Thermal Decomposition of Siderite and Characterization of the Decomposition Products under O2 and CO2 Atmospheres

Abstract

Siderite (FeCO3) is an iron-bearing carbonate mineral that is the most abundant sedimentary iron formation on Earth. Mineralogical alteration of four siderite samples annealed at temperatures 200 °C, 300 °C, 400 °C, 500 °C, 750 °C, and 1000 °C in an O2 and a CO2 atmosphere were investigated using such tools as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), the X-ray fluorescence (XRF) method, differential scanning calorimetry and thermogravimetric analysis (DSC/TGA), and Mössbauer spectroscopy measurements. The decomposition of three siderite samples with similar iron content in the oxygen atmosphere took place in the temperature range of 340–607 °C. This process begins at approximately ~100 °C higher under a reducing atmosphere, but it is completed just above 600 °C, which is a temperature comparable to decomposition in an oxidizing atmosphere. These processes are shifted toward higher temperatures for the fourth sample with the lowest iron but the highest magnesium content. Magnetite, hematite, and maghemite are products of siderite decomposition after annealing in the oxygen atmosphere in the temperature range 300–500 °C, whereas hematite is the main component of the sample detected after annealing at 750 °C and 1000 °C. Magnetite is the main product of siderite decomposition under the CO2 atmosphere. However, hematite, maghemite, wüstite, and olivine were also present in the samples after annealing above 500 °C in this atmosphere.