Chromium is a relatively hard metal with grayish black color- a steel with the symbol Cr, atomic number 24, atomic weight 51.9961, density of 7.19 grams per cubic centimeter, inflexibility of 8.5 in the Mose scale with high shine and furbish, different fusion quality, corrosion resistant and with a boiling point of 2672 degrees centigrade and a melting point of 1857 degrees centigrade. Chromium is placed in group VI of the periodic table of elements under the transition metals in the period 4.
Chromite is the only commercial source of chromium with the formula of FeCr2O4. Chromite is the only mineral containing chromium. Its formula has been shown as FeO,CrO2 or FeCr2O4. In some chromite samples elements such as zinc, nickel, manganese, titanium, and vanadium have been detected in small amounts. The amount of Cr2O3 in commercial chromites is variable between 25 to 65 percent. The mineral is formed in the ultra-basic rocks. Half of the chromite mines of the world are located in South Africa and the rest in Kazakhstan, India and Turkey.
This metal is formed in the solid crust of the earth almost within basic igneous rocks related to the primary stage of magma crystallization and the serpentine resulting from their change. In these rocks chromium usually goes with iron (2, 3) nickel and cobalt. Basic silicates such as olivine, diopside and hornbland may also have chromium in their compounds.
This mineral generally exists in ultra-basic rocks (harsborjite, pyridutite, dunite, gabro, norite and pyroxinite). Strip texture, panther skin texture, cokade texture are the most common textures of chromites.
General belief about formation of chromite says that at the time of elevation of infiltrating magma as a result of heat exchange, the temperature comes down gradually causing chromite seeds to form. In studying the condition of formation and genesis of the chromite minerals, recognition of the tectonic origin of ophiolite is of particular importance which we will be surveyed here.
Destructive tectonic margins: In this tectonic regime two adjacent crusts move toward each other and the edge of the crust with a higher density and speed and with a more appropriate shape goes under the other plane resulting in the following three general modes in view of the type of the crust:
The plunging zone of the margin of continents: That includes the plunge of a continental crust under the crust of another continent and the most important plunging zones are in Latin America, as well as Iran, Turkey and Romania as a result of which the ophiolite complexes of Iran, Turkey, Oman, Cyprus, former Yugoslavia, and Greece have formed containing important chromite minerals and some mass sulfide deposits.
Arched Islands zone: That includes the plunge of and oceanic plane under another oceanic plane or under a young continental crust. The arched islands of the western Pacific Ocean are an example in which major and giant ophiolite complexes of the Philippines and New Caledonia have formed.
Two-continent collision zone: Two continents collide with each other in the last stage of elimination of the oceanic crust whereby one continent goes beneath the other one in particular conditions. An example of this case is seen in the Alpine and Himalayan mountains with the Alpine ophiolites serving as an evident example.
Chromites (chromite minerals) formed in two ways:
A. From primary reduction of basaltic magma
B. From reduction of the remaining magma
The chromite minerals can be divided into different groups based on the type of usage, amount of deposit, formation conditions and genesis,…
A. Division based on the amount of deposit
B. Division based on the conditions of formation and genesis
Division of ophiolites of Iran in terms of structure:
A. Ultra-basic complexes
B. Colored blend complexes
Generally, chromites are formed in the alternation of mafic – ultramafic rocks and their deposits are
divided into two Alpine and stratified types. The most important explorative criterion is geology and for the Alpine deposits there is no proper geophysical and geochemical method. Whereas the place of deposit and distribution of these rocks during the geological periods is clear the primary choice of the targets is relatively easy.
The chromite deposits known in Iran are of the Alpine type and according to the existing geological evidences there is no probability of existence of the deposits of stratified type. In view of the extension of the alternation of ophiolites and ophiolite mélanges and their specified geographical situation the areas with cleavage of ophiolites strips can be introduced as the potential areas. Indeed in Azerbaijan too, traces of infiltrative mafic-ultramafic masses have been observed with least importance after ophiolites. Nevertheless, these infiltrations would need more comprehensive studies.
The major application of chromites is in metallurgical, chemical and refractory die-cast industries. The chemical and physical specifications of chromite determine its application in any of the above usages. In this way chromites can be classified into three groups of per-chromium used in metallurgy, per-iron used in metallurgy and chemical industries and per-aluminum used in refractory industries. The metallurgical industries alone account for 85 percent of the global consumption. The main role of chromite in these industries is its usage in production of ferrous chromium alloys for stainless steels. The stainless steel is the most important alloyed metal of steel which accounts for 1 to 2 percent of the global steel production. The amount of chromium used in this type of steel is about 12 to 36 percent. Some 75 percent of chromite and more than 90 percent of ferrous chromium are consumed in stainless steel production. No major change has taken place in the type of chromite consumption over the recent years.