Chromite

Authors: Lisandro, Ploy, and Billie

Mineral Name: Chromite

Chemical Composition: CrFe2O4, though it can have a variable composition with magnesium and aluminum too,(Fe,Mg,Al)Cr2O4

Color: Dark Gray to Black, Brownish-Black, Greenish-Black

Streak: Dark brown

Hardness: 5.5 to 6


Cleavage/Fracture: Even though chromite does not have a cleavage to it may exhibit parting. Parting is when the mineral will split generally at a point of stress as well as well as the boundary between where two crystals grow together or twin. When it comes to fracture, chromite show both conchoidal and uneven fractures. A conchoidal fracture looks rounded and has a shape like a shell while an uneven fracture has a rough and bumpy.(9)

Crystal Form: Chromite is an Isometric crystal. Isometric is a crystal system that has all three axes of the crystal equal to 90° from each other and all sides are equal distances.(9) The shape that isometric crystals takes are octahedral, an eight sided shape, and dodecahedral, which is a 12 sided shape, that has sides similar to rhombuses or octagons. Though, with chromite, it is more likely to get a crystal that is grainy, veiny and has rounded grains to it rather than a perfect crystal. (9)

Luster: Metallic to dull

Special Features: Chromite’s special features include its specific gravity being 4.2-5, and brown streak. Another interesting thing about Chromite is that it is paramagnetic, which means it is attracted to magnets.

Varieties: One variety of chromite is aluminian chromite, which is an aluminum variant of chromite. Berezovskite is a magnesian variant while alumoberezovite is an magnesian aluminian variation. There is also ferrian chromite and ferrich chromite which are a part of the chromite-magnetite series and contain different amounts of Fe3+. (2) Leoparenzer is a variety that has a leopard like pattern with patches of black on a lighter surface. Additionally, there is a titanium variant called titaniferous chromite.(2)

Mineral Group: Chromite is a part of the spinel group. The spinel group consists of minerals that have a specific chemical makeup of AB2O4, where A and B are different metallic elements and O is oxygen. The metals that make up chromite are Cr, chromium and Fe, iron.(11)

Environment: Chromite can be commonly found in the ultramafic portion of a layered mafic intrusion. It can be commonly found in meteorites too. (4) Chromite forms when magma is slowly cooling inside the Earth's crust; but, because of the density of the mineral, it falls to the bottom of the magma chamber, which is where it most concentrated. These layered magma chamber deposits are called stratiform deposits. (1) Most of the world’s chromite is formed in these stratiform deposits. Podiform deposits, also known as ophiolites, are slabs of the Earth's oceanic crust that can also contain chromite. (1) You can also get chromite deposits on beaches; this is because rocks that contain chromite are weathered by water and eventually end up on the beach as a part of the sand. (1)

Associated Rock types: Peridotite and dunite are igneous rocks that are accessory components to chromite while chromitite and olivine-chromite-rock have chromite as essential components. Some metamorphic rock that have chromite as essential parts are birbitite, boetonite, and Martian basaltic breccia. Rocks containing chromite can also be found in meteorites. (2)

Occurrence in North America: Areas within North America that chromite can be found include Maryland, Montana, New Jersey, New York, North Carolina, Pennsylvania and Texas, for the US as well as Ontario in Canada. (1)

Economic Uses: Chromite is used for its silver, shiny appearance. Chromium, whose main ore is chromite, is used to create chrome surfaces to protect cars, stoves and other appliances from corrosion as well as improve their looks. (6) Chrome surfaces are created by electroplating and chromizing. Electroplating has 2 methods for either just decorative uses or for protection that both use chromic acid solutions. (6) Chromizing has a few different methods, one of which uses condensed chromium capos (which is essentially a clamp) on the surface and is diffused into the metal with heat. Another method of chromizing requires a layer of chromium to be fused to the surface and then diffused into the surface. (6)

Industrial Uses: Chromite is used in metals to strengthen its hardness, toughness and to give it chemical resistance. (1) Through the process of alloying chromite with other metals it creates stainless steel. Also, when chromite and nickel or iron are alloyed it produces something called nichrome, which is resistant to high temperatures. (1) Chromite is also use in jet engines because it is resistant to high temperature, corrosion, and high stress environments, like jet engines. (1) Being able to resist high heat and being chemically stable, means chromite can be used as a means of refractory. Chromite is used as a refractory material in blast furnaces and crucibles, this means it is used as insulations, generally for metals.(1)

First Notable Identification: The first deposits were found in Baltimore, Maryland in 1800.(1) Originally this was where all the world’s chromite was coming from until 1850.(1) In 1798, this mineral was originally named fer chromate aluminum by Louis Nicolas Vauquelin, who also discovered chromium.(4) Chromite was finally named in 1845 by Wilhelm Haidingerand.(4)

How We Identified It: Careful observation of hardness, streak, and density distinguish chromite in hand samples. Chromite has this black, but chrome look to it under bright light.

Don’t Confuse It With: Chromite sometimes has a slight magnetic field. This may cause confusion between the minerals magnetite, chromite and ilmenite. Chromite can be confused with magnetite because the color of magnetite is closer to chromite, but magnetite has a darker streak than chromite. (2) Another mineral that can be mistaken for chromite is magnesiochromite but magnesiochromite has a greyer streak than chromite does. (2)



Bibliography

  1. King, Hobart M. “Chromite.” Geology, https://geology.com/minerals/chromite.shtml

  2. “Chromite: The Mineral Chromite Information and Pictures.” Chromite: The Mineral

  3. Chromite Information and Pictures, www.minerals.net/mineral/chromite.aspx.

  4. Hudson Institute. “Chromite.” Chromite: Mineral Information, Data and Localities., www.mindat.org/min-1036.html.

  5. Barthelmy, Dave. “Mineralogy Database.” Chromite Mineral Data, 2014, webmineral.com/data/Chromite.shtml#.XOSmRS-ZPRZ.

  6. Frederick E. Bacon, James H. Downing, The Editors of Encyclopaedia. “Chromium processing.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 25 Jan. 2018, https://www.britannica.com/technology/chromium-processing.

  7. Koleli Demir. “Chromite.” ScienceDirect, Academic Press, 29 Apr. 2016, www.sciencedirect.com/science/article/pii/B9780128038376000111.

  8. “THE MINERAL CHROMITE.” CHROMITE (Iron Chromium Oxide), www.galleries.com/Chromite.

  9. Amethyst Galleries. “The Spinel Group of Minerals.” Mineral Gallery - The Spinel Group, 1995, www.galleries.com/spinel_group.

  10. Chromite. (n.d.). Retrieved from https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/chromite