Peridotite is a dense, coarse-grained
igneous rock, consisting mostly of the minerals
olivine and
pyroxene. Peridotite is
ultramafic, as the rock contains less than 45%
silica. It is high in
magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable
iron. Peridotite is derived from the
Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered
igneous complexes vary widely, reflecting the relative proportions of
pyroxenes,
chromite,
plagioclase, and
amphibole.