  For binary fluid mixtures of spherical particles in which the two species are
sufficiently different in size, the dominant wavelength of oscillations of the
pair correlation functions is predicted to change from roughly the diameter of
the large species to that of the small species along a sharp crossover line in
the phase diagram [C. Grodon, M. Dijkstra, R. Evans & R. Roth, J.Chem.Phys.
121, 7869 (2004)]. Using particle-resolved colloid experiments in 3d we
demonstrate that crossover exists and that its location in the phase diagram is
in quantitative agreement with the results of both theory and our Monte-Carlo
simulations. In contrast with previous work [J. Baumgartl, R. Dullens, M.
Dijkstra, R. Roth & C. Bechinger, Phys.Rev.Lett. 98, 198303 (2007)], where a
correspondence was drawn between crossover and percolation of both species, in
our 3d study we find that structural crossover is unrelated to percolation.
