Non-ideality and defectivity of the åkermanite-gehlenite solid solution: An X-ray diffraction and TEM study

This paper reports a structure anal. of the akermanite-gehlenite solid soln. This soln. is non-ideal with a neg. excess vol. in the entire compositional range. X-ray diffraction shows anomalous behavior of the cell parameters close to the gehlenite end-member (ak08ge92, ak35ge65). This behavior is correlated with an excess of Si and deficiency of Al with respect to the Mg content, which implies a defective, non-stoichiometric structure with Ca vacancies. Electron microscopy images have confirmed an increase in the defectivity on the at. scale for Al-rich compns., and single-crystal structure refinements show a correlated decrease of the tetrahedral vol. preferentially occupied by Si. The incommensurate modulation, characteristic of akermanite, has been obsd. also in ak95ge05, and it is still visible as diffuse scattering in ak75ge25. (210) twinning has been obsd. in the entire compositional range.