Why does magnetite gain weight upon ignition?

Magnetite (Fe3O4) gains weight upon ignition primarily due to the oxidation of iron ions. When heated, some of the iron in magnetite can be converted from Fe²⁺ to Fe³⁺, resulting in the formation of additional iron oxides, such as hematite (Fe2O3). This oxidation process increases the overall mass of the sample because it incorporates more oxygen from the atmosphere, leading to a heavier product. Thus, the weight gain is a result of the chemical reaction with oxygen during ignition.