Structural investigations of the two polymorphs of synthetic Fe-cordierite and Raman spectroscopy of hexagonal Fe-cordierite

The crystal structures of synthetic hexagonal and orthorhombic Fe-cordierite polymorphs with the space groups P6/mcc and Cccm were refined from single-crystal X-ray diffraction data to R _{1, hex}?=?3.14 % and R _{1, ortho}?=?4.48 %. The substitution of the larger Fe²⁺ for Mg leads to multiple structural changes and an increase of the unit cell volumes, with a, c (hex)?=?9.8801(16) Å, 9.2852(5) Å and a, b, c (ortho)?=?17.2306(2) Å, 9.8239(1) Å, 9.2892(1) Å in the end-members. Furthermore Fe incorporation results in an increase of the volumes of the octahedra, although the diameters of the octahedra in direction of the c-axis decrease in both polymorphs. X-ray powder diffraction analysis indicates a high degree of Al/Si ordering in the orthorhombic polymorph, the Miyashiro distortion index is ~0.24. Estimations of site occupancies based on the determined tetrahedral volumes result in the following values for hexagonal Fe-cordierite: ~73 % Al for T₁ and ~28 % Al for T₂. For the first time Raman spectroscopy was performed on the hexagonal Fe-cordierite polymorph. In the hexagonal Fe-cordierite polymorph most Raman peaks are shifted towards lower wavenumbers when compared with the Mg-end-member.