Averaging 8.1% (w/w) of the earth's crust, aluminum is the most highly abundant metal in our biosphere, yet has long been thought to serve no essential biological function. In aqueous solutions, aluminum salts and hydroxides are exceptionally potent aggregators of biological molecules, often coalescing molecular species to the point that they precipitate out of solution. A biological function for aluminum is proposed in which this abundant, high charge density metal cation has a significant role in biomolecular compaction. Sometimes, molecules ectopically aggregated by aluminum are associated with pathological conditions. The data further suggests that a specific consequence of 'aluminum biocompaction' may be particularly important in the condensation of A+T-rich chromatin domains, and in silencing the expression of specific kinds of genetic information.