Effects of O2- or OH- ion incorporation on the cage geometry in 12CaO center dot 7Al(2)O(3) (C12A7) crystals are examined by the maximum entropy method (MEM)/Rietveld analyses of synchrotron powder X-ray diffraction data. Presence of an extraframework anion in a cage causes inward displacements of two Ca2+ ions axially coordinating to the anion. The OH- ion locates at the cage center and induces smaller symmetric displacements of the two Ca2+ ions, while the O2- ion likely locates on off-axis positions and induces larger asymmetric Ca2+ ion displacements.