Electron ordering in ilvaite, a mixed-valence iron silicate: crystal structure refinement at 138 K

Ilvaite, a mixed valence iron silicate with the chemical composition, Ca(Fe²⁺, Fe³⁺)Fe²⁺Si₂O₈(OH), shows considerable thermally activated electron delocalization among Fe²⁺ and Fe³⁺ ions located in octahedral A sites. An electron ordering induced displacive phase transition takes place from orthorhombic (space group Pnam) to the monoclinic (space group P2₁/a) symmetry upon cooling through the range 333–343 K (Ghose et al, 1984a). The crystal structure of ilvaite from Elba, Italy [a = 13.008(1), b = 8.784(1) c = 5.836(1)Å, β = 90.21(1)°] has been refined at 138 K based on 1750 reflections measured on an automatic X-ray single crystal diffractometer (R = 0.024). The average octahedral distances at Ao and Am sites are Ao–O = 2.101Å, Am–O = 2.048Å. Calculated effective valence charges compared to earlier room temperature results (Finger et al., 1982) indicate a slight increase of Fe³⁺ content at the Am site. Concomittant with electron ordering in A sites, the two silicate tetrahedra undergo slight, but significant changes in configuration including small changes of Si–O bond lengths.