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A comprehensive thermodynamic assessment has been carried out on the phases SiO₂(L), including SiO₂(glass), the amorphous monoxide SiO(am) and on the solubility of oxygen in solid and liquid silicon, jointly with the equilibrium partition coefficient k. Contradictions in the sparse thermodynamic data of SiO(am) are pointed out. Our key experiments were performed on the decomposition and condensation of SiO(am). X-ray diffraction on samples heated in evacuated silica capsules was used. The condensation experiment was performed in local equilibrium on a silica rod in a commercial CZ-Si apparatus. With the Calphad method a complete set of Gibbs energy equations was developed for the phase SiO₂(L), comprising glass, supercooled liquid and stable liquid silica, the phase SiO(am) and the solid and liquid silicon solution phases. Combining this with the data for crystalline silica and the gas phase, a …