Local structural relaxation around Co2+ along the hardystonite–Co-åkermanite melilite solid solution

Six pure compounds belonging to the hardystonite (Ca2ZnSi2O7)–Co-åkermanite (Ca2CoSi2O7) solid solution were investigated by the combined application of X-ray powder diffraction and electronic absorption spectroscopy. Structural refinements of the XRPD data revealed a negative excess volume of mixing due to the single isovalent substitution of Co for Zn in the tetrahedral site. In agreement with the diffraction data, deconvolution of the optical spectra showed a progressive decreasing of the crystal field strength parameter 10Dq moving toward the Co-åkermanite end-member, meaning that the local cobalt–oxygen bond distance, $$ \langle {\text{Co}}{-}{\text{O}}\rangle^{\text{local}} $$, increased along the join with the amount of cobalt. The calculated structural relaxation coefficient around the fourfold coordinated Co2+ in the Ca2(Zn1−x Co x )Si2O7 join was ε = 0.69, very far from the one predicted by the Vegard’s law (ε = 0) and at variance with ε = 0.47 previously found for tetrahedrally coordinated Co2+ in gahnite–Co-aluminate spinel solid solution. This difference is consistent with the largest constraints existing on the spinel structure, based on cubic closest packing, compared to the more flexible layered melilite structure.