Thermal decomposition and dehydration mechanism of kurnakovite MgB3O3(OH)5·5H2O

Abstract The thermal decomposition and dehydration mechanism of kurnakovite MgB3O3(OH)5·5H2O, a hydrated Mg borate mineral with potential industrial applications, were investigated using thermal analysis, in-situ high-temperature synchrotron single-crystal X-ray diffraction (30–140 °C), and ex-situ high-temperature synchrotron powder X-ray diffraction (20–1000 °C). Kurnakovite exhibited anisotropic thermal expansion up to 120 °C, attributed to the increase in lengths of the hydrogen bonds and thermal expansion of the Mg octahedra. The linear thermal expansion coefficients obtained between 30 and 120 °C for lattice parameters a, b, and c were αa = 3.73 × 10–5 K–1, αb = 2.14 × 10–5 K–1, and αc = 3.13 × 10–5 K–1, respectively. Therefore, the thermal expansibilities were largest along the a axis, followed by the c and b axes. Subsequently, a rapid mass loss of 25.6% occurred at 142 °C, corresponding to dehydration of four H2O molecules. Analyses of isotropic and anisotropic displacement parameters indicated that the dehydration of two distinct types of H2O molecules exhibiting pronounced thermal vibrations led to the disconnection of hydrogen bonds and significantly disrupted the electrostatic balance between the Mg-centered interstitial complex and borate fundamental building blocks (FBBs), resulting in rapid amorphization. Upon further heating, the amorphous material recrystallized at 761 °C, and ex-situ XRD revealed that the anhydrous Mg borate phases derived from kurnakovite evolved through four distinct stages containing m-Mg2B2O5, t-Mg2B2O5, MgB4O7, and the unidentified phases X, Y1, Y2, and Z. Stage I between 700 and 850 °C consisted of m-Mg2B2O5, MgB4O7, and phase X; stage II between 850 and 900 °C of t-Mg2B2O5, MgB4O7, and phase Y1; stage III between 900 and 950 °C of t-Mg2B2O5, MgB4O7, and phase Y2; and stage IV between 950 and 1000 °C of t-Mg2B2O5 and phase Z. The endothermic event at 993 °C is attributed to partial melting of MgB4O7 and/or phase Y2 during the transition from stage III to stage IV.