  Bifurcations in kinetic pathways decide the evolution of a system. An example
is crystallization, in which the thermodynamically stable polymorph may not
form due to kinetic hindrance. Here, we use confined self-assembly to
investigate the interplay of thermodynamics and kinetics in the crystallization
pathways of finite clusters. We report the observation of decahedral clusters
from colloidal particles in emulsion droplets and show that these decahedral
clusters can be thermodynamically stable just like icosahedral clusters. Our
hard sphere simulations reveal how the development of the early nucleus shape
passes through a bifurcation that decides the cluster symmetry. A geometric
argument explains why decahedral clusters are kinetically hindered and why
icosahedral clusters can be dominant even if they are not the thermodynamic
ground state.
