  Nucleation plays a critical role in the birth of crystals and is associated
with a vast array of phenomena such as protein crystallization and ice
formation in clouds. Despite numerous experimental and theoretical studies,
many aspects of the nucleation process like the polymorph selection mechanism
in the early stages are far from being understood. Here, we show that the
excess of particles in a face-centred-cubic (fcc)-like environment with respect
to those in a hexagonal-close-packed (hcp)-like environment in a crystal
nucleus of hard spheres as observed in simulations and experiments can be
explained by the higher order structure in the fluid phase. We show using both
simulations and experiments that, in the metastable fluid phase, fivefold
symmetry clusters -- pentagonal bipyramids (PBs) -- known to be inhibitors of
crystal nucleation, transform into a different cluster -- Siamese dodecahedra
(SDs). Due to their geometry, these clusters form a bridge between the fivefold
symmetric fluid and the fcc crystal, thus lowering its interfacial free energy
with respect to the hcp crystal, and shedding new light on the polymorph
selection mechanism.
