Orthopyroxenedominated

Orthopyroxenedominated is a descriptive term used in geology and petrology to characterize rocks in which orthopyroxene is the principal pyroxene. In these rocks, the pyroxene component is largely orthopyroxene (enstatite or ferrosilite) rather than clinopyroxene, with little or no clinopyroxene present.

Occurrences and examples include ultramafic rocks such as harzburgite, where olivine and orthopyroxene are the dominant minerals and clinopyroxene is scarce or absent, and orthopyroxenite, a rock dominated by orthopyroxene. The term can also apply to certain high-temperature metamorphic rocks, including granulites, where orthopyroxene becomes a stable and abundant phase. In mantle xenoliths and other cumulate or crustal rocks formed under conditions that favor orthopyroxene stability, orthopyroxenedominated assemblages are commonly observed.

Mineralogy and identification: Orthopyroxene is typically magnesium-iron silicate (enstatite or ferrosilite). Rocks described as orthopyroxenedominated may lack significant quantities of clinopyroxene. In thin section, identification relies on optical properties such as refractive behavior and extinction, as well as crystal habit and associated minerals. Distinguishing orthopyroxene from clinopyroxene is a routine part of petrographic analysis and is important for interpreting crystallization and metamorphic histories.

Formation conditions and significance: The dominance of orthopyroxene reflects specific chemical and physical conditions, such as Mg-rich bulk composition, high-temperature stability, and fluid activity that can suppress clinopyroxene. These assemblages provide insights into mantle processes, crystallization sequences, and metamorphic pathways, and they help distinguish particular tectonothermal environments in both igneous and metamorphic contexts.

See also: orthopyroxene, clinopyroxene, harzburgite, orthopyroxenite, dunite.