Neutron Powder Diffraction Study of the Akermanite-Gehlenite Solid-Solution Series
Physics and Chemistry of Minerals 19(3): 185-195
The crystal structures of members of the akermanite-gehlenite solid solution series have been refined from high resolution neutron powder diffraction data. Synthetic samples of 0, 25, 50, 75 and 100% akermanite composition were used. The results are consistent with the ordering scheme for the tetrahedrally-coordinated cations represented by the structural formula Ca2[MgxAl1-x]1[Si1+xAl1-x]2O7 where the superscripts denote inequivalent tetrahedral sites. No evidence was found for the existence of phase transitions in the samples with x less-than-or-equal-to 0.75, in contrast to the behaviour found in pure akermanite. The relationship between the cell parameters of meilite and the tetrahedral conformations and the resulting non-ideality of the solid-solution are discussed.