http://www.space.com/31030-cosmic-crashes-change-earth-composition.html

Earth formed by a collection of bits of matter in space that combined and with the help of heat formed an iron rich core.  A question posed is why earth has a different isotropic composition than it does a chemical composition.  This may be caused by primitive meteorites made of a mix of isotopes that collected.  The earth is low in silicon potassium and silicon and rich in magnesium, calcium and aluminum.  Expirements has modeled how a hot forming earth may have left the rocks of the earths crust rich in silicon and low in magnesiem but cosmic impacts may have taken off this crust leaving a low level of silicon with the escape of other elements.