DeVore, George W. Department of Geology, Florida State University, Tallahassee, Florida.
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The name given to the monoclinic pyroxenes of the general formula (Mg,Fe)SiO3 with some augite in solid solution. Pigeonite bears the same relation to the orthorhombic pyroxenes as augite does to the diopside-hedenbergite series. Pigeonite is the orthorhombic pyroxene equivalent in the volcanic rocks. Most high-temperature metamorphic and igneous orthorhombic pyroxenes were probably originally pigeonite. The small optic angle (2V) distinguishes the mineral from augite and the inclined extinction distinguishes it from the orthorhombic pyroxenes. Pigeonite forms black, brown, or dark-green short stubby crystals with the 87° pyroxene (110) cleavages. The slower cooling rates of the igneous and metamorphic rocks usually permit the augitic materials in solution to exsolve and the remaining monoclinic pyroxene to invert to the orthorhombic form. The original augitic material is evident by the oriented exsolution lamellae in the host orthorhombic pyroxene. The faster cooling rates of the volcanic rocks quenches in the augitic material and thereby preserves the metastable pigeonite at surface temperatures. See also: Augite; Diopside; Enstatite; Orthorhombic pyroxene; Pyroxene
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