Crystal chemistry of aeschynite-(Y) from the Western Alps: residual electron density on difference-Fourier map

Crystal structure and chemical composition of five crystals of non-metamict aeschynite-(Y) from the Western Alps were investigated. In the whole the s tructure previously determined on heat-treated crystals of aeschynite was c onfirmed. Nevertheless an interesting feature emerged from the structure re finements concerning the-presence of a residual density maximum of approxim ately 3 divided by 5 e(-)/Angstrom(3) (C site) in the difference Fourier ma p. The addition of the C site to the structural model significantly improve d 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 o f atoms in the C-site is coupled with structural vacancies in A. The implic ations in the calculation of chemical formulae are discussed.