  Crystallization often proceeds through successive stages that lead to a
gradual increase in organization. Using molecular simulation, we determine the
nucleation pathway for solid solutions of copper and gold. We identify a new
nucleation mechanism (liquid$\to$$L1_2$~precursor$\to$solid solution),
involving a chemically ordered intermediate that is more organized than the end
product. This nucleation pathway arises from the low formation energy of $L1_2$
clusters which, in turn, promote crystal nucleation. We also show that this
mechanism is composition-dependent since the high formation energy of other
ordered phases precludes them from acting as precursors.
