Neutron and X-ray total scattering study of hydrogen disorder in fully hydrated hydrogrossular, Ca 3 Al 2 (O 4 H 4 ) 3

The structure of fully hydrated grossular, or katoite, contains an unusual arrangement of four O–H bonds within each O4 tetrahedra. Neutron and X-ray total scattering from a powdered deuterated sample have been measured to investigate the local arrangement of this O4D4 cluster. The O–D bond length determined directly from the pair distribution function is 0.954 A, although the Rietveld-refined distance between average O and D positions was slightly smaller. Reverse Monte Carlo refinement of supercell models to the total scattering data show that other than the consequences of this correctly determined O–D bond length, there is little to suggest that the O4D4 structure is locally significantly different from that expected based on the average structure determined solely from Bragg diffraction.