Chromite Cr occurs exclusively in mafic and ultramafic igneous rocks Papp and Lipin 2006 Although some Cr based minerals such as crocoite PbCrO 4 vauquelinite Pb 2 Cu CrO 4 PO 4 uvarovite Ca 3 Cr 2 SiO 4 3 and merumite 4 Cr Al 2 O 3 ·3H 2 O are well known chromite FeCr 2 O 4 is the only one that is a commercially recoverable source of Cr Sueker 2006 Glastonbury et