Crystal structure of spinelloid II in the system Fe (sub 3) O (sub 4) -Fe (sub 2) SiO (sub 4)

The crystal structure of Fe (sub 2.57) Si (sub 0.43) O (sub 4) synthesized at 1200 degrees C and 4.0 GPa has been determined. It is isotypic with the spinelloid II structure found in the NiAl (sub 2) O (sub 4) -Ni (sub 2) SiO (sub 4) and the NiGa (sub 2) O (sub 4) -Ni (sub 2) SiO (sub 4) systems; space group Imma; a 5.85928, b 17.9801, c 8.38395 Aa, V 883.252 Aa. The structure contains two symmetrically distinct tetrahedral sites which together form T (sub 3) O (sub 10) groups; both tetrahedra are partially occupied by both Fe (super 3+) and Si (super 4+) , with the central tetrahedral site of each group being enriched in Si (super 4+) relative to the terminal sites. The only octahedral site bonded to the bridging oxygen of the T (sub 3) O (sub 10) groups appears to be occupied by Fe (super 2+) alone, while the remaining three distinct octahedral sites are smaller and appear to contain both Fe (super 2+) and Fe (super 3+) .