The results of partial melting experiments with iron-rich and iron-poor mantle compositions have been used to determine the pressure at which pigeonite is stable at mantle solidus temperatures. Pigeonite is stable at the solidus at lower pressure in an iron-rich mantle (Mg♯75, ~1.0. GPa, Mg♯=atomic (Mg/(Mg+Fe))*100), than in an iron-poor mantle (Mg♯ 90, >2.0 GPa). The stabilization of pigeonite in a peridotite mantle will result in the absence or low modal abundance of orthopyroxene in the subsolidus assemblage. The melting reaction which takes place at the solidus of a pigeonite-bearing mantle assemblage produces orthopyroxene. Partial melting increases the mode of orthopyroxene in the residue. Orthopyroxene may be absent from igneous eclogites because they are the crystallization product of melts from a pigeonite-bearing source. The high modal abundance of orthopyroxene in South African cratonic peridotites may be evidence of their origin as the residue of a high pressure, >2.0 GPa, partial melting event. ¿ American Geophysical Union 1993