In situ and ex situ studies on thermal decomposition process of hydromagnesite Mg5(CO3)4(OH)2·4H2O

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/c0 > a/a0$$\gg$$b/b0. 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.