The structures of (Sr,Ba)[(Al,Ga)2(SiGe)2O8]. V. The crystal structure of the synthetic feldspar BaAl2Ge2O8

The structure of the feldspar modification of BaGa2Ge2Oa has been found from diffractometer intensities and refined by Fourier and least-squares methods. The structure is similar to that of celsian: space group I2/c, Z = 8. The mean values for the four independent tetrahedra of the asymmetric unit are: (Ge(1) (0)-O)= 1.74, (Ge(2) (z)-O)= 1.74, (Ga(1) (z)-O)= 181, (Ga(2) (0)-O)= 1.82 A. These figures are compatible with the postulate of nearly complete Ga/Ge order. The strains in the tetrahedral bond angles are very marked, the angles varying between 96 and 118 °. The Ba ion can be considered sevencoordinated. The influence of different T and M cations on the framework of the 14 A monoclinic feldspars is examined.