Colour index

Rocks can be classified according to their colour index (CI). This describes the proportion of ferromagnesian minerals, such as olivine, pyroxenes, amphiboles and biotite mica, they contain. Colour index is measured in volume percent. The terms used to describe the varying proportions of ferromagnesian minerals are:

• hypermelanic: >90 vol% dark (mafic) minerals
• melanocratic: >60 vol% dark (mafic) minerals
• mesocratic: 30-60 vol% dark (mafic) minerals
• leucocratic: <30 vol% dark (mafic) minerals