  Studies of nucleation generally focus on the properties of the critical
cluster, but the presence of defects within the crystal lattice means that the
population of nuclei necessarily evolve through a distribution of pre-critical
clusters with varying degrees of structural disorder on their way to forming a
growing stable crystal. To investigate the role pre-critical clusters play in
nucleation, we develop a simple thermodynamic model for crystal nucleation in
terms of cluster size and the degree of cluster order that allows us to alter
the work of forming the pre-critical clusters without effecting the properties
of the critical cluster. The steady state and transient nucleation behaviour of
the system are then studied numerically, for different microscopic ordering
kinetics. We find that the models exhibits a generic order-disorder transition
in the pre-critical clusters. Independent of the type of ordering kinetics,
increasing the accessibility of disordered pre-critical clusters decreases both
the steady state nucleation rate and the nucleation lag time. Furthermore, the
interplay between the free energy surface and the microscopic ordering kinetics
leads to three distinct nucleation pathways.
