Hexaferrum and