Phase equilibria on the pseudobinary join NaFeO 2SiO 2 have been determined for SiO 2 contents in the range 0-20 mole% and for temperatures in the range of 700-1350°C, using thermal analysis methods and high-temperature X-ray diffraction. A new cubic polymorph, δ-NaFeO 2, has been found to be stable from 1250°C up to the melting point. The δ-phase forms solid solutions with SiO 2, extending over the full composition range studied. The lower-temperature γ and β polymorphs of NaFeO 2 form limited solid solutions with SiO 2, containing up to 5 and 11 mole% SiO 2, respectively. A new solid solution, γ' ss, was identified with a lower SiO 2 composition limit of 7.5 mole%, at 1050°C, that extends beyond the upper SiO 2 content considered in this study. The γ' ss members have orthorhombic symmetry with unit cell parameters a γ' ⋍ a γ, b γ' ⋍ 2b γ, c γ' ⋍ 2c γ. X-ray diffraction data are given for the various silica-containing ferrite phases as a function of temperature and SiO 2 content, and a new mechanism is proposed for the β to γ structure transformation. The applicability of the results to a new process for regeneration of spent paper-pulp liquors is discussed.