Chromium is the 24th most abundant element in Earth's crust with an average concentration of 100 ppm. Chromium compounds are found in the environment, due to erosion of chromium-containing rocks and can be distributed by volcanic eruptions. The concentrations range in soil is between 1 and 3000 mg/kg, in sea water 5 to 800 µg/liter, and in rivers and lakes 26 µg/liter to 5.2 mg/liter. Chromium is mined as chromite (FeCr2O4) ore. About two-fifths of the chromite ores and concentrates in the world are produced in South Africa, while Kazakhstan, India, Russia, and Turkey are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern Africa.
Although rare, deposits of native chromium exist. The Udachnaya Pipe in Russia produces samples of the native metal. This mine is a kimberlite pipe, rich in diamonds, and the reducing environment helped produce both elemental chromium and diamond.
The relation between Cr(III) and Cr(VI) strongly depends on pH and oxidative properties of the location, but in most cases, the Cr(III) is the dominating species, although in some areas the ground water can contain up to 39 µg/liter of total chromium of which 30 µg/liter is present as Cr(VI).
Other articles related to "occurrence, occurrences":
... Co-occurrence networks are generally used to provide a graphic visualization of potential relationships between people, organizations, concepts or other entities represented within written ... The generation and visualization of co-occurrence networks has become practical with the advent of electronically stored text amenable to text mining ... By way of definition, co-occurrence networks are the collective interconnection of terms based on their paired presence within a specified unit of text ...
... Typical occurrence is as an alteration and vesicle filling mineral in basalts ... The calcium variety was first described in 1821 for an occurrence in Dalsnipa, Sandoy, Faroe Islands ... The sodium variety was described in 1997 for an occurrence in Chojabaru, Nagasaki Prefecture, Japan ...
... Mediterranean North African cities of Ceuta and Melilla (respectively registered 10,410 and 7,982 occurrences), due to immigration from Morocco ... boys, thus it is not unusual to encounter a man named Mohamed Mohamed Mohamed the first occurrence is the name, the second occurrence is the paternal surname, and the third ...
Famous quotes containing the word occurrence:
“One is absolutely sickened, not by the crimes that the wicked have committed, but by the punishments that the good have inflicted; and a community is infinitely more brutalised by the habitual employment of punishment than it is by the occasional occurrence of crime.”
—Oscar Wilde (18541900)