- α-phase: light blue -- forms at 1 atm below 23.8 K, monoclinic crystal structure
- β-phase: faint blue to pink -- forms at 1 atm below 43.8 K, rhombohedral crystal structure (at room temperature and high pressure, transforms to tetraoxygen, O4)
- γ-phase: faint blue -- forms at 1 atm below 54.36 K, cubic crystal structure
- δ-phase: orange -- forms at room temperature at pressure of 9 GPa
- ε-phase: dark-red to black -- forms at room temperature at pressures greater than 10 GPa
- ζ-phase: metallic -- forms at pressures greater than 96 GPa
Image: The octaoxygen or O8 molecule, which forms red crystals.