Color index (geology)

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The color index or the color number (Abbr. M ' = the percentage of mafic minerals or FZ ), English color index ( CI ) is in the lithology used parameter of the fraction of dark minerals in igneous rocks indicates. All mafic minerals minus muscovite , apatite and igneous carbonate minerals are considered dark minerals in the determination of M ' . The sum of the volume percentages of these minerals corresponds to the color index of the rock. The color index can be estimated on the handpiece and is therefore a simple aid for classifying the rock. Exact values ​​are determined by counting a thin section of rock under the microscope with the aid of a point counter.

Igneous rocks with a color index below 90 can be classified according to segment iron in the QAPF diagram . A further specification can be made on the basis of the color number. Other classification diagrams must be used for values> 90.

M '0 to 10 Hololeucocrates rock ( completely light )
M '10 to 35 Leucocrates rock ( light )
M '35 to 65 Mesocrates rock ( medium color )
M '65 to 90 Melanocrates rock ( dark )
M '90 to 100 Holomelanocrates, ultramafic rock , naming according to another classification necessary (ultramafitite diagrams)


  • RW Le Maitre (Ed.): Igneous Rocks - A Classification and Glossary of Terms. 2nd edition. Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Cambridge University Press 2002, Cambridge, ISBN 0-521-61948-3 .