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

Abstract

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.