CRYSTAL-STRUCTURE OF SPINELLOID-II IN THE SYSTEM FE3O4-FE2SIO4

The crystal structure of Fe2.57Si0.43O4 synthesised at 1200 degrees C and 3.0 GPa has been determined by single-crystal X-ray diffraction at room conditions. It is isotypic with the spinelloid II structure foun d in the NiAl2O4-Ni2SiO4 and the NiGa2O4-Ni2SiO4 systems. The space gr oup is Imma and the cell parameters are: a = 5.85928(10) Angstrom, b = 17.9801(3) Angstrom, c = 8.38395(14) Angstrom, V = 883.252(17) Angstr om(3). The structure contains two symmetrically distinct tetrahedral s ites which together form T2O10 groups. Both tetrahedra are partially o ccupied by both Fe3+ and Si4+, with the central tetrahedral site of ea ch group bring enriched in Si4+ relative to the terminal sites. The on ly octahedral site bonded to the bridging oxygens of the T3O10 groups appears to be occupied by Fe2+ alone, while the remaining three distin ct octahedral sites are smaller and therefore appear to contain both F e2+ and Fe3+.