Commensurate structure of Ca2CoSi2O7, a new twinned orthorhombic structure

The crystal structure of the commensurate phase of Ca2CoSi2O7, dicalcium co balt disilicate, has been derived from the modulated structure described in (3 + 2)-dimensional space. The structure is orthorhombic P2(1)2(1)2; a = 2 3.510 (4), b = 23.510 (4), c = 5.025 (1) Angstrom (at 170 K), Z = 18. Since the crystal is twinned and the apparent diffraction symmetry is 4/mmm, the parameters were refined by a newly developed leasts-quares program for the refinement of twinned crystals. The structure is essentially similar to th e known structure of the melilite group, but with regular arrangement of th e bundles along [001] formed with four arrays of the sixfold coordinated Ca polyhedra and an array of CoO4 tetrahedra. The distribution of the bundles found in the present structure is different from that reported by Riester et al. [(2000), Z. Kristallogr. 215, 102-109].