  The phase behaviour of colloidal dispersions is interesting for fundamental
reasons and for technological applications such as photonic crystals and
electronic paper. Sedimentation, which in everyday life is relevant from blood
analysis to the shelf life of paint, is a means to determine phase boundaries
by observing distinct layers in samples that are in sedimentation-diffusion
equilibrium. However, disentangling the effects due to interparticle
interactions, which generate the bulk phase diagram, from those due to gravity
is a complex task. Here we show that a line in the space of chemical potentials
$\mu_i$, where $i$ labels the species, represents a sedimented sample and that
each crossing of this sedimentation path with a binodal generates an interface
under gravity. Complex phase stacks can result, such as the sandwich of a
floating nematic layer between top and bottom isotropic phases that we observed
in a mixture of silica spheres and gibbsite platelets.
