  Binary mixtures of lamellar colloids represented by hard platelets are
studied within a generalization of the Zwanzig model for rods, whereby the
square cuboids can take only three orientations along the $x$, $y$ or $z$ axes.
The free energy is calculated within Rosenfeld's ''Fundamental Measure Theory''
(FMT) adapted to the present model. In the one-component limit, the model
exhibits the expected isotropic to nematic phase transition, which narrows as
the aspect ratio $\zeta=L/D$ ($D$ is the width and $L$ the thickness of the
platelets) increases. In the binary case the competition between nematic
ordering and depletion-induced segregation leads to rich phase behaviour.
