The crystal structure of calcium oxalate trihydrate: Ca(H₂O)₃(C₂O₄)

The crystal structure of calcium oxalate trihydrate, grown on whitlockite by reaction with oxalic acid, was determined utilizing 697 independent reflections and converged to R = 0.034. Important parameters are triclinic space group P1, a 7.145(6), b 8.600(7), c 6.099(5), α 112.30(5), β 108.87(5), γ 89.92(5), with 2Ca(H₂O)₃(C₂O₄) in the unit cell.

The structure is based on dimers of edge-linked square antiprisms of composition $∞0[Ca2ϕ14]$ where ϕ = oxygen ligand which is linked to oxalate groups or water molecules to form a sheet parallel to {100}. In weddellite, square antiprisms link to form a $∞1[Ca2ϕ12]$ chain and in whewellite a related polyhedron of order 8 links to form $∞2[Ca2ϕ10]$ sheet. It is not topologically possible to construct the series of structures by condensation of polyhedra alone since in each case some rearrangement of the water molecules is necessary.