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American Mineralogist; October 2007; v. 92; no. 10; p. 1685-1694; DOI: 10.2138/am.2007.2380
© 2007 Mineralogical Society of America
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Non-ideality and defectivity of the åkermanite-gehlenite solid solution: An X-ray diffraction and TEM study

Mauro Gemmi1,*, Marco Merlini1, Giuseppe Cruciani2 and Gilberto Artioli1

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




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M. Merlini, M. Gemmi, M. Hanfland, and W. Crichton
High-pressure behavior of akermanite and gehlenite and phase stability of the normal structure in melilites
American Mineralogist, May 1, 2009; 94(5-6): 704 - 709.
[Abstract] [Full Text] [PDF]


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