[S-MP23] Physics and Chemistry of Minerals

[SMP23-P04] Thermal decomposition of ulexite NaCaB₅O₆(OH) ₆･5H₂O and reconstitution under a humid atmosphere

*Kosuke Yamaguchi¹, Atsushi Kyono¹ (1.Unibersity of Tsukuba, Life and Environmental Sciences)

Keywords:ulexite, thermal decomposition, fundamental building block, rehydration reaction

The crystal structures of borate minerals contain three and four-coordinate boron cations, which behave as the fundamental building blocks (FBBs). Hydrated borate minerals are well known to undergo a number of phase transformations by changing their FBBs during thermal decomposition. For example, borax [Na₂B₄O₅(OH)₄·8H₂O], one of the major borate minerals, transforms into tincalconite [Na₂B₄O₅(OH)₄·3H₂O] at 25℃, and into anhydrous α-Na₂B₄O₇ at 650℃ through γ- Na₂B₄O₇ at 600℃. Tincalconite further reversibly transforms back into borax at 25℃ at relative humidities above 60%. In this study, synchrotron powder XRD, single-crystal XRD, and TG-DTA analyses were performed on ulexite [NaCaB₅O₆(OH)₆·5H₂O], one of the major borate minerals, treated with heating and humidification experiments to investigate the FBB changes during the thermal decomposition and reconstitution processes. Synchrotron powder XRD and TG-DTA analyses showed that ulexite transformed into the dehydrated phase [NaCaB₅O₆(OH)₆·3H₂O] at 100℃, and then became an amorphous phase at 174℃. Subsequently, it crystallized into the anhydrous phase [NaCaB₅O₉] at 647℃, and finally transformed into calcium borate [CaB₂O₄] at 875℃. The crystal structure of the dehydrated phase determined by single crystal XRD showed that the FBB of the dehydrated phase possesses the same linkage as ulexite. The change of FBB in the thermal decomposition process from ulexite to the calcium borate was as follows: the FBB of ulexite (ulexite-type FBB) was maintained in the phase change between ulexite and the dehydrated phase. In the change from the dehydrated phase to the anhydrous phase, however, the ulexite-type FBB was partially changed and a part of ulexite-type FBB was inherited to the anhydrous phase (anhydrous phase-type FBB). In the change from the anhydrous phase to the calcium borate, the anhydrous phase-type FBB was also partially changed and a part of the anhydrous phase-type FBB was inherited to the calcium borate. Humidification experiments revealed that at 20℃ and relative humidity of 100% ulexite is reconstituted from the all thermally produced phases; dehydrated, amorphous, and anhydrous phases. Particularly, the dehydrated phase with the ulexite-type FBB readily revered to ulexite. In addition, the author found that the amorphous phase formed not only ulexite but borax under a humid environment. This might be because the local FBB of the amorphous phase possesses the same linkage as borax. This study confirms that dehydrated phases of the major borate minerals such as borax and ulexite are reversibly transformed back into the original phases under a humid environment. This suggests that the major borate minerals would repeat the dehydration and rehydration cycle with extreme weather events and climate change between dry and wet conditions.