Even though it is hotter than the surface of the Sun, the crystallized iron core of the Earth remains solid. Spinning within Earth’s molten core is a crystal ball – actually a mass formation of almost pure crystallized iron – nearly the size of the moon.
As with all metals, the atomic-scale crystal structures of iron change depending on the temperature and pressure the metal is exposed to. Atoms are packed into variations of cubic, as well as hexagonal formations. At room temperatures and normal atmospheric pressure, iron is in what is known as a body-centered cubic (BCC) phase, which is a crystal architecture with eight corner points and a center point. But at extremely high pressure the crystalline structures transform into 12-point hexagonal forms, or a close packed (HCP) phase.
At Earth’s core, where pressure is 3.5 million times higher than surface pressure – and temperatures are some 6,000 degrees higher – scientists have proposed that the atomic architecture of iron must be hexagonal.
At low temperature BCC is unstable and crystalline planes slide out of the ideal BCC structure. But at high temperatures, the stabilization of these structures begins much like a card game – with the shuffling of a “deck”. In the extreme heat of the core, atoms no longer belong to planes because of the high amplitude of atomic motion.
Such a shuffling leads to an enormous increase in the distribution of molecules and energy – which leads to increasing entropy, or the distribution of energy states. That, in turn, makes the BCC stable.
Normally, diffusion destroys crystal structures turning them into liquid. In this case, diffusion allows iron to preserve the BCC structure.
He says that this diffusion also explains why the Earth’s core is anisotropic – that is, it has a texture that is directional – like the grain of wood. Anisotropy explains why seismic waves travel faster between the Earth’s poles, than through the equator.
— source kth.se