Reversals locating the equilibrium orthoenstatite = protoenstatite + diopside in CaO-MgO-SiO2 demonstrate that orthoenstatite cannot exist stably above 1005¿10 ¿C at one atmosphere pressure. This in turn suggests that the orthorhombic low-Ca phase found near 1400 ¿C at atmospheric pressure in this system (formerly identified as orthoenstatite) is in fact a previously unrecognized pyroxene polymorph.