Φ as function of composition for a binary SiO₂-MgO melt (lines, colored by fO₂), assuming T_irr = 2500 K, 1 M_⊕ and 1 R_⊕, compared to Φ for a atmosphere-free, pure black body planet (black dashed line). The Φ of the multicomponent-melts of the archetypes in Table 2 are overlaid (markers). The distribution of SiO₂/MgO of hypothetical exoplanet mantles is indicated as histogram (top), with the 16, 50 and 80 percentile shown as grey dotted lines.