Composition
Peridotites are rich in magnesium, reflecting the high proportions of magnesium-rich olivine. The compositions of peridotites from layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole. Minor minerals and mineral groups in peridotite include plagioclase, spinel (commonly the mineral chromite), garnet (especially the mineral pyrope), amphibole, and phlogopite. In peridotite, plagioclase is stable at relatively low pressures (crustal depths), aluminous spinel at higher pressures (to depths of 60 km or so), and garnet at yet higher pressures.
Pyroxenites are related ultramafic rocks, which are composed largely of orthopyroxene and/or clinopyroxene; minerals that may be present in lesser abundance include olivine, garnet, plagioclase, amphibole, and spinel.
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