The notion of I-element of a lattice is introduced, where I is an arbitrary lattice identity. This notion generalizes practically all types of special elements of lattices considered earlier. It is proved that, if a semigroup variety is an I-element of the lattice of all semigroup varieties for some nontrivial lattice identity I and is different from the variety of all semigroups, then it is a periodic variety. It is established that the converse is not true.