  When white dwarfs freeze the plasma mixtures inside them undergo separation
processes which can produce radical changes in the composition profile of the
star. The abundance of neutron rich elements, such as $^{22}$Ne or $^{56}$Fe,
determines whether or not the first crystals are more or less dense than the
surrounding fluid and thus whether they sink or float. These processes have now
been studied for C-O-Ne and C-O-Fe mixtures, finding that distillation and
precipitation processes are possible in white dwarfs. In this work, we
calculate the phase diagram of more complicated O-Ne-Fe mixtures and make
predictions for the internal structure of the separated white dwarf. There are
two possible outcomes determined by a complicated interplay between the Ne
abundance, the $^{22}$Ne fraction, and the $^{56}$Fe abundance. Either Fe
distills to form an inner core because the first O-Ne solids are buoyant, or an
O-Ne inner core forms and Fe accumulates in the liquid until Fe distillation
begins and forms a Fe shell. In the case of an Fe shell, a Rayleigh-Taylor
instability may arise and overturn the core. In either case, Fe distillation
may only produce a cooling delay of order 0.1 Gyr as these processes occur
early at high white dwarf luminosities. Fe inner cores and shells may be
detectable through asteroseismology and could enhance the yield of neutron rich
elements such as $^{55}$Mn and $^{58}$Ni in supernovae.
