  The gelation of water suspension of a synthetic clay (Laponite) has been
studied by dynamic light scattering in a wide range of clay weight
concentration (Cw = 0.003-0.031). At variance with previous determination,
indicating a stable liquid phase for Cw < Cw*=0.015-0.018, we find that the
gelation takes actually place in the whole examined Cw range. More importantly,
we find that Cw* marks the transition between two different routes to gelation.
We hypothesize that at low concentration Laponite suspension behaves as an
attractive colloid and that the slowing down of the dynamics is attained by the
formation of larger and larger clusters while at high concentration the basic
units of the gel could be the Debye Huckel spheres associated to single
Laponite plates.
