Pyroxenites are a diffuse heterogeneity in the upper mantle and represent key lithologies in melting processes and mantle deformation. Mantle peridotites exposed in ultramafic massifs are commonly veined by pyroxenites. The latter experienced the same metamorphic evolution as host peridotite and may develop substantially different phase assemblages in response to the different bulk composition. Although several experimental studies focused on melting relations in pyroxenites, subsolidus phase relations are still poorly known. We provide new experimental constraints on phase stability and mineral chemistry for a natural mantle pyroxenite. Piston-cylinder experiments were conducted from 0.7 to 1.5 GPa, 1100–1250 °C. Al-rich spinel, clinopyroxene, orthopyroxene and olivine are ubiquitous phases within the whole pressure range investigated. At 1100 °C, plagioclase is stable up to 0.9 GPa; anorthite content [An = Ca/(Ca + Na)] decreases as a function of pressure from 0.70 at 0.7 GPa to 0.61 at 0.9 GPa. Maximum plagioclase modal abundance of 14 wt% forms at 0.7 GPa; this amount is more than twice as experimentally determined at the same P–T conditions in fertile lherzolite (5–6 wt%). At intermediate pressure (1.0–1.4 GPa), modal spinel is almost constant (4–5 wt%). A pyrope-rich garnet is stable at 1.5 GPa and its modal abundance increases from 5 to 10 wt% when temperature decreases from 1230 °C to 1150 °C, from 1230 °C to 1150 °C. The Al content in pyroxenes varies significantly across the plagioclase-out and garnet-in transitions and is not pressure-dependent in the spinel-pyroxenite field. At 1100 °C, the plagioclase-out boundary occurs at comparable pressures in the pyroxenite and in fertile lherzolites. On the contrary, the garnet-in curve is located at significantly lower pressure than for mantle peridotites.