Ultramafic rocks are especially poor in Si and have high Mg:Fe ratios; Below table gives some examples. Peridotite is a general term used for all of them. Pyroxene (either orthopyroxene or clinopyroxene), olivine, and plagioclase are the dominant minerals in peridotites. We call rocks composed nearly entirely of pyroxene pyroxenite, and those composed nearly entirely of olivine dunite (Figure a). Many dunites look like light-green equivalents of sandstone. Because of their very high melting temperatures and mafic compositions, ultramafic magmas must come from deep within the Earth. They rarely reach the surface to produce volcanic rocks, but some Precambrian terranes contain spectacular examples of ultramafic lava flows called komatiites.
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Ultramafic Igneous Rocks
| Plutonic Rock Name | Volcanic Rock Name | Major Minerala | Minor Minerals |
|---|---|---|---|
| Peridotite | Komatiite | Olivine, Clinopyroxene, Orthopyroxene | Hornblende |
| Pyroxenite | Does not exist | Clinopyroxene, Orthopyroxene | Olivine, Hornblende |
| Dunite | Does not exist | Olivine | Spinel |
Ultramafic minerals are most stable at high temperature and tend to alter by reaction with water or carbon dioxide when exposed to normal Earth surface conditions. Consequently, finding fresh, unaltered ultramafic rock is difficult. Variable amounts of secondary serpentine, chlorite, talc, brucite, or calcite are nearly always present. Rocks called serpentines in which serpentine has replaced all mafic minerals, often result. Many ultramafic rocks including kimberlites, ultramafic rocks associated with diamonds—have been altered -so-much that we cannot determine the original mineralogy.
 Figure (a)
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 Figure (b)
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| Dunite In hand specimen and thin section: (a) 0.5-cm wide view of dunite in hand specimen, showing only olivine; (b) dunite in thin section showing high relief olivine grains (1-mm-wide view). |