Enthalpy of formation of katoite Ca₃Al₂[(OH)₄]₃: Energetics of the hydrogarnet substitution

The silicon-free end member of the hydrogrossular solid solution series, katoite Ca₃Al₂[(OH)₄]₃, was synthesized in a solid-media, piston-cylinder apparatus. The enthalpy of formation from the component oxides was measured by high-temperature oxide melt calorimetry and found to be ΔH(f) = -255.6 ± 12.2 kJ/mol; the resulting enthalpy of formation from the elements is ΔH(f) = -5551.5 ± 16.4 kJ/mol. From this value, enthalpies for breakdown reactions of hydrogrossular were calculated and the relative energetic stability of katoite evaluated.