page_title: Ultramafic rock
section_title: Intrusive ultramafic rocks
hierarchical_section_title: Ultramafic rock / Intrusive ultramafic rocks
caption_attribution_description: English: Ternary / triangle diagram of peridotites. Axes are olivine, orthopyroxene and clinopyroxene. Common peridotite compositions highlighted. Deutsch: Ternäres Diagramm / Dreiecksdiagramm. Achsen sind Olivin, Orthopyroxen, Klinopyroxen. Typische Peridotitzusammensetzungen markiert.
context_page_description: Ultramafic rocks are igneous and meta-igneous rocks with a very low silica content, generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals. The Earth's mantle is composed of ultramafic rocks. Ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as carbonatites and ultrapotassic igneous rocks.
context_section_description: Intrusive ultramafic rocks are often found in large, layered ultramafic intrusions where differentiated rock types often occur in layers. Such cumulate rock types do not represent the chemistry of the magma from which they crystallized. The ultramafic intrusives include the dunites, peridotites and pyroxenites. Other rare varieties include troctolite which has a greater percentage of calcic plagioclase. These grade into the anorthosites. Gabbro and norite often occur in the upper portions of the layered ultramafic sequences. Hornblendite and, rarely phlogopite, are also found.
caption: IUGS Classification diagram for intrusive ultramafic rocks based on modal percentages of mafic minerals. Green area represents typical mantle peridotite.
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