This is because pyroxenes stoichiometrically contain more silica than olivine. SiO 2-poor compositions lie close to the top vertex (dunite), whereas SiO 2-rich compositions occur at the base of the diagram. This diagram defines very well the first-order chemical variability of ultramafic rocks. ‘Olivine’ is specified in the name in pyroxenite varieties with an olivine content between 5 and 40% (e.g. Websterite: consisting of clinopyroxene and orthopyroxene.Ĭlinopyroxenite: with clinopyroxene > 90%. Orthopyroxenite: with orthopyroxene > 90%. Wehrlite: consisting largely of olivine and clinopyroxene. Lherzolite: a peridotite with olivine, orthopyroxene, and clinopyroxene. Harzburgite: consisting predominantly of olivine and orthopyroxene. Two broad fields are separated based on the modal content of olivine: peridotites (olivine > 40%) and pyroxenites (olivine 90%). For ultramafic rocks containing hornblende (extremely rare) the diagram below should be used instead. Most ultramafic rocks consist of the three minerals in this diagram: olivine, ferromagnesian pyroxene (orthopyroxene, enstatite), and calcium pyroxene (clinopyroxene, diopside). IUGS classification of ultramafic rocks with olivine, orthopyroxene, and clinopyroxene.
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