Kenji Hagiya

The incommensurate modulation in a synthetic iron-bearing åkermanite, Ca₂(Mg, Fe)Si₂O₇, has been determined at room temperature by the five-dimensional refinements of the structure. The basic structure is tetragonal P-42₁m with unit-cell dimensions a=7.8679(3), c=5.0144(2)Å, V=310.41(2)ų, Z=2, M=286.69, D_x=3.067Mg·m⁻³, MoKα with graphite monochromator, μ=3.199mm⁻¹, F(000)=284.49, Mg/(Mg+Fe)=0.554(6), R=0.065 for 770 unique reflections. The modulated structure is also tetragonal P_P4mg^p-42_1m, k₁=0.295(2)×(a^*+b^*), k₂=0.295(2)×(-a^*+b^*), where k₁, 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 O 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)O₄ tetrahedra. The number of the clusters in the present material is less than that in Co-åkermanite. The reason may be attributed to the disorder of the modulation amplitudes and wavelength.