Orthorhombic pyroxene

Article By:

Tracy, Robert J. Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia.

Last reviewed:December 2019

DOI:https://doi.org/10.1036/1097-8542.477600

A group of minerals having the general chemical formula XYSi₂O₆, in which the Y site contains iron (Fe) or magnesium (Mg) and the X site contains Fe, Mg, manganese (Mn), or a small amount of calcium (Ca; up to about 3%). They are characterized by orthorhombic crystal symmetry, which makes it impossible for significant amounts of larger cations such as Ca or sodium (Na) to enter the X position. The most important compositional variation in the orthopyroxene series is variation in the Fe-Mg ratio, leading to a solid solution series bounded by the end members enstatite (Mg₂Si₂O₆; abbreviated En) and ferrosilite (Fe₂Si₂O₆; Fs). Names that are used for intermediate members of the series are enstatite (Fs_0-50) and ferrosilite (Fs_50-100). More Fs-rich enstatite may be called ferroan enstatite and more enstatite-rich ferrosilite may be called magnesian ferrosilite. In addition to the major elements noted above, minor components in orthopyroxene may include aluminum (Al), titanium (Ti), chromium (Cr), nickel (Ni) and ferric iron (Fe³⁺), although only Al occurs in substantial amounts. Conditions of formation may be important in determining orthopyroxene compositions: for example, Al content is greater at higher temperatures and the most Fe-rich compositions are stable only at very high pressures. Further, there is a limited solid solution of orthopyroxene toward calcic clinopyroxene, noted above, which increases with increasing temperature. See also: Enstatite; Solid solution

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