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pyroxenite An ultrabasic, igneous rock consisting of essential clinopyroxene, orthopyroxene, and olivine. The ferromagnesian minerals are magnesium-rich, the pyroxenes dominating olivine, which is usually less than 40% by volume. Different types of pyroxenites are defined using the ratio of clinopyroxene to orthopyroxene to olivine. These include orthopyroxenite, websterite, clinopyroxenite, and olivinebearing versions of these. Pyroxenites are found as cumulate horizons within layered basic intrusions and as components of the sub-oceanic mantle in obducted fragments of ocean crust known as ophiolites.