How can heating magnetite in air change it into hematite?

Heating magnetite in air facilitates its transformation into hematite through an oxidation process.1

Magnetite contains both ferrous ($F{e}^{2+}$) and ferric ($F{e}^{3+}$) ions in a 1:2 ratio, whereas hematite is the fully oxidized form containing only $F{e}^{3+}$ ions. Because hematite is the most stable form of iron oxide in air, magnetite—which is stable primarily in low-oxygen environments—will undergo a chemical change to hematite when exposed to heat or air for a prolonged period. This redox behavior is a fundamental property of iron oxides, where heating or exposure to oxidizing gases drives the transition toward the more stable, fully oxidized state.12