  While the realistically modeling of the thermodynamic behavior of fluids
usually demands elaborated atomistic models, much have been learned from
simplified ones. Here, we investigate a model where point-like particles (with
activity $z_0$) are mixed with molecules that exclude their first and second
neighbors (i.e., cubes of lateral size $\lambda=\sqrt{3}a$, with activity
$z_2$), both placed on the sites of a simple cubic lattice with parameter $a$.
Only hard-core interactions exist among the particles, so that the model is
athermal. Despite its simplicity, the grand-canonical solution of this model on
a Husimi lattice built with cubes revels a fluid-fluid demixing, yielding a
phase diagram with two fluid phases (one of them dominated by small particles -
$F0$) and a solid-like phase coexisting at a triple-point. Moreover, the
fluid-fluid coexistence line ends at a critical point. An anomaly in the total
density ($\rho_T$) of particles is also found, which is hallmarked by minima in
the isobaric curves of $\rho_T$ versus $z_0$ (or $z_2$). Interestingly, the
line of minimum density cross the phase diagram starting inside the region
where both fluid phases are stable, passing through the $F0$ one and ending
deep inside its metastable region, in a point where the spinodals of both fluid
phases cross each other.
