A commensurate-incommensurate phase transition in iron-bearing åkermanites

Synthetic melilites on the join Ca₂MgSi₂O₇ (åkermanite) — Ca₂FeSi₂O₇ (iron åkermanite) with Fe/(Fe+Mg) from 0.0 to 0.7 exhibit, at room temperature, an incommensurate phase with a rectangular modulation of a wavelength of about 19 Å in the 110] direction. Upon increase of temperature, they transform to a commensurate melilite structure at about 80° C for Fe/(Fe+Mg)=0.0 and about 250° C for Fe/(Fe+Mg)=0.6. In addition to the T(2) positions of the melilite structure filled by Si, the incommensurate phase exhibits two distinguishable T(1) sites containing the Mg and Fe²⁺. These two sites merge into one site during the phase transition from the incommensurate to the commensurate phase. A structural model for the incommensurate phase is based on the misfit between the tetrahedral (Mg, Fe²⁺)Si₂O ₇ ^4? sheets and the Ca²⁺ ions.