PYROXENE

any of a group of important rock-forming silicate minerals of variable composition, among which calcium-, magnesium-, and iron-rich varieties predominate. Common pyroxenes belong to either the low-calcium enstatite(ortho)ferrosilite series, (Mg,Fe)SiO3, or high-calcium diopsidehedenbergite series, Ca(Mg,Fe)Si2O6. The pyroxenes have a crystal structure composed of single chains of silicon tetrahedrons sharing two oxygen atoms with adjacent tetrahedrons. Their crystals are typically elongated and show two good cleavages at about 90. The metal ions are in six-fold coordination between chains in two symmetry-distinct sites. Low-calcium, high-magnesium pyroxenes usually have orthorhombic symmetry and are termed orthopyroxene or hypersthene, while high-calcium and low-calcium, moderate-iron pyroxenes have monoclinic symmetry and are termed clinopyroxene or augite. Pyroxenes with compositions intermediate between augite and hypersthene are termed pigeonite and are common in basic volcanic rocks. Submicroscopic to visible lamellar intergrowths are a common feature owing to unmixing of low- and high-calcium pyroxenes at subsolidus temperatures. Rapidly cooled rocks such as basalts show a wide range in composition from early magnesium-rich to late iron-rich within a single grain. In contrast, slowly cooled igneous and metamorphic rocks typically show narrow compositional ranges. Enstatite is an important constituent of the upper mantle of the Earth. Other rare pyroxenes include jadeite, aegirine (or acmite), and johannsenite. any of a group of important rock-forming silicate minerals of variable composition, among which calcium-, magnesium-, and iron-rich varieties predominate. Additional reading W.A. Deer, R.A. Howie, and J. Zussman, Rock-forming Minerals, 5 vol. (196263), with a 2nd ed. in progress (1978 ); Manual of Mineralogy (after James D. Dana), 20th ed. by Cornelis Klein and Cornelius S. Hurlbut, Jr. (1985); J.J. Papike and M. Cameron, Crystal Chemistry of Silicate Minerals of Geophysical Interest, Reviews of Geophysics and Space Physics, 14(1):3780 (1976); Charles T. Prewitt (ed.), Pyroxenes (1980). William B. Simmons