Melanocrates rock

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Close-up of "Mayener Basaltlava" from the Quaternary of the Eifel. This rock is classified petrographically with the Tephrites and is therefore a typical melanocrates rock.
Handpiece with dunite , coated with basanite , from the island of Réunion . Confusingly, here the melanocrate rock (basanite) is darker than the holomelanocrate rock (dunite), since olivine , the mineral of which dunite consists almost exclusively, is lighter than the clinopyroxenes , which dominate the Mafit part of the basanite.

Melanocrate rocks (from ancient Greek μέλας mélas , German 'black' ), also called melage stones for short , are igneous rocks with a relatively dark hue. The reason for this is that the rock has a relatively high volume proportion of dark mafic minerals ( color index M ′ ).

In a narrower sense, the attribute “melanocrat” applies to rocks with M ′ between 65 and 90. Rocks with M ′> 90 are called holomelanocrat ( ultramafic ) and rocks with M ′ between 35 and 65 are called mesocrat or mesotype . Melanocrate rocks can be found e.g. B. within the gabbro and basalt families.

In addition, it is common for igneous rocks to prefix a rock name with the prefix mela- if the rock in question is darker than the average rock of the same rock family. The decisive factor here is the average value for M ′ within the rock family under consideration. For example, M ′ in a Melagranite is still so low (> 20) that it would generally be classified as a leucocrates , while M ′ in a Melagabbro is significantly higher at> 65.

literature

  • RW Le Maitre (Eds.), A. Fahrtisen, B. Zanettin, MJ Le Bas, B. Bonin, P. Bateman, G. Bellieni, A. Dudek, S. Efermova, J. Keller, J. Lameyre, PA Sabine , R. Schmid, H. Sørensen, AR Woolley: Igneous Rocks: A Classification and Glossary of Terms. Recommendations of the International Union of Geological Sciences, Subcommission on the Systematics of Igneous Rocks. Cambridge University Press, 2002, ISBN 978-0-521-66215-4
  • Walter Schumann: The new BLV stone and mineral guide . 6th edition, BLV Verlags GmbH, Munich (2002), ISBN 3-405-16441-9
  • Wolfhardt Wimmenauer: Petrography of igneous and metamorphic rocks. Enke-Verlag, Stuttgart 1985, p. 106 ff., ISBN 3-432-94671-6

Notes and individual references

  1. ↑ Usually, a volume fraction is always given in percent. However, if the volume fraction of dark Mafite is specified as the color index M ′, the percentage symbol is omitted.
  2. ^ Volkmar Trommsdorff, Volker Dietrich: Fundamentals of Earth Sciences: Lecture for the University and ETH Zurich. 6. revised Ed., Vdf Hochschulverlag AG at the ETH Zurich, Zurich 1999, ISBN 3-7281-2078-2 , p. 57

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