Thermal decomposition and reversible transformation of ulexite NaCaB<sub>5</sub>O<sub>6</sub>(OH)<sub>6</sub>·5H<sub>2</sub>O

Borate minerals, particularly hydrated borates, undergo structural transformations during thermal decomposition, which is an important process for understanding their thermodynamic stability and structural properties.In this study, we investigated reversible transformations of the dehydration/dehydroxylation phases of ulexite NaCaB5O6(OH)65H2O under high-humidity conditions.Remarkably, the dehydrated ulexite NaCaB5O6(OH)63H2O rapidly transformed back to ulexite after exposure to highhumidity conditions, indicating a high rehydration capacity caused by minimal structural reconfiguration.In contrast, the amorphous phase obtained at the temperature of 180-500C exhibited multiple recrystallization pathways, forming both ulexite and borax Na2B4O5(OH)48H2O, likely due to dissolution-reprecipitation mechanisms.Anhydrous ulexite NaCaB5O9 formed by further heating of the amorphous phase at 600C reversibly transformed to ulexite through progressive rehydroxylation of its fundamental building blocks (FBBs).Furthermore, we confirmed that the thermal transformation of ulexite follows the principle of FBB inheritance, where a structural unit of the FBBs persists across successive phases.Ulexite fully preserved the FBB to dehydrated ulexite, whereas dehydrated ulexite passed a partial FBB to anhydrous ulexite.Even after the phase decomposition, a part of the FBB in anhydrous ulexite was retained in CaB2O4.