  We present a theoretical realization of quantum spin and quantum valley Hall
effects in silicene. We show that combination of an electric field and
intrinsic spin-orbit interaction leads to quantum phase transitions at the
charge neutrality point. This phase transition from a two dimensional
topological insulator to a trivial insulating state is accompanied by a
quenching of the quantum spin Hall effect and the onset of a quantum valley
Hall effect, providing a tool to experimentally tune the topological state of
silicene. In contrast to graphene and other conventional topological
insulators, the proposed effects in silicene are accessible to experiments.
