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.

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