Reactive crystallization in HfO2 exposed to molten silicates

•MSiO4 phases crystallize from the reaction of Hf/ZrO2 with acidic silicate melts.•Reactive crystallization is more favorable in HfO2 relative to ZrO2 based systems.•YO1.5 addition degrades MSiO4 formation in favor of non-equilibrium (M,Y)O2.•A secondary phase can constrain grain growth, preventing microcracking in HfO2.•Dissolution and diffusion kinetics of Hf4+ and Zr4+ are quantified and compared.