| Chromite | |
|---|---|
 Octahedral chromite crystal from the Freetown Layered Complex inSierra Leone,Africa (size: 1.3 x 1.2 x 1.2 cm) | |
| General | |
| Category | Oxide minerals Spinel group Spinel structural group |
| Formula | (Fe, Mg)Cr2O4 |
| IMA symbol | Chr[1] |
| Strunz classification | 4.BB.05 |
| Crystal system | Isometric |
| Crystal class | Hexoctahedral (m3m) H-M symbol: (4/m3 2/m) |
| Space group | Fd3m (no. 227) |
| Unit cell | a = 8.344 Å; Z = 8 |
| Identification | |
| Color | Black to brownish black; brown to brownish black on thin edges in transmitted light |
| Crystal habit | Octahedral rare; massive to granular |
| Twinning | Spinel law on {Ill} |
| Cleavage | None, parting may develop along {III} |
| Fracture | Uneven |
| Tenacity | Brittle |
| Mohs scale hardness | 5.5 |
| Luster | Resinous, Greasy, Metallic, Sub-Metallic, Dull |
| Streak | Brown |
| Diaphaneity | Translucent to opaque |
| Specific gravity | 4.5–4.8 |
| Optical properties | Isotropic |
| Refractive index | n = 2.08–2.16 |
| Other characteristics | Weakly magnetic |
| References | [2][3][4][5] |
Chromite is a crystallinemineral composed primarily ofiron(II) oxide andchromium(III) oxide compounds. It can be represented by the chemical formula ofFeCr2O4. It is anoxide mineral belonging to thespinel group. The elementmagnesium can substitute foriron in variable amounts as it forms asolid solution with magnesiochromite (MgCr2O4).[6] Substitution of the elementaluminium can also occur, leading tohercynite (FeAl2O4).[7] Chromite today is mined particularly to make stainless steel through the production offerrochrome (FeCr), which is an iron-chromium alloy.[8]
Chromite grains are commonly found in largemafic igneous intrusions such as theBushveld in South Africa and India. Chromite is iron-black in color with a metallicluster, a dark brownstreak and a hardness on theMohs scale of 5.5.[9]
Chromite minerals are mainly found in mafic-ultramaficigneous intrusions and are also sometimes found inmetamorphic rocks. The chromite minerals occur in layered formations that can be hundreds of kilometres long and a few meters thick.[10] Chromite is also common iniron meteorites and form in association withsilicates andtroilite minerals.[11]
The chemical composition of chromite can be represented as FeCr2O4, with the iron in the +2oxidation state and the chromium in the +3 oxidation state.[5] The structure of theore can be seen as platy, with breakages along planes of weakness. Chromite can also be presented in a thin section. The grains seen in thin sections are disseminated with crystals that areeuhedral tosubhedral.[12]
Chromite contains Mg, ferrous iron [Fe(II)], Al and trace amounts ofTi.[5] Chromite can change into different minerals based on the amounts of each element in the mineral. Chromite is a part of thespinel group, which means that it is able to form a completesolid solution series with other members in the same group. These include minerals such as chenmingite (FeCr2O4),xieite (FeCr2O4), magnesiochromite (MgCr2O4) andmagnetite (Fe2+Fe3+2O4). Chenmingite and xieite arepolymorphs of chromite while magnesiochromite and magnetite areisostructural with chromite.[5]
Chromite occurs as massive and granular crystals and very rarely asoctahedral crystals.Twinning for this mineral occurs on the {III} plane as described by thespinel law.[5]
Grains of minerals are generally small in size. However, chromite grains up to 3 cm have been found. These grains are seen to crystallize from the liquid of ameteorite body where there are low amounts of chromium and oxygen. The large grains are associated with stablesupersaturated conditions seen from the meteorite body.[11]
Chromite is an important mineral in helping to determine the conditions that rocks form. It can have reactions with various gases such asCO andCO2. The reaction between these gases and the solid chromite grains results in the reduction of the chromite and allows for the formation of iron and chromiumalloys. There could also be a formation of metalcarbides from the interaction with chromite and the gases.[13]
Chromite is seen to form early in thecrystallization process. This allows for chromite to be resistant to the alteration effects of high temperatures and pressures seen in themetamorphic series. It is able to progress through themetamorphic series unaltered. Other minerals with a lower resistance are seen to alter in this series to minerals such asserpentine,biotite andgarnet.[14]
Chromite is found asorthocumulate lenses inperidotite from the Earth'smantle. It also occurs inlayered,ultramafic intrusive rocks.[15] In addition, it is found in metamorphic rocks such as someserpentinites.Ore deposits of chromite form as early magmatic differentiates. It is commonly associated witholivine,magnetite,serpentine andcorundum.[16] The vastBushveld Igneous Complex ofSouth Africa is a large layeredmafic toultramaficigneous body with some layers consisting of 90% chromite, forming the rare rock typechromitite (cf. chromite the mineral and chromitite, a rock containing chromite).[17] TheStillwater Igneous Complex inMontana also contains significant chromite.[3]
Chromite suitable for commercial mining is found in just a handful of very substantial deposits. There are 2 main types of chromite deposits:stratiform deposits and podiform deposits. Stratiform deposits in layered intrusions are the main source of chromite resources and are found inSouth Africa,Canada,Finland, andMadagascar. Chromite resources from podiform deposits are mainly found inKazakhstan,Turkey, andAlbania.Zimbabwe is the only country that contains notable chromite reserves in both stratiform and podiform deposits.[18]
Stratiform deposits are formed as large sheet-like bodies, usually formed in layeredmafic toultramaficigneous complexes. This type of deposit is used to obtain 98% of the worldwide chromite reserves.[19]
Stratiform deposits are typically seen to be ofPrecambrian in age and are found incratons. Themafic toultramaficigneous provinces that these deposits are formed in were likely intruded intocontinental crust, which may have containedgranites orgneisses. The shapes of these intrusions are described as tabular or funnel-shaped. The tabular intrusions were placed in the form ofsills with the layering of these intrusions being parallel. Examples of these tabular intrusions can be seen in theStillwater Igneous Complex andBird River. The funnel-shaped intrusions are seen to be dipping towards the center of the intrusion. This gives the layers in this intrusion asyncline formation. Examples of this type of intrusion can be seen in theBushveld Igneous Complex and theGreat Dyke.[19]
Chromite can be seen in stratiform deposits as multiple layers which consist ofchromitite. Thicknesses for these layers range between 1 cm and 1 m. Lateral depths can reach lengths of 70 km. Chromitite is the main rock in these layers, with 50–95% of it being made of chromite and the rest being composed ofolivine,orthopyroxene,plagioclase,clinopyroxene, and the various alteration products of these minerals. An indication of water in the magma is determined by the presence of brownmica.[19]
Podiform deposits are seen to occur within theophiolite sequences. The stratigraphy of the ophiolite sequence is deep-ocean sediments,pillow lavas,sheeted dykes,gabbros andultramafictectonites.[19]
These deposits are found in ultramafic rocks, most notably in tectonites. It can be seen that the abundance of podiform deposits increase towards the top of the tectonites.[19]
Podiform deposits are irregular in shape. "Pod" is a term given by geologists to express the uncertain morphology of this deposit. This deposit showsfoliation that is parallel to the foliation of the host rock. Podiform deposits are described as discordant, subconcordant and concordant. Chromite in podiform deposits form asanhedral grains. The ores seen in this type of deposit have nodular texture and are loosely-packed nodules with a size range of 5–20 mm. Other minerals that are seen in podiform deposits areolivine,orthopyroxene,clinopyroxene,pargasite,Na-mica,albite, andjadeite.[19]
Chromium extracted from chromite is used on a large scale in many industries, including metallurgy, electroplating, paints, tanning, and paper production. Environmental contamination with hexavalent chromium is a major health and environmental concern. Chromium is most stable in itstrivalent (Cr(III)) form, seen in stable compounds such as natural ores. Cr(III) is an essential nutrient, required forlipid andglucose metabolism in animals and humans. In contrast, the second most stable form,hexavalent chromium (Cr(VI)), is generally produced through human activity and rarely seen in nature (as incrocoite), and is a highly toxic carcinogen that may kill animals and humans if ingested in large doses.[20]
Health effects
When chromiteore is mined, it is aimed for the production offerrochrome and produces a chromiteconcentrate of a high chromium to iron ratio.[21] It can also be crushed and processed. Chromite concentrate, when combined with areductant such ascoal orcoke and a high temperature furnace can produceferrochrome. Ferrochrome is a type offerroalloy that is analloy in between chromium and iron. This ferroalloy, as well as chromiteconcentrate can introduce various health effects. Introducing a definitive control approach and distinct mitigation techniques can provide importance related to the safety of human health.[22]
