In situ and ex situ studies on thermal decomposition process of hydromagnesite Mg 5 (CO 3 ) 4 (OH) 2 ·4H 2 O

We investigated crystal structure and the local structure changes during the thermal decomposition of hydromagnesite by using in situ high-temperature XRD and ex situ high-temperature X-ray total scattering measurements. Hydromagnesite displayed anisotropic thermal expansion up to 220 °C. The a and c lattice parameters exhibited an increase trend with temperature, whereas the b lattice parameter and β angle did not show a regular trend with temperature. The relative expansion between 25 and 220 °C followed the c / c 0 > a / a 0 ≫ b / b 0 . At 260 °C, the a , b , and c lattice parameters significantly decreased. Above 280 °C, hydromagnesite underwent a structural collapse with dehydration and dehydroxylation reactions, but was never accompanied by nucleation and growth of crystal phases up to 425 °C. During the thermal decomposition from hydromagnesite to periclase, the Mg atoms maintained the octahedral coordination environments in the structure.