Chromite

Chromite is the most important chromium ore mineral. It forms a complete solid solution series with many other members of the group, eg. in the Chromite-Hercynite Series, Chromite-Spinel Series, Chromite-Magnetite Series and the Chromite-Magnesiochromite Series. It is the iron analogue of Zincochromite, Cochromite, Manganochromite and Magnesiochromite and the Cr analogue of Hercynite, Coulsonite and Magnetite It usually contains Mg, ferric iron [Fe(III)], Al and often trace amounts of Ti. "Ferritchromite", i.e. Fe(III) substitutions for Al and Cr, usually in grain rims, may give high total Fe contents and a false chromite identification. Of course, the definition requires dominant Fe(II)>Mg and Cr>Fe(III). (e.g. photo ID: 514894). "Chromite" is commonly used for any Cr-rich mineral of the spinel group, particularly for the chromite-magnesiochromite series, and much of the mineral referred to as "chromite" in geological and petrological papers and mining statistics is magnesiochromite. The Fe-dominant species - chromite itself - can be FOUND, although uncommonly, in many "chromite" deposits, chromitite (rock name), and other "chromite"-bearing occurrences connected with hyperbasic rocks. As a rule of thumb, a member of the chromite-magnesiochromite series is twice as likely to be a magnesiochromite instead of a chromite. "Chromites" associated with olivine or serpentines, which are Mg-rich rocks, frequently have Mg-rich associated minerals. True chromite is common as inclusions in diamonds from kimberlites (South Africa and Yakutia). Members of the spinel group may be enriched Cr and V in marbles (Slyudyanka), massive sulphide ores (Outokumpu) and also in some types of meteorites. In any case, "chromite" always needs ANALYTICAL CONFIRMATION. Note: Chromite, that is near end-member, has a higher specific gravity than near end-member magnesiochromite.