olelog

First a few words about iron crystals.

In contrast to most metals, which come in only one crystalline form, iron (Fe) can have two standard forms:

• Body centred cubic (bcc) iron, also called alpha iron or the ferritic phase, and
• Face centred cubic (fcc) iron, also called gamma iron or the austenitic phase.

Both structures are built from cubes with atoms at each corner. The bcc crystal structure has Fe atoms also at the centre of each cube, whereas the fcc structure has additional atoms in the sides of each cube.

At ambient conditions, pure iron is body-centred cubic (bcc). Above approximately 920°C, it becomes face centred cubic (fcc). At higher pressures the atoms may get even closer packed together into a crystal form of iron known as hexagonal close-packed (hcp), in which the atoms are arranged in planes like racked pool balls, each atom touching six others, and the planes of hexagons are stacked up, with alternating planes slightly offset, to give the three-dimensional structure.

At high temperature, an iron atom in the hcp structure is surrounded by 12 identical neighbours. An iron atom in the bcc structure is surrounded by eight atoms at a distance slightly shorter than that in the hcp structure and by six atoms located at a slightly larger distance compared to the nearest neighbour separation in the hcp structure.

And now to Earth’s inner core.

The Earth's inner core - a spherical body in the centre of Earth with a radius of about 1200 km - is a solid ball mainly of iron, but maybe with a bit of nickel as well. It has even been suggested (some ten years ago) that Earth's inner core may be a single crystal of (hcp-)iron.

Seismic observations have shown that elastic waves pass more rapidly through this core in directions that are parallel to the earth’s axis of rotation than in directions parallel to the equator. In the single crystal theory this is explained by a single hcp iron crystal, because hexagonal crystals have a unique directionality.

In 2003, however, Swedish researchers published strong theoretical proof that the earth’s core assumes the body centred cubic (bcc) crystal structure, a structure that despite its high degree of symmetry evinces a surprisingly high level of elastic anisotropy, that is, its elastic properties are contingent on direction. This theory about the crystal structure directly contradicted the then prevailing view, but since then the theory has found both experimental and theoretical support. The body-centred cubic crystal structure forms a cube with atoms in each corner and a further atom in the middle of this cube. It is oriented in such a way that its great diagonal is directed along the earth’s axis of rotation, which makes it possible for the iron to evince wave propagations with the velocities observed (seismic waves propagate by 3 to 4% faster in the direction of Earth’s spin axis than in the direction aligned with the equatorial plane).

In a new study published in Science of 8 February 2008 the same Swedish researchers present further evidence and an explanation based on computer simulations.

Why is the crystal structure important? Well, the earth’s heat balance, like its magnetic field, is dependent on the amount of heat that is stored in the inner core of the earth. These conditions, in turn, are dependent on the crystal structure of the iron in the inner core.

Another matter is that it recently was suggested that the inner core consists of two parts. The central part seems to react differently from the rest of the core on seismic waves. Such a different behaviour could maybe be explained in terms of two iron phases, one of which (bcc) is anisotropic and the other (hcp) isotropic.

Reference:
Article: Anatoly B. Belonoshko, Natalia V. Skorodumova, Anders Rosengren, Börje Johansson, Elastic Anisotropy of Earth's Inner Core, Science 8 February 2008: Vol. 319. no. 5864, pp. 797 - 800 DOI: 10.1126/science.1150302

• http://www.sciencemag.org/cgi/content/short/319/5864/797
• http://www.physorg.com/news121692398.html
• http://www.scientificblogging.com/news_releases/magnetic_mystery_at_the_earths_core
• http://www.nature.com/nature/journal/v424/n6952/abs/nature01954.html