and exported to Europe and North America as a stone. Especially in the 1960s Charnockite were used in Germany as grave stones. In the countries of origin, they are also partly used for gravel and road construction. The base for the world-famous Christ the Redeemer statue in Rio de Janeiro consists of a Charnockite, the Verde Ubatuba.
Charnockite can brighten as polished natural stones at an outdoor use, such as the Verde Ubatuba. In general, these rocks are frost - and acid-resistant and polished.