Origins and Nature
Chromium appeared in the universe due to explosions of white dwarfs and explosions of massive stars.
Chromium is a fairly common element in the Earth's crust - 0.03% by mass[3][4].
The main chromium compound is chromium ironstone (chromite) FeO-Cr2O3. The second most important mineral is crocoite PbCrO4.
Deposits
The largest deposits of chromium are in South Africa (1st place in the world), Kazakhstan, Russia, Zimbabwe and Madagascar. There are also deposits in Turkey, India, Armenia,[5] Brazil and the Philippines[6].
6] The main chrome ore deposits in Russia are known in the Urals (Donskoye and Saranovskoye).
Explored reserves in Kazakhstan amount to more than 350 million tons (2nd place in the world)[6]. World production in 2012 was about 9 million tons of chromium.
Geochemistry and mineralogy
The average chromium content in various igneous rocks is sharply variable. In ultrabasic rocks (peridotites) it reaches 2 kg/t, in basic rocks (basalts, etc.) - 200 g/t, and in granites tens of g/t. Clark chromium in the Earth's crust is 83 g/t. It is a typical lithophilic element and is almost all contained in minerals of the chromospinelide type. Chromium together with iron, titanium, nickel, vanadium, and manganese constitute one geochemical family.
There are three major chromium minerals: magnochromite (Mg, Fe)Cr2O4, chrompicotite (Mg, Fe)(Cr, Al)2O4 and aluminochromite (Fe, Mg)(Cr, Al)2O4. They are indistinguishable in appearance, and are inaccurately called "chromites." Their composition is variable:
Cr2O3 18-62 %,
FeO 1-18 %,
MgO 5-16 %,
Al2O3 0.2 - 0.4 (up to 33 %),
Fe2O3 2 - 30 %,
TiO2 impurity up to 2 %,
V2O5 up to 0,2 %,
ZnO up to 5 %,
MnO up to 1%; Co, Ni, etc. are also present.
Chromite proper, i.e. FeCr2O4, is relatively rare. In addition to various chromites, chromium is a part of a number of other minerals: chromic mica (fuchsite), chromic chlorite, chromic vesuvian, chromic diopside, chromic tourmaline, chromic garnet (uvarovite) and others, which often accompany ores, but have no industrial significance themselves. In exogenous conditions, chromium, as well as iron, migrates in the form of suspensions and may accumulate in clays. The most mobile form is chromates.
Physical properties
In free form - bluish-white metal with a cubic volumetric centered lattice, a = 0.28845 nm. Below 38 °C it is antiferromagnetic, above it goes to paramagnetic state (point of Neel).
Chromium has a hardness of 8.5 on the Mohs scale[7], Pure chromium is a brittle metal and breaks when struck with a hammer. It is also the hardest of the pure metals (only tungsten is harder). Very pure chromium is quite machinable.
Isotopes
Main article: Chromium isotopes
Known isotopes of chromium with mass numbers from 42 to 67 (number of protons 24, neutrons 18 to 43) and 2 nuclear isomers.
Natural chromium consists of four stable isotopes (50Cr (isotopic distribution 4.345%), 52Cr (83.789%), 53Cr (9.501%), 54Cr (2.365%)).
Among artificial isotopes, 51Cr is the longest-lived (half-life of 27 days). The half-life of the others does not exceed one day.
