The bulk thermodynamic properties of proteins originate from a varied and complex combination of interactions. We propose a simple model for the formation of ordered two-dimensional aggregates based on the interactions between pairs of annexin V molecules. Simulations of this model are shown to reproduce the experimental observations of a honeycomb (p6) and a triangular (p3) crystalline phase. The simulations indicate that the transition between these two phases is first order. While this model is extremely simple in that it relies only on hard body and short-range directional interactions, it nevertheless captures the essential physics of the interactions between the protein molecules and reproduces the phase behavior observed in electron microscopy and atomic force microscopy experiments.