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 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 (super -) /Aa ³ (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.