<p>The thermal decomposition of four iron carbonates (siderites) and their decomposition products in a vacuum (~ 2·10^− 5&#xa0;Pa) was investigated for the first time using in-situ transmission ⁵⁷Fe Mössbauer spectroscopy. These measurements were supported by X-ray diffraction, X-ray fluorescence, Raman spectroscopy, magnetic measurements, and transmission electron microscopy. Mössbauer spectra were collected from room temperature (RT) to 750&#xa0;°C, and the sample was then cooled to RT. The initial samples contained siderite as the only Fe-mineral, and its content in the analyzed samples was greater than 80%. Heated samples contained hematite and magnetite as the only Fe-bearing phases. Siderite decomposition occurs in the temperature range of 300–500&#xa0;°C. The analysis shows that iron carbonates transform directly into magnetite. Then hematite generation occurs as long as siderite is present in the sample because, during its decomposition, oxygen is produced, which enables the oxidation of Fe²⁺ ions in magnetite and the formation of hematite. Changes in hyperfine parameters, such as isomer shift, quadrupole splitting, and hyperfine magnetic field, as a function of temperature were obtained for iron carbonates and iron oxides (hematite, magnetite). These results have not yet been published and allow the identification of iron carbonates and iron oxides in other materials exposed to high temperatures. The measurements also show that magnetite formed during the decomposition of siderite consists of particles of various sizes with a low degree of crystallization.</p>