order parameters S$_{1}$ , S$_{2}$ vary smoothly with increasing composition and hence so does S$_{tot}$ . Where a re-entrant feature occurs, all three order parameters along the nematic branch of the binodal drop sharply. As λ increases, S$_{2}$ becomes smaller at x = 1, reaching < 0 . 2 for λ = 2. We do not find any stable I - I demixing for the range of λ in this investigation. Through the range of λ values considered, a striking observation is that whilst FMT predicts the occurence of phase boundaries at locations quantitatively different from Onsager theory, the topology of the phase diagrams for a particular choice of λ is the same as for Onsager theory. Coupled with the fact that the I - N transition for the monodisperse case agrees well with simulation results [25], we gain confidence that the phase diagrams predicted by FMT are quantitatively more reliable since we assume that Onsager theory predicts the correct physics qualitatively. Furthermore, we have confidence that the order parameter profiles predicted by FMT are closer to those that would be observed from simulation studies, since the monodisperse limit in the present theory yields results for the I - N transition and nematic order parameter that is in good agreement with simulation results. Whether the differences to the true transition densities for the case of binary mixtures increases remains to be seen. The results from Onsager theory cannot be obtained by some scaling of the results that exist for binary mixtures of rods (thick and thin [27] or long and short [28]). For example, for there to be a mapping between thick and thin rods [27] and the present system, we would require simultaneously that (1+ d ) = λ $^{2}$+ λ and d = λ $^{3}$, where d = D$_{2}$/D$_{1}$ > 1 is the diameter ratio of the thick and thin rods. These requirements are obtained from consideration of the free energy for thick and thin rods (compare Eq. (2) of Ref. [27] with Eq. (13) of the present investigation). Clearly these conditions cannot be satisfied simultaneously except for the trivial case λ = d = 1. In the monodisperse limit, the mapping of concentrations from rods to platelets is c$_{rod}$ = ( π $^{2}$/ 2) c$_{platelet}$ where c$_{rod}$ = ( π/ 4) L $^{2}$D is the conventional dimensionless concentration for rods ( L is the rod length, D is the rod diameter). Clearly, simulation results for the systems studied in the present paper are most desirable. The predictions of the present work could also be tested experimentally. For example, for gibbsite (having typical experimental radius of approximately 100nm) and hydrotalcite (typically between about 25 and 75 nm) [8] one has values of λ between 1 . 3 and 4, which are in the range of the present work. We have taken mean radii here. Of course in experiments there will be effects due to polydispersity and our model ignores effects of finite platelet thickness. 23