  We have only rules of thumb with which to predict how a material will
crystallize, chief among which is Ostwald's rule of stages. It states that the
first phase to appear upon transformation of a parent phase is the one closest
to it in free energy. Although sometimes upheld, the rule is without
theoretical foundation and is not universally obeyed, highlighting the need for
microscopic understanding of crystallization controls. Here we study in detail
the crystallization pathways of a prototypical model of patchy particles. The
range of crystallization pathways it exhibits is richer than can be predicted
by Ostwald's rule, but a combination of simulation and analytic theory reveals
clearly how these pathways are selected by microscopic parameters. Our results
suggest strategies for controlling self-assembly pathways in simulation and
experiment.
