a mineral, the product yielded upon conversion of magnetite, Fe₃O₄ into hematite, Fe₂O₃. The crystals and crystalline grains of magnetite, upon conversion into martite, are usually replaced, without alteration of their original form, by finely aggregated hematite and exhibit the physical properties characteristic of the latter. Martite forms during the oxidation of magnetite in the weathering zone of the earth’s crust as well as deep within the earth under conditions characterized by increased oxidation potential of hydrothermal solutions surrounding the previously formed magnetite. Martite ores are high-grade iron ores.