R2: Crystal structure, crystal chemistry, physical properties of minerals, crystal growth and applied mineralogy

5:00 PM - 5:15 PM JST(8:00 AM - 8:15 AM UTC)

[R2-16] Change in thermal expansion behavior during the stepwise dehydration of kernite Na₂B₄O₆(OH)₂·3H₂O

*Atsushi KYONO¹, Kodai Shibuya¹, Rio Midorikawa¹, Kosuke Yamaguchi¹, Sumit Ranjan Maity², Kouhei Ichiyanagi², Yuiga Nakamura² (1. University of Tsukuba, 2. Japan Synchrotron Radiation Research Institute)

Keywords:

Kernite,Stepwise dehydration,Thermal expansion,Synchrotron X-ray diffraction,H-site disorder

Kernite, Na₂B₄O₆(OH)₂·3H₂O, is an important hydrated sodium borate mineral whose dehydration behavior is closely related to the stability of borate minerals in evaporitic environments. In this study, the dehydration behavior and high-temperature structural response of kernite were investigated by thermogravimetry–differential thermal analysis, in-situ and ex-situ synchrotron powder X-ray diffraction, and in-situ synchrotron single-crystal X-ray diffraction. TG-DTA showed two-stage dehydration, corresponding to the release of approximately 1.5 H₂O and 1.0 H₂O per formula unit, respectively. In situ powder XRD showed that diffraction peaks of kernite persisted up to 200 °C, although a broad background developed at higher temperatures. Ex-situ heating for 1 h resulted in pronounced amorphization, and no distinct crystalline dehydrated phase, including the previously proposed “phase I”, was detected under the present experimental conditions. In-situ single-crystal XRD refinements were successfully performed up to 120 °C, whereas single-crystal coherence was lost at 140 °C. The water oxygen sites O8, O10, and O11 retained occupancies close to unity up to 120 °C, indicating that the average structure obtained from Bragg diffraction does not record uniform water loss from specific crystallographic sites. The most characteristic structural change is a marked decrease in the volumetric thermal expansion coefficient from 1.53 × 10^-4 K^-1 at 30–60 °C to 4.96 × 10^-5 K^-1 at 80–120 °C. This change is associated with suppressed expansion of Na coordination polyhedra and borate polyhedra, rather than with systematic elongation of selected H···O hydrogen bonds. Difference-Fourier maps indicate enhanced unresolved H-site disorder around the O8 and O11 water molecules above approximately 80 °C. These results suggest that the first dehydration stage of kernite involves local H-site disorder, geometrical constraint of the Na–water coordination framework, and progressive dehydration-induced amorphization, rather than formation of a long-range ordered crystalline dehydrated phase.