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

Abstract

This paper reports a structure analysis of the åkermanite-gehlenite solid solution. This solution is non-ideal with a negative excess volume in the entire compositional range. X-ray diffraction shows anomalous behavior of the cell parameters close to the gehlenite end-member (åk 08 ge 92 , åk 35 ge 65 ). 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 atomic scale for Al-rich compositions, and single-crystal structure refinements show a correlated decrease of the tetrahedral volume preferentially occupied by Si. The incommensurate modulation, characteristic of åkermanite, has been observed also in åk 95 ge 05 , and it is still visible as diffuse scattering in åk 75 ge 25 . (210) twinning has been observed in the entire compositional range.