Estimates of the Earth's inner core grain size

Estimates of the Earth's inner core grain size range from 1200 km to 5 mm. A review of the experimental data shows that the single crystal elastic anisotropy of hexagonal closest-packed (hcp) titanium and zirconium, thought to be good analogs for high pressure hcp iron, increases continuously with temperature, from 4-5 % at 4 K to about 11% at the body-centered cubic (bcc) transition. Thus, in order to match the seismic elastic anisotropy of about 4%, the inner core cannot behave elastically as a single crystal. Several lines of evidence suggest a 5 mm grain size is unrealistically small. They include that core solidification promotes forming non-equiaxed grains extrapolated to have a smallest dimension of hundreds of meters, that should deformation result in dynamic recrystallization the minimum steady state grain size is likely to be .1 to 10 meters for geophysically plausible stresses, and that iron meteorites over a meter in extent have typically cooled from single crystals. By assuming the observed seismic attenuation anisotropy is due to scattering off cylindrically radial columnar grain boundaries, the shorter grain dimension is again estimated to be hundreds of meters.