Condensation temperatures (T_C) of the rock-forming elements Mg, Si, Ca, and Al, plotted against the molar C/O ratio in disks, based on stellar ϵ_C/ϵ_O values (top). Solar C/O = 0.55 (Asplund et al. 2009) is indicated as a vertical dashed line. All elements show a transition from high T_C at low C/O to lower T_C at higher C/O. For Mg, colouring condensation temperature by the disk Mg/O (= ϵ_Mg / ϵ_O; bottom) reveals that the C/O value at which this transition occurs depends on Mg/O. Condensation temperatures are derived from condensation sequence calculations at P = 10⁻⁴ bar.