any one mineral of the spinel group of the class of multiple oxides. Chrome spinellids are systems of solid solutions of variable composition, with the general formula (Mg, Fe)(Cr, Al, Fe)2O4. They include about 20 minerals and their varieties. The principle minerals are magnochromite, (Mg, Fe)Cr2O4, chrome picotite, (Mg, Fe) (Cr, A1)2O4, and alumochromite, Fe(Cr, A1)2O4. Chromite proper, FeCr2O4, is a miner-alogical rarity and has been found only in meteorites. Chrome spinellids are characterized by isomorphic admixtures of V, Ti, Mg, or Zn. The high Ti content is related to microinclusions of ulvospinel and ilmenite. Chrome spinellids often contain mechanical admixtures of minerals of the platinum group.
Chrome spinellids crystallize in the isometric system, and their crystal structure is similar to that of ordinary spinel. The minerals usually occur in the form of black granular masses, although specimens with small octahedral crystals are occasionally found. Their hardness on Mohs’ scale ranges between 5.5 and 7.5, and the density varies from 4,200 to 5,100 kg/m3. The minerals exhibit no cleavage. The parameter of the unit cell a0 ranges from 8.30 to 8.39 angstroms. The physical properties are determined by the ratio of cations and the nature of their distribution in the octahedral and tetrahedral vacancies of the spinel structure. Chromite is paramagnetic at room temperature and has a Curie point of 90°K. The infrared spectrum of chrome spinellids has two distinct bands of 617 and 532 cm–1. Chrome spinellids with a high FeO and Fe2O3 content have ferromagnetic properties.
Chrome spinellids are thermally unstable and when heated break down, forming Fe2O3, Cr2O3, and A12O3. Fe2O3 is released at a temperature of about 300°C, and Cr2O3, at about 520°C. A solid solution of Cr2O3═ Fe2O3 occurs at 800°C, and magnetite forms at 1,000°C. The melting points range from 450° to 2,180°C, increasing proportionally with an increase in the content of MgO and Cr2O3.
Deposits of chrome spinellids are primarily connected with ultrabasic magmatic rocks. In addition, chrome spinellids are found in talc schists, chlorite schists, dolomites, iron meteorites, and lunar basalts. They are stable in alluvial deposits. Chrome spinellids are the main minerals of chromium ores. Synthetic chrome spinellids, called chrome ferrites, are used in radio electronics, computer technology, and elsewhere.
REFERENCESMineraly: Spravochnik, vol. 2, fasc. 3. Moscow, 1967.
Malakhov, I. A., T. A. Shilova, and B. A. Telegin. “Khromity.” In Geologiia SSSR, vol. 12. Moscow, 1973.
G. P. KUDRIAVTSEVA