|Chemical formula||iron magnesium chromium oxide: (Fe, Mg)Cr2O4|
|Color||Black to brownish black|
|Crystal habit||Octahedral rare; massive to granular|
|Crystal system||Isometric; hexoctahedral|
|Mohs Scale hardness||5.5|
|Specific gravity||4.5 - 4.8|
|Refractive index||Subtranslucent to opaque|
|Other characteristics||Weakly magnetic|
Chromite is iron magnesium Chromium oxide: (Fe, Mg)Cr2O4. It is an oxide mineral belonging to the spinel group. Magnesium can substitute for iron in variable amounts; also, aluminium and ferric iron commonly substitute for chromium.
Chromite is found in peridotite from the Earth's mantle. It also occurs in layered ultramafic intrusive rocks. In addition, it is found in metamorphic rocks such as some serpentinites. Ore deposits of chromite form as early magmatic differentiates. It is commonly associated with olivine, magnetite, serpentine, and corundum. The vast Bushveld igneous complex of South Africa is a large layered mafic to ultramafic igneous body with some layers consisting of 90% chromite making the rare rock type, chromitite.
Chromite is also used as a refractory material, because it has a high heat stability.
The only ore of chromium is the mineral chromite. The two main products of Chromite refining are ferrochromium and metallic chromium, for those products the ore smelter process differs considerably. For the production of ferrochromium the chromite ore (FeCr2O4) is reduced with either aluminium or silicon in a aluminothermic reaction and for the production of pure chromium the iron has to be separated from the chromium in a two step roasting and leaching process.
In 2002 14,600,000 metric tons of chromite have been mined. The largest producers have been South Africa (44%) India (18%), Kazakhstan (16%) Zimbabwe (5%), Finland (4%) Iran (4%) and Brazil (2%) with several other countries producing the rest of less than 10% of the world production. 
In Pakistan, Chromite is mined from the ultramafic rocks in mainly the Muslim Bagh area of Zhob District of Balochistan. Most of the chromite is of metallurgical grade with Cr2O3 averaging 40% and a chrome to iron ratio of 2.6:1. Afghanistan has significant deposits of high grade Chromite ore.
Recently, the biggest user of Chromite ore has been China, importing large quantities from South Africa, Pakistan and other countries. The concentrate is used to make ferrochromium, which is in turn used to make Steel.
|silvery metallic |
|Name, symbol, number||chromium, Cr, 24|
|Element category||transition metal|
|Group, period, block||6, 4, d|
|Standard atomic weight||51.9961(6) g·mol−1|
|Electron configuration||[Ar] 3d5 4s1|
|Electrons per shell||2, 8, 12, 2 (Image)|
|Density (near r.t.)||7.19 g·cm−3|
|Liquid density at m.p.||6.3 g·cm−3|
|Melting point||2180 K, 1907 °C, 3465 °F|
|boiling point||2944 K, 2671 °C, 4840 °F|
|Heat of fusion||21.0 kJ·mol−1|
|Heat of vaporization||339.5 kJ·mol−1|
|Specific heat capacity||(25 °C) 23.35 J·mol−1·K−1|
|Oxidation states||6, 5, 4, 3, 2, 1, -1, -2 |
(strongly acidic oxide)
|Electronegativity||1.66 (Pauling scale)|
|Ionization energies |
|1st: 652.9 kJ·mol−1|
|2nd: 1590.6 kJ·mol−1|
|3rd: 2987 kJ·mol−1|
|Atomic radius||128 pm|
|Covalent radius||139±5 pm|
|Crystal structure||body-centered cubic|
|Magnetic ordering||AFM (rather: SDW)|
|Electrical resistivity||(20 °C) 125 nΩ·m|
|Thermal conductivity||(300 K) 93.9 W·m−1·K−1|
|Thermal expansion||(25 °C) 4.9 µm·m−1·K−1|
|Speed of sound (thin rod)||(20 °C) 5940 m/s|
|Young's modulus||279 GPa|
|Shear modulus||115 GPa|
|Bulk modulus||160 GPa|
|Vickers hardness||1060 MPa|
|Brinell hardness||1120 MPa|
|CAS registry number||7440-47-3|
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Chromium (pronounced /ˈkroʊmiəm/) is a chemical element which has the symbol Cr and atomic number 24. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable. The name of the element is derived from the Greek word "chrōma" (χρωμα), meaning color, because many of its compounds are intensely colored. It was discovered by Louis Nicolas Vauquelin in the mineral crocoite (lead chromate) in 1797. Crocoite was used as a pigment, and after the discovery that the mineral chromite also contains chromium this latter mineral was used to produce pigments as well.
