Ultra-basic rock

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Ultrabasic peridotite, with serpentine weathering, Serra da Capelada , Galicia

An ultra- basic rock is igneous rock with an SiO 2 content below 45 percent by weight . Rocks with a higher SiO 2 content are called basic (up to 52%), intermediate (52–63%) or acidic rocks .


An ultra-basic rock consists predominantly of low-silicon mafic minerals such as olivine , pyroxene , amphibole and melilite . The mass fraction of less than 45% characterizes it as SiO 2 - undersaturated , therefore ultrabasic rock never contains free SiO 2 in the form of quartz or cristobalite . Instead, olivine and minerals from the group of foids are common.

Typical representatives of the group of ultra-basic rocks are dunite (a plutonite ), foidite and melilithite ( volcanic rock ).

The term ultrabasic rock relates to the chemical composition of the rock, that of the ultramafic rock to the proportion of dark minerals ( color index M '> 90); the two terms are therefore not synonymous .


  • RW Le Maitre et al .: Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. 252 pp., Cambridge University Press 2005. ISBN 978-0-521-61948-6