Thermogravimetric analysis–mass spectrometry (TGA–MS) of hydromagnesite from Dujiali Lake in Tibet, China

Thermogravimetric analysis–mass spectrometry, in situ X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy were used to characterize hydromagnesite [Mg₅(CO₃)₄(OH)₂·4H₂O] from Dujiali Lake in Tibet, China. This study describes the variations in the thermal decomposition mechanisms of hydromagnesite at varying heating rates and under either helium (He) or carbon dioxide (CO₂) atmospheres. In a He atmosphere, only two decomposition stages were observed; the loss of the crystalline water followed by the combined dehydroxylation and decarbonation. However, under a CO₂ atmosphere, the dehydroxylation and decarbonation occur separately as the inert CO₂ gas prevents the decomposition of the MgCO₃ component of hydromagnesite. Overall, the thermal decomposition is an endothermic process. A distinctly exothermic process occurs at about 540 °C under conditions of high partial pressure of CO₂ or high heating rate and implies the crystallization of magnesite (MgCO₃). We propose that the release of H₂O and CO₂ at different stages likely results from the complicated hydrogen bonds and different carbonate groups in the crystal structure of hydromagnesite.