Chromium was regarded with great interest because of its high corrosion resistance and hardness. A major development was the discovery that steel could be made highly resistant to corrosion and discoloration by adding chromium and nickel to form stainless steel. This application, along with chrome plating (electroplating with chromium) are currently the highest-volume uses of the metal. Chromium and ferrochromium are produced from the single commercially viable ore, chromite, by silicothermic or aluminothermic reaction or by roasting and leaching processes. Although trivalent chromium (Cr(III)) is required in trace amounts for sugar and lipid metabolism in humans and its deficiency may cause a disease called chromium deficiency, hexavalent chromium (Cr(VI)) is a toxin and a carcinogen, so that abandoned chromium production sites need environmental cleanup.
Chromium is the 21st 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. 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 of total chromium of which 30 µg is present as Cr(VI).
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.
Though native chromium deposits are rare, some native chromium metal has been discovered. 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.
Naturally occurring chromium is composed of three stable isotopes; 52Cr, 53Cr and 54Cr with 52Cr being the most abundant (83.789% natural abundance). Nineteen radioisotopes have been characterized with the most stable being 50Cr with a half-life of (more than) 1.8x1017 years, and 51Cr with a half-life of 27.7 days. All of the remaining radioactive isotopes have half-lives that are less than 24 hours and the majority of these have half-lives that are less than 1 minute. This element also has 2 meta states.
53Cr is the radiogenic decay product of 53Mn. Chromium isotopic contents are typically combined with manganese isotopic contents and have found application in isotope geology. Mn-Cr isotope ratios reinforce the evidence from 26Al and 107Pd for the early history of the solar system. Variations in 53Cr/52Cr and Mn/Cr ratios from several meteorites indicate an initial 53Mn/55Mn ratio that suggests Mn-Cr isotopic composition must result from in-situ decay of 53Mn in differentiated planetary bodies. Hence 53Cr provides additional evidence for nucleosynthetic processes immediately before coalescence of the solar system.
The isotopes of chromium range in atomic mass from 43 u (43Cr) to 67 u (67Cr). The primary decay mode before the most abundant stable isotope, 52Cr, is electron capture and the primary mode after is beta decay.
|Oxidation states |
of chromium[note 1]
Chromium is a member of the transition metals, in group 6. Chromium(0) has an electronic configuration of 4s13d5, due to the lower energy of the high spin configuration. Chromium exhibits a wide range of possible oxidation states. The most common oxidation states of chromium are +2, +3, and +6, with +3 being the most stable. +1, +4 and +5 are rare.
Chromium compounds of oxidation state +6 are powerful oxidants. All (except the hexafluoride and chromium hexacarbonyl) stable chromium compounds of the oxidation state +6 contain oxygen as ligand, for example the chromate (CrO42-) and Chromyl chloride (CrO2Cl2).
Chemistry and compounds
The oxidation state 3+ is the most stable one and therefore a large number of chromium(III) compounds is known. Chromium(III) can be obtained by dissolving chromium in acids like hydrochloric acid or sulfuric acid. The aluminium(III) (ion radius 0.50 Å) and chromium(III) (ion radius 0.63 Å) can replace each other in some compounds, for example chrome alum and alum. Another example is aluminium oxide (corundum, Al2O3) where by replacement, the red colored ruby is formed.
Chromium tends to form complexes, for example with water molecules (hydrates); the chromium ions in water are usually octahedrally coordinated. The commercially available chromium(III) chloride hydrate is the dark green complex [CrCl2(H2O)4]Cl, but two other forms are known, viz., pale green [CrCl(H2O)5]Cl2 and violet [Cr(H2O)6]Cl3. If water-free green chromium(III) chloride is dissolved in water then the green solution turn violet after some time. This color change is due to the substitution of water for chloride in the inner coordination sphere. This kind of reactions is also observed in chrome alum solutions and some other water soluble chromium(III) salts, and the reverse reaction can be induced by heating the solution. The chromium(III) hydroxide Cr(OH)3 shows amphoteric reactions and dissolves in acid water by forming [Cr(H2O)6]3+ and in basic water by forming [Cr(OH)6]3-. By heating the chromium(III) hydroxide it is transformed into the green chromium(III) oxide (Cr2O3), which is the stable oxide (melting point of 2275 °C) with the crystal structure identical to that of corundum.