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Chromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. It can be represented by the chemical formula of IronChromium2Oxygen4. It is an oxide mineral belonging to the spinel group. The element magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite (MagnesiumChromium2Oxygen4). Substitution of the element aluminium can also occur, leading to hercynite (IronAluminum2Oxygen4). Chromite today is mined particularly to make stainless steel through the production of ferrochrome (IronChromium), which is an iron-chromium alloy.
Chromite grains are commonly found in large mafic igneous intrusions such as the Bushveld in South Africa and India. Chromite is iron-black in color with a metallic luster, a dark brown streak and a hardness on the Mohs scale of 5.5.
Chromite contains Mg, ferrous iron Fe(II), Al and trace amounts of Titanium. Chromite can change into different minerals based on the amounts of each element in the mineral. 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. These include minerals such as chenmingite (FeCr2O4), xieite (FeCr2O4), magnesiochromite (MgCr2O4) and magnetite (Fe2+Fe3+2O4). Chenmingite and xieite are polymorphs of chromite while magnesiochromite and magnetite are isostructural with chromite.
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 a meteorite body where there are low amounts of chromium and oxygen. The large grains are associated with stable Supersaturation conditions seen from the meteorite body.
Chromite is seen to form early in the crystallization process. This allows for chromite to be resistant to the alteration effects of high temperatures and pressures seen in the Metamorphic rock series. It is able to progress through the Metamorphic rock series unaltered. Other minerals with a lower resistance are seen to alter in this series to minerals such as serpentine, biotite and garnet.
Chromite suitable for commercial mining is found in just a handful of very substantial deposits. There are 2 main types of chromite deposits: Stratum deposits and podiform deposits. Stratiform deposits in layered intrusions are the main source of chromite resources and are found in South Africa, Canada, Finland, and Madagascar. Chromite resources from podiform deposits are mainly found in Kazakhstan, Turkey, and Albania. Zimbabwe is the only country that contains notable chromite reserves in both stratiform and podiform deposits. (2016). 9780128039069 ISBN 9780128039069
Stratiform deposits are typically seen to be of Precambrian in age and are found in . The mafic to Ultramafic rock Igneous rock provinces that these deposits are formed in were likely intruded into continental crust, which may have contained or . The shapes of these intrusions are described as tabular or funnel-shaped. The tabular intrusions were placed in the form of sills with the layering of these intrusions being parallel. Examples of these tabular intrusions can be seen in the Stillwater Igneous Complex and Bird River. The funnel-shaped intrusions are seen to be dipping towards the center of the intrusion. This gives the layers in this intrusion a syncline formation. Examples of this type of intrusion can be seen in the Bushveld Igneous Complex and the Great Dyke.
Chromite can be seen in stratiform deposits as multiple layers which consist of chromitite. 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 of olivine, Pyroxene, plagioclase, Pyroxene, and the various alteration products of these minerals. An indication of water in the magma is determined by the presence of brown mica.
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.
Podiform deposits are irregular in shape. "Pod" is a term given by geologists to express the uncertain morphology of this deposit. This deposit shows foliation that is parallel to the foliation of the host rock. Podiform deposits are described as discordant, subconcordant and concordant. Chromite in podiform deposits form as anhedral 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 are olivine, Pyroxene, Pyroxene, pargasite, Mica, albite, and jadeite.
Health effects
When chromite ore is mined, it is aimed for the production of ferrochrome and produces a chromite concentrate of a high chromium to iron ratio. It can also be crushed and processed. Chromite concentrate, when combined with a Reducing agent such as coal or coke and a high temperature furnace can produce ferrochrome. Ferrochrome is a type of ferroalloy that is an alloy in between chromium and iron. This ferroalloy, as well as chromite
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When chromite ore is exposed to surface conditions, weathering and redox can occur. The element chromium is most abundant in chromite in the form of trivalent (Cr-III). When chromite ore is exposed to aboveground conditions, Cr-III can be converted to Cr-VI, which is the hexavalent state of chromium. Cr-VI is produced from Cr-III by means of dry milling or grinding of the ore. This is due to the moistness of the milling process as well as the atmosphere 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. Potential Toxic Effects of Chromium, Chromite Mining and Ferrochrome Production : A Literature Review
Production of ferrochrome is observed to emit into the air such as , Oxocarbon and Sulfur oxide, as well as dust particulates with a high concentration of heavy metals such as chromium, zinc, lead, nickel and cadmium. During high temperature smelting of chromite ore to produce Ferrochrome, Cr-III is converted to Cr-VI. As with chromite ore, Ferrochrome is milled and therefore produces Cr-VI. Cr-VI is therefore introduced into the dust when the Ferrochrome is produced. This introduces health risks such as inhalation potential and 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.
Environmental effects
Chromite mining, chromium, and ferrochrome production can be toxic for the environment. Chromite mining is necessary when it comes to the production of economic Commodity.Das, P.K., Das, B.P. & Dash, P. Chromite mining pollution, environmental impact, toxicity and phytoremediation: a review. Environ Chem Lett
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. Plant studies have shown that toxic effects on plants from chromium include things such as wilting, narrow leaves, delayed or reduced growth, a decrease in chlorophyll production, damage to root membranes, small root systems, death and many more. Chromium's structure is similar to other essential elements which means that it can impact the mineral nutrition of plants.
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. 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). Certain types of chromium such as Chromium 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.
In aquatic environments, chromium could experience things such as dissolution, sorption, precipitation, oxidation, reduction, and desorption. In aquatic ecosystems chromium Bioaccumulation 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.
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Applications
Chromite can be used as a refractory material because it has a high Thermostability. The chromium extracted from chromite is used in chrome plating and alloying for production of corrosion resistant , nichrome, and stainless steel. Chromium is used as a pigment for glass, glazes, and paint, and as an oxidizing agent for tanning leather. It is also sometimes used as a gemstone.Tables of Gemstone Identification By Roger Dedeyne, Ivo Quintens, p.189 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.
Porcelain tile pigmentation
are often produced with many different colours and . The usual contributor to colour in fast-fired porcelain tiles is black pigment, which is fairly expensive and is synthetic. Natural chromite allows for an inexpensive and inorganic pigmentation alternative to the expensive and allows for the microstructure and mechanical properties of the tiles to not be substantially altered or modified when introduced.
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