Abstract The solid solution of MgO in Ca 2FeAlO 5 and the resulting change in anisotropic absorption have been studied. The solid solution of MgO was investigated by directly determining the chemical formulas of the solid solutions of the system Ca 2FeAlO 5MgO by electron probe microanalysis. Mg ions replace exclusively Fe and Al ions in the octahedral sites in the structure; they neither replace Ca ions nor enter the interstitial sites. The solubility limit of MgO at 1350 °C was 0.75wt%, which was half the amount hitherto reported. Similarly, incorporation of Si ions in the tetrahedral sites has been confirmed. The optic orientation of Ca 2FeAlO 5 and its solid solutions is X = b, Y = c, Z = a. Absorption anisotropy is weak for Ca 2FeAlO 5 with Y, Z (reddish orange) > X (yellowish orange). With incorporation of MgO, absorption becomes extremely strong for light vibrating in the YZ-plane while remains practically unchanged for light vibrating parallel to the X-axis. The strong absorption in the presence of Mg ions has been ascribed to the occurrence of tetravalent iron ions under oxidizing atmospheric conditions, which compensates the deficiency of positive charge caused by the substitution of Mg for Fe and Al.