  We present experimental evidence for a first-order freezing/melting phase
transition in a nonequilibrium system -- an oscillated two-dimensional isobaric
granular fluid. The steady-state transition occurs between a gas and a crystal
and is characterized by a discontinuous change in both density and temperature.
It is suppressed if the number of particles is incommensurate with the cell
size, shows rate-dependent hysteresis, and obeys the Lindemann criterion for
melting. Further, the measured equation of state both above and below the phase
transition compares well with theory.