When chromite ore is exposed to surface conditions,weathering andoxidation can occur. The element chromium is most abundant in chromite in the form of trivalent (Cr-III). When chromiteore is exposed to aboveground conditions, Cr-III can be converted toCr-VI, which is the hexavalent state of chromium. Cr-VI is produced from Cr-III by means of drymilling or grinding of the ore. This is due to the moistness of the milling process as well as theatmosphere in which the milling is taking place. A wet environment and a non-oxygenated atmosphere are ideal conditions to produce less Cr-VI, while the opposite is known to create more Cr-VI.[23]
Production offerrochrome is observed to emitpollutants into the air such asnitrogen oxides,carbon oxides andsulfur oxides, as well as dustparticulates with a high concentration ofheavy metals such aschromium,zinc,lead,nickel andcadmium. During high temperaturesmelting of chromiteore to produceFerrochrome, Cr-III is converted to Cr-VI. As with chromite ore, Ferrochrome ismilled and therefore produces Cr-VI. Cr-VI is therefore introduced into the dust when theFerrochrome is produced. This introduces health risks such as inhalation potential andleaching of toxins into the environment. Human exposure to chromium is ingestion, skin contact, and inhalation. Chromium-III and VI will accumulate in the tissues of humans and animals. The excretion of this type of chromium from the body tends to be very slow which means that elevated concentrations of chromium can be seen decades later in human tissues.[23]
Environmental effects
Chromite mining, chromium, and ferrochrome production can be toxic for the environment.[23] Chromitemining is necessary when it comes to the production of economiccommodities.[24]
As a result of leaching of soils and the explicit discharge from industrial activities,weathering of rocks that contain chromium will enter the water column. The route of chromium uptake in plants is still ambiguous, but because it is a nonessential element, chromium will not have a distinct mechanism for that uptake which is independent from chromium speciation.[25] Plant studies have shown that toxic effects on plants from chromium include things such as wilting, narrow leaves, delayed or reduced growth, a decrease inchlorophyll production, damage to root membranes, small root systems, death and many more.[23] Chromium's structure is similar to other essential elements which means that it can impact the mineral nutrition of plants.[25]
During industrial activities and production things such as sediment, water, soil, and air all become polluted and contaminated with chromium. Hexavalent chromium has negative impacts towards soil ecology because it decreases soil micro-organism presence, function, and diversity.[23] Chromium concentrations in soil diversify depending on the different compositions of the sediments and rocks that the soil is made from. The chromium present in soil is a mixture of both Cr(VI) and Cr(III).[25] Certain types of chromium such asChromium-VI has the capability to pass into the cells of organisms. Dust particles from industry operations and industrial wastewater contaminate and pollute surface water, groundwater, and soils.[23]
In aquatic environments, chromium could experience things such asdissolution,sorption,precipitation,oxidation,reduction, anddesorption.[25] In aquatic ecosystems chromiumbioaccumulates in invertebrates, aquatic plants, fish, and algae. These toxic effects will operate differently because things such as sex, size, and the development stage of an organism may vary. Things such as the temperature of the water, its alkalinity, salinity, pH, and other contaminants will also impact these toxic effects on organisms.[23]
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Chromite can be used as arefractory material because it has a highheat stability.[26] The chromium extracted from chromite is used inchrome plating and alloying for production of corrosion resistantsuperalloys,nichrome, andstainless steel.[27] Chromium is used as apigment for glass, glazes, and paint, and as anoxidizing agent for tanning leather.[28] It is also sometimes used as agemstone.[29] Most shiny car trim is chromium plated. Superalloys that contain chromium allow jet engines to run under high stress, in a chemically oxidizing environment, and in high-temperature situations.[27]
Porcelain tiles are often produced with many different colours andpigmentations. The usual contributor to colour in fast-fired porcelain tiles is black(Fe,Cr)
2O
3 pigment, which is fairly expensive and issynthetic. Natural chromite allows for an inexpensive and inorganic pigmentation alternative to the expensive(Fe,Cr)
2O
3 and allows for themicrostructure and mechanical properties of the tiles to not be substantially altered or modified when introduced.[30]
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