  In a mixture of two species of grains of equal size but different mass,
placed in a vertically vibrated shallow box, there is spontaneous segregation.
Once the system is at least partly segregated and clusters of the heavy
particles have formed, there are sudden peaks of the horizontal kinetic energy
of the heavy particles, that is otherwise small. Together with the energy peaks
the clusters rapidly expand and the segregation is partially lost. The process
repeats once segregation has taken place again. Depending on the experimental
or numerical parameters, the energy bursts can occur either randomly or with
some regularity in time. An explanation for these events is provided based on
the existence of a fixed point for an isolated particle bouncing with only
vertical motion. The horizontal energy peaks occur when the energy stored in
the vertical motion is partly transferred into horizontal energy through a
chain reaction of collisions between heavy particles. A necessary condition for
the observed regularity of the events is that chain reactions involve most of
the heavy particles.
