The copper sulfate hydration cycle. Crystal structures of CuSO4 (Chalcocyanite), CuSO4·H2O (Poitevinite), CuSO4·3H2O (Bonattite) and CuSO4·5H2O (Chalcanthite) at low temperature using non-spherical atomic scattering factors

Sky blue CuSO 4 ·3H 2 O is the midpoint of the copper sulfate hydration cycle. The progression from colourless CuSO 4 to bright blue CuSO 4 ·5H 2 O is intimately linked to the relative number of sulfato versus aqua ligands coordinated to copper.