Magnesium-rich orthopyroxenes and calcium-rich clinopyroxenes are important constituents of basalts, gabbros, peridotites, and norites. They are the major minerals in pyroxenites. Magnesium-rich orthopyroxenes occur in the earlier-formed rocks of layered ultramafic complexes.
Iron-rich orthopyroxenes are found in metamorphosed iron formations in association with the amphibole grunerite. At higher grades of regional metamorphism, the amphibole anthophyllite breaks down to form magnesium-iron orthopyroxenes. The orthopyroxene enstatite occurs in high-temperature and high-pressure granulite facies rocks such as quartz-rich, garnet-bearing granulites.
The key differences between metamorphic and igneous orthopyroxenes are primarily related to their formation processes and resulting chemical compositions:
These differences are crucial for geologists when interpreting the geological history and conditions of rock formation.
Dear Massoud Rahimpour: let me add that the metamorphic orthopyroxene (which used to be called hypersthene) is Fe rich, but also can have important proportions of Mn, this is quite obvious when the pleochroic scheme of the mineral goes from green to pink, as it happens in the orthopyroxene in the Imataca Complex (granulites and charnockites), located in the northern edge of the Guiana Shield, in Venezuela. These rocks, probably of volcanic origin, underwent Neoarchean granulitic metamorphism some 2,8-2,7 Ga ago), Regards, Sebastian.
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