A single iron crystal has been considered as an explanation for a few types
of anisotropy measured in the core.
While a single, huge iron crystal may sound incredible, it is actually
fairly plausible. Most substances exhibit allotropes and crystal
restructuring that only occur under extremes of heat and pressure, and most
of which are unstable under "normal" conditions (diamond is an example).
Iron at ordinary pressures changes its crystalline structure back and forth
between FCC and BCC depending on the temperature. I would imagine that
center of earth conditions could be replicated in the laboratory to verify
if iron undergoes any type of interesting organizational change under those
conditions. I might add that the high temp/pressure conditions expected in
the core are ideal for large metal crystal formation, as there is enough
energy to lubricate the reorganization of the atoms and enough pressure to
control their organization.
-James Rogers
jamesr@best.com