Updated About encyclopedia.com content Print Article
colour index The total volume percent of mafic (coloured) silicate minerals in an igneous rock. The mafic minerals would include the olivine group, pyroxene group, and amphibole group of minerals, as well as biotite. Colour index is one of many methods used for classifying igneous rocks. If a rock has 60% modal mafic minerals it has a colour index of 60. Rocks with colour indices between 50 and 90 are termed ‘mafic’ rocks (e.g. basalts).