  We present a statistical mechanical description of randomly packed spherical
particles, where the average coordination number is treated as a macroscopic
thermodynamic variable. The overall packing entropy is shown to have two
contributions: geometric, reflecting statistical weights of individual
configurations, and topological, which corresponds to the number of
topologically distinct states. Both of them are computed in the thermodynamic
limit for isostatic packings in 2D and 3D, and the result is further expanded
to the case of "floppy" particle clusters. The theory is directly applicable to
sticky colloids, and in addition, generalizes concepts of granular and glassy
configurational entropies for the case of non-jammed systems.
