The commensurately modulated structure of the lock-in phase of synthetic Co-åkermanite, Ca₂CoSi₂O₇

Abstract

This paper deals with the structure determination of the lock-in phase of synthetic modulated Co-åkermanite, Ca 2 CoSi 2 O 7 . The structure determination of the (3 × 3 × 1) superstructure is based on X-ray data of a twinned single crystal taken at 130 K. The deformation modes in the commensurate lock-in phase are derived and compared with the deformations found by other authors in the incommensurate phase. A model for the driving force of the modulation in melilites in general is proposed: In order to release internal stress domains with clusters of 6-and 7-fold coordinated calcium are formed which are arranged in octagons.