Transition metal sulfates form a variety of hydrates, each of which crystallizes in only one form.  The sulfate group often binds to the metal, especially for those salts with fewer than six aquo ligands.  The heptahydrates, which are often the most common salts, crystallize as monoclinic and the less common orthorhombic forms.  In the heptahydrates, one water is in the lattice and the other six are coordinated to the ferrous center.  Many of the metal sulfates occur in nature, being the result of weathering of mineral sulfides.  Many monohydrates are known.