orthopyroxene

1. (mineralogy) Any pyroxene that forms orthorhombic crystals.Examples: "Biotite from lava flows is invariably rimmed or totally replaced by an exceedingly fine-grained, almost isotropic intergrowth of K-feldspar, orthopyroxene, cordierite and ilmenite, probably as a result of dehydration of the magma on eruption. Cordierite and orthopyroxene (5% each) occur as euhedral phenocrysts 1 to 2 mm in diameter, but cordierite can be up to 5 mm across and occur as anhedral ovoids."; "The meteorite contains abundant, distinct to poorly defined chondrules (about 40 volume %). They are enclosed in a recrystallised, heavily iron-stained, but coherent matrix of irregular fragments of olivine and orthopyroxene, together with fragmented chrondrules and small grains and interstitial areas of oligoclase."; "However, the relatively low Al₂O₃ content of the co-existing orthopyroxenes (2.2 wt%) is not consistent with an upper mantle origin (Binns et al. 1970) and therefore probably crystallised at crustal pressures."