PHASE-DIAGRAM OF IRON BY IN-SITU X-RAY-DIFFRACTION - IMPLICATIONS FOREARTH CORE

The phase diagram of iron has been studied to 130 gigapascals (1 gigap ascal = 10(4) atmospheres) and 3500 kelvin by a combined laser-heated diamond-anvil cell and x-ray diffraction technique that provides direc t identification of the solid phases. Iron in the hexagonal close-pack ed (hcp) phase (epsilon-Fe) is stable from 50 to at least 110 gigapasc als at high temperatures. The wide stability field of epsilon-Fe indic ates that this polymorph should currently be considered the most relev ant solid phase for Earth's core. The triple point between the gamma, epsilon, and liquid phases is located at 2500 +/- 200 kelvin and 50 +/ - 10 gigapascals, There is evidence for a phase with a double hcp stru cture below 40 gigapascals and for another transition above 110 gigapa scals and 3000 kelvin.