Siderite Decomposition Kinetics—Influence of Time, Temperature, and Isomorphous Impurities

Siderite (FeCO3) is an iron-bearing carbonate mineral that is the most abundant sedimentary iron formation on Earth. The influence of time, temperature, and isomorphous impurities on the kinetics of siderite decomposition in air was studied using Mössbauer spectroscopy, X-ray diffraction, X-ray fluorescence spectroscopy, and the Transmission Electron Microscopy method. Two rock siderite samples were used for investigations, one containing a significant amount of magnesium. The siderite decomposition process begins at a temperature of 300 °C. In a sample containing practically no magnesium, complete decomposition occurs at a temperature of 450 °C, and in a sample with magnesium at a temperature of 500 °C. The annealing time does not affect the width of the temperature range where siderite decomposition occurs. Still, it affects the degree of siderite decomposition: the longer the annealing time, the greater the amount of siderite decomposition. The decomposition products of siderite annealed in air are iron oxides. In the sample containing practically no Mg, this oxide was mainly hematite, and in the sample containing magnesium, it was magnesioferrite. Iron oxides formed directly in siderite decomposition are poorly crystalline, and we can treat them as iron oxide nanoparticles. They maintain this form in a wide temperature range, especially in a magnesium-containing sample. The presence of this element significantly slows down the process of magnesioferrite crystallization.