The composition Zn2.33Sb0.67O4 (or Zn7Sb2O12) exists in two polymorphic forms. The thermodynamically stable, low-temperature orthorhombic beta form transforms to the high-temperature cubic alpha-polymorph with a spinel structure at 1225 degrees +/- 25 degrees C. The transformation is fully reversible but slower in the alpha -> beta direction and therefore, it is easy to preserve the high-temperature a-polymorph to lower temperatures where it is kinetically stable but thermodynamically metastable. It is also possible to synthesize the alpha-polymorph directly at low temperatures, e.g., 900 degrees C. This synthesis, of a phase that is thermodynamically stable only at high temperatures, but which has sufficient kinetic stability to exist metastably at low temperatures, represents an example of Ostwald's law of successive reactions in which the first phase to crystallize from a reaction mixture is not necessarily the equilibrium phase of lowest free energy. The crystal structure of the alpha-polymorph has been confirmed by Rietveld refinement of X-ray powder diffraction data to be an inverse spinel, (Zn)[Sb2/3Zn4/3]O-4, in which octahedral sites contain a disordered, random mixture of Zn and Sb and tetrahedral sites are fully occupied by Zn.