The structure of anorthite, CaAl2Si2O8. I. Structure analysis

Anorthite CaA12Si20 s has the same point group, I, as a simple felspar, but four times the volume per lattice point; its unit cell is primitive, with a 14 Jk c-axis, while that of albite, for comparison, is C-face-centred with a 7 A c-axis.Anorthite approximates much more closely to a C-face-centred cell with a 14 A c-axis than to either a body-centred cell with a 14 tix c-axis or a primitive cell with a 7 /~_ c-axis.The work was done in two stages.A synthesis using only main reflections gives elongated peaks or pairs of peaks whose centres of gravity define an 'average structure'.In the different subcells, atoms have displacements from the average positions whose magnitude and direction are given by the elongation (or 'splitting') and whose signs are found from the difference reflections (which also provide a check for magnitude and direction).The Ca peak has the most conspicuous elongation, and can be used for a heavy-atom method of determining the signs of the difference reflections.In the first stage, only 'C-type difference reflections were used, and the 14/~_ C-facecentred approximation was obtained; a repeated application of the method using 'b' and 'd' reflections gave the true primitive structure.The final refinement was done by successive differential syntheses.Coordinates of the 52 independent atoms are given; their standard deviations are 0.0007 A for Ca, 0.0015/~ for Si and A1, 0.0038 A for O. 'c' splittings or the 'b' splittings respectively.