Iron Chromium Oxide
FeCr2O4
The principle ore of chromium, as a refractory component, as a dye and as mineral specimens.
The Present Scenario
Notable Occurrences is several mines in North Carolina, Montana, Maryland, Oregon, Texas, California and Wyoming, USA also found in Turkey; South Africa; Philippines and Russia.
Chromite is the most important ore of chromium from which it derives its name. Chromium is an important metal and has a wide range of industrial uses.
Chromite forms in deep ultra-mafic magmas and is one of the first minerals to crystallize. It is because of this fact that chromite is found in some concentrated ore bodies. While the magma is slowly cooling inside the Earth's crust, chromite crsytals are forming and because of their density, fall to the bottom and are concentrated there.
Although its primary origin is ultra-mafic rocks such as peridotites, chromite is also found in metamorphic rocks such as serpentites. Chromite, as is indicated by its early crystallization is resistant to the altering affects of high temperatures and pressures. Thus it is capable of going through the metamorphic processes unscathed, while other minerals around it are being altered to serpentine, biotite and garnets. This characteristic also explains chromites use as a refractory component in the bricks and linings of blast furnaces.
Usually magnesium is present in chromite substituting for the iron and in fact a solid solution series exists between chromite and the much rarer mineral magnesiochromite. All chromite specimens in nature contain some magnesium, likewise all natural magnesiochromites contain some iron. Magnesiochromite is grayer in color and in streak and has a slightly lower density than chromite at a specific gravity of 4.2 to 4.4.
| Hardness | Associated Minerals | Chemical/Typical composition | Colour | characteristics | Luster | Field Indicators |
| 5.5 | olivine talc serpentine uvarovite pyroxenes biotite magnetite anorthite | Oxides and Hydroxides | brownish black to a deep dark black | Weakly magnetic and an octahedral parting is sometimes seen | metallic to greasy | crystal habit, streak, associations with ultra-basic minerals and parting |
Uses
The following uses for chromium are gathered from a number of sources as well as from anecdotal comments. I'd be delighted to receive corrections as well as additional referenced uses (please use the feedback mechanism to add uses).
- used to harden steel, to manufacture stainless steel, and to form alloys
- used in plating to produce a hard, beautiful surface and to prevent corrosion.
- used to give glass an emerald green colour. It is responsible for the green colour of emeralds and the red colour of rubies
- wide use as a catalyst
- dichromates such as K2Cr2O7 are oxidising agents and are used in quantitative analysis and also in tanning leather
- lead chromate as chrome yellow is a pigment
- compounds are used in the textile industry as mordants
- used by the aircraft and other industries for anodising aluminium
- the refractory industry uses chromite for forming bricks and shapes, as it has a high melting point, moderate thermal expansion, and stable crystalline structure
- tanning leather