Minerals & Mining

Chromium

Content Cover Image

Pure chromium piece, about 20 grams. Original size in cm: 2 x 2. Source: images-of-elements.com

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Next Element: Manganese
24

Cr

51.9961
Physical Properties
Color silvery-white
Phase at Room Temperature solid
Density (g/cm3) 7.14
Hardness (Mohs) 9
Melting Point (K) 2130.2
Boiling Point (K) 2963
Heat of Fusion (kJ/mol) 15.3
Heat of Vaporization (kJ/mol) 347
Heat of Atomization (kJ/mol) 397
Thermal Conductivity (J/m sec K) 93.9
Electrical Conductivity (1/mohm cm) 77.519
Source Chromite (oxide)
Atomic Properties
Electron Configuration [Ar]4s13d5
Number of Isotopes 4
Electron Affinity (kJ/mol) 64.3
First Ionization Energy (kJ/mol) 652.8
Second Ionization Energy (kJ/mol) 1592
Third Ionization Energy (kJ/mol) 2987.2
Electronegativity 1.66
Polarizability (Å3) 11.6
Atomic Weight 51.9961
Atomic Volume (cm3/mol) 7.3
Ionic Radius2- (pm) ---
Ionic Radius1- (pm) ---
Atomic Radius (pm) 128
Ionic Radius1+ (pm) ---
Ionic Radius2+ (pm) 90.5
Ionic Radius3+ (pm) 75.5
Common Oxidation Numbers +3,+6,-2,-1
Other Oxid. Numbers +1, +2,+4,+5
Abundance
In Earth's Crust (mg/kg) 1.02×102
In Earth's Ocean (mg/L) 3×10-4
In Human Body (%) 0.00002%
Regulatory / Health
CAS Number 7440-47-3
OSHA Permissible Exposure Limit (PEL) TWA: 1 mg/m3
OSHA PEL Vacated 1989 TWA: 1 mg/m3
NIOSH Recommended Exposure Limit (REL) TWA: 0.5 mg/m3
IDLH: 250 mg/m3
Sources:
Mineral Information Institute
Jefferson Accelerator Laboratory
EnvironmentalChemistry.com
 

Chromium is a hard, bluish metallic element (Cr) with an atomic number of 24. In the mid-1700’s, chemical analysis of a mineral from Siberia showed that it contained lead. This mineral, crocoite (PbCrO4, lead chromate), was known as “red lead” because of the beautiful orange-red color of its crystals. It also contained another, then-unknown material. This material was identified as chromium oxide (CrO3) by Louis-Nicholas Vauquelin. In 1797, he heated this oxide with charcoal to remove the oxygen (chemists call this reaction a reducing process) which left the metal chromium. caption Chromite, an ore containing Chromium. (Source: MII)

Shortly after Vauquelin’s discovery, a German chemist name Tassaert discovered chromium in an ore that geologists now call chromite (FeCr2O4, ferrous chromic oxide). Chromite forms in an igneous environment.

Name

The name chromium was derived from the Greek word chroma which means color, in reference to the fact that chromium is known to cause a number of colors in a variety of materials. For example, the green color of emerald is caused by the presence of very small amounts of chromium in the crystal.

Sources

The only ore of chromium is the mineral chromite. United States chromium consumption is equivalent to about 14% of all the chromite ore mined each year. In the western hemisphere, chromite ore is produced only in Brazil and Cuba; the United States, Mexico and Canada do not produce chromite. (The Stillwater Complex in Montana is the biggest chromium deposit in the United States, however it is not producing chromite ore at this time.) By comparison, about 80% of world production of chromite comes from India, Kazakhstan, Turkey and southern Africa. Southern Africa itself produces about half of this.

Geologists estimate that there are about 11 billion tons of chromium ore (chromite) in the world that could be mined. Most of these resources are found in southern Africa. This is enough chromium ore to meet world demand for hundreds of years into the future.

Uses

Chromium is alloyed (that is, mixed) with steel to make it corrosion resistant or harder. An example is its use in the production of stainless steel, a bright, shiny steel that is strong and resistant to oxidation (rust). Stainless steel production consumes most of the chromium produced annually. Chromium is also used to make heat-resisting steel. So-called "superalloys" use chromium and have strategic military applications.

Chromium also has some use in the manufacture of certain chemicals. For example, chromium-bearing chemicals are used in the process of tanning leather. Chromium compounds are also used in the textiles industries to produce a yellow color.

Substitutes and Alternative Sources

There is no good alternative for chromium in the manufacture of steel or chromium chemicals. Scrap metal that contains chromium can be recycled as an alternative source. The natural abundance of chromite in the Earth’s crust makes alternative sources unnecessary at this time.

Further Reading

Glossary

Citation

Institute, M. (2011). Chromium. Retrieved from http://www.eoearth.org/view/article/51cbed3a7896bb431f690ae8

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