  We studied the phase behavior of charged and sterically stabilized colloids
using confocal microscopy in a less polar solvent (dielectric constant 5.4).
Upon increasing the colloid volume fraction we found a transition from a fluid
to a body centered cubic crystal at 0.0415+/-0.0005, followed by re-entrant
melting at 0.1165+/-0.0015. A second crystal of different symmetry, random
hexagonal close-packed, was formed at a volume fraction around 0.5, similar to
that of hard spheres. We attribute the intriguing phase behavior to particle
interactions that depend strongly on volume fraction, mainly due to changes in
the colloid charge. In this low polarity system the colloids acquire charge
through ion adsorption. The low ionic strength leads to fewer ions per colloid
at elevated volume fractions and consequently a density-dependent colloid
charge.
