The structure of bytownite ( Anss ) . A new refinement

Bytownite (Cao.ssNao.l4AI1.83Si2.l60s) (a = 8.188A, b = 12.882A,c = 14.196A,1X = 93.37°,{3 = 116.04°,)' = 90.87°) from a satellite dyke of the Traversella stock (Italian Western Alps) showing only 'a' and 'b' type reflections was refined in space group II using 3756 diffractometer measured intensities. An anisotropic refinement carried out with only CajNa split half-atoms gave R = 0.034; an alternative model with T and ° atoms also treated as isotropic split half-atoms gave R = 0.038. Both refinements confirm, in general, the results of the isotropic refinement (R = 0.118) by Fleet et al. (1966). The more accurate definition of the atomic coordinates permits discussion of the variations in the CajNa cation positions for the average structures oflow-temperature calcic plagioclases in the range AnsoAnlOO' The mean T ° distances in the non-split model (R = 0.034) are: Tl(OO)= 1.622,Tl(OZ)= 1.746,Tl(mO) = 1.729,Tl(mz) = 1.615,TiOO) = 1.721, T2(Oz) = 1.618, TimO) = 1.622, Tirnz) = 1.735A. The AI,Si configuration shows a significantly larger degree of order with respect to the Anss bytownite thermally treated at 1450°C (Facchinelli et aI., 1979), but if the 7 A average structure is considered, the degree of order appears identical.