IUGS Classification of Ultramafic Rocks


IUGS Classification of Ultramafic Rocks

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Within the IUGS classification of igneous rocks, rocks that contain more than 90% of mafic minerals are called ultramafic rocks (there are no "mafic rocks").
If their mineral composition can be determined, they are classified independently from the QAPF scheme by their relative content (in volume percent) of either
- olivine (Ol), orthopyroxene (Opx) and clinopyroxene (Cpx), or
- olivine (Ol), pyroxenes (Px) and amphiboles (Hbl).

Amphiboles are represented as hornblende (Hbl) and the rocks are called hornblendites because the name amphibolite is already in use for another rock.

The figure shows the two corresponding ternary diagrams with the ultramafic rock names.

Ultramafic rocks are all plutonic rocks, there are no volcanic counterparts, as they are for rocks that are classified in the QAPF diagram. The reason is that these rocks form as cumulates by segregation of solid crystals from magma, and not by solidification of the magma itself.

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Photo added: 20th Jun 2012