However, chromite grains up to 3 cm have been found. Grains of minerals are generally small in size. Twinning for this mineral occurs on the plane as described by the spinel law. Crystal size and morphology Ĭhromite occurs as massive and granular crystals and very rarely as octahedral crystals. Chenmingite and xieite are polymorphs of chromite while magnesiochromite and magnetite are isostructural with chromite. These include minerals such as chenmingite (FeCr 2O 4), xieite (FeCr 2O 4), magnesiochromite (MgCr 2O 4) and magnetite (Fe 2+Fe 3+ 2O 4). Chromite is a part of the spinel group, which means that it is able to form a complete solid solution series with other members in the same group. Chromite can change into different minerals based on the amounts of each element in the mineral. Ĭhromite contains Mg, ferrous iron, Al and trace amounts of Ti. The grains seen in thin sections are disseminated with crystals that are euhedral to subhedral. Chromite can also be presented in a thin section. The structure of the ore can be seen as platy, with breakages along planes of weakness. Chromite, when presented as an ore, or in massive form, forms as fine granular aggregates. The chemical composition of chromite can be represented as FeCr 2O 4, with the iron in the +2 oxidation state and the chromium in the +3 oxidation state. Chromite is also common in iron meteorites and form in association with silicates and troilite minerals. The chromite minerals occur in layered formations that can be hundreds of kilometres long and a few meters thick. Properties Ĭhromite minerals are mainly found in mafic-ultramafic igneous intrusions and are also sometimes found in metamorphic rocks. Chromite is iron-black in color with a metallic luster, a dark brown streak and a hardness on the Mohs scale of 5.5. Ĭhromite grains are commonly found in large mafic igneous intrusions such as the Bushveld in South Africa and India. Chromite today is mined particularly to make stainless steel through the production of ferrochrome (FeCr), which is an iron-chromium alloy. A substitution of the element aluminium can also occur, leading to hercynite (FeAl 2O 4). The element magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite (MgCr 2O 4). It is an oxide mineral belonging to the spinel group. It can be represented by the chemical formula of FeCr 2O 4. Resinous, Greasy, Metallic, Sub-Metallic, DullĬhromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. Octahedral chromite crystal from the Freetown Layered Complex in Sierra Leone, Africa (size: 1.3 x 1.2 x 1.2 cm)īlack to brownish black brown to brownish black on thin edges in transmitted light
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