Compressional study of katoite [Ca₃Al₂(O₄H₄)₃] and grossular garnet

End‐member synthetic katoite and grossular were studied by energy‐dispersive X ray diffraction in a diamond anvil cell to 42.3 GPa and 18.4 GPa, respectively. A Birch‐Murnaghan fit to the katoite data gives K 0 = 66 ± 4 GPa and K 0 ′ = 4.1 ± 0.5. For grossular we determined K 0 = 168 ± 25 GPa with K 0 ′ = 6.2 ± 4. A better constrained fit is obtained using K 0 = 168.4 GPa as determined by Brillouin spectroscopy (Bass, 1989) and the present data to give K 0 ′ = 6.1 ± 1.5. The low bulk modulus of katoite is attributed largely to the highly compressible tetrahedral O‐O distances in the structure.