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

¹ Dipartimento di Scienze della Terra "A. Desio," Università degli Studi di Milano, via Botticelli, 23, I-20133 Milano, Italy
² Dipartimento di Scienze della Terra, Università di Ferrara, via Saragat 1, I-44100 Ferrara, Italy

Correspondence: ^* E-mail: mauro.gemmi{at}unimi.it

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 (åk₀₈ge₉₂, åk₃₅ge₆₅). 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 åk₉₅ge₀₅, and it is still visible as diffuse scattering in åk₇₅ge₂₅. (210) twinning has been observed in the entire compositional range.

Key Words: Melilite • solid solution • X-ray diffraction • electron microscopy