Thermal decomposition and reversible transformation of ulexite NaCaB₅O₆(OH)₆·5H₂O

抄録

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)₆·5H₂O under high-humidity conditions. Remarkably, the dehydrated ulexite NaCaB₅O₆(OH)₆·3H₂O rapidly transformed back to ulexite after exposure to high-humidity conditions, indicating a high rehydration capacity caused by minimal structural reconfiguration. In contrast, the amorphous phase obtained at the temperature of 180–500°C exhibited multiple recrystallization pathways, forming both ulexite and borax Na₂B₄O₅(OH)₄·8H₂O, likely due to dissolution–reprecipitation mechanisms. Anhydrous ulexite NaCaB₅O₉ formed by further heating of the amorphous phase at 600°C 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. Dehydrated ulexite, its amorphous phase, and anhydrous ulexite produced by thermal decomposition of ulexite can revert to ulexite under high-humidity conditions. This transformation property implies that the inherited and retained FBBs are reorganized upon rehydration into the crystal structure of ulexite, which represents the thermodynamically most stable phase. This observation also indicates that reconstitution of FBBs can occur readily in water-rich environments. These insights have significant implications for the thermodynamic modeling of borate systems, geological boron cycle involving the bonding mode inheritance of FBB, and the development of borate-based functional materials.