chromites

chromites Chromite (FeCr₂O₄) is the only ore mineral of chromium (Cr) and is found, usually in small amounts, in mafic and ultramafic rocks (igneous rocks containing a large proportion of olivine and pyroxene). Chromite deposits are of two main types:

(1) ‘Magmatic sediment layers’ in layered basic intrusions where the chromite ores appear to have segregated during crystallization of the magma body, e.g. Bushveld complex, South Africa. These layers vary from a few centimetres to several metres in thickness and often extend laterally for considerable distances. Many show features analogous to sedimentary rocks. Occurrences are isolated, but are of economic significance; approximately 97 per cent of the world's chromite reserves occur in such layered deposits in South Africa and Zimbabwe.(2) ‘Podiform chromites’ are found in peridotite or serpentinized masses associated with mountain belts (sometimes termed Alpine-type), e.g. the Perm mining district in the Ural Mountains and deposits in Turkey and Albania. Such deposits are found in highly unstable tectonic environments. The deposits are believed to have formed initially within the oceanic lithosphere and to have been incorporated subsequently into the continental crust during mountain building.

Elizabeth H. Bailey