Crystal chemistry of aeschynite-(Y) from the Western Alps: residual electron density on difference-Fourier map
Bonazzi, Paola; Menchetti, Silvio
European Journal of Mineralogy Volume 11 Number 6 (1999), p. 1043 - 1050
published: Nov 29, 1999
manuscript accepted: Jun 15, 1999
manuscript received: Apr 19, 1999
ArtNo. ESP147051106013, Price: 29.00 €
Abstract Crystal structure and chemical composition of five crystals of non-metamict aeschynite-(Y) from the Western Alps were investigated. In the whole the structure previously determined on heat-treated crystals of aeschynite was confirmed. Nevertheless an interesting feature emerged from the structure refinements concerning the presence of a residual density maximum of approximately 3 ÷-5 e-/Å3 (C site) in the difference Fourier map. The addition of the C site to the structural model significantly improved the final R factors in all refined samples. The C site is asymmetrically located within an eight-fold cavity, similar in shape to that occupied by A cations. The short A-C distances led to the assumption that the presence of atoms in the C-site is coupled with structural vacancies in A. The implications in the calculation of chemical formulae are discussed.