On variety of the Ca coordination in the incommensurate structure of synthetic iron - bearing akermanite , Ca 2 ( Mgo . ss , Feo . 4 s ) Si 207

The incommensurate modulation in a synthetic iron-bearing akermanite, Ca2(Mg, Fe )Si207, has been determined at room tempe~ature by the five-dimensional refinements of the structure. The basic structure is tetragonal P4 21m with unit-cell dimensions a =7.8679(3), c ~5.0144(2)A, V=31OAl(2)A3, 2=2, M=286.69, Dx =3.067Mg . m-3, MoKa with graphite monochromator, J1 =3.199mm-l, F(000)=284A9, Mg/(Mg+Fe)=0.554(6), R=0.065 for 770 unique reflections. The modulated structure is also tetragonal P~~~km, kt = 0.295(2)x (a*+b*), k2=0.295(2)x(-a*+b*), where kt. k2 are the wave vectors and a*, b* the reciprocal lattice vectors of the basic structure. R=0.136 for 3965 unique reflections. The modulation is caused mainly by the shifts of Ca and 0 atoms, and substitutional modulation at (Mg, Fe) sites was not detected in the present studies. Six-coordinated Ca forming a distorted oxygen octahedra exists in the modulated structure and most of them form clusters together with the flattened (Mg,Fe )04 tetrahedra. The number of the clusters in the present material is less than that in Co-akermanite. The reason may be attributed to the disorder of the modulation amplitudes and wavelength.