 Likewise, at conditions more reducing than the IW (iron-wustite) buffer, minerals such as pyroxene can still contain Fe3+.The redox buffers therefore are only approximate guides to the proportions of Fe2+ and Fe3+ in minerals and rocks.Terrestrial igneous rocks commonly record crystallization at oxygen fugacities more oxidizing than the WM (wüstite-magnetite) buffer and more reduced than a log unit or so above the nickel-nickel oxide (NiNiO) buffer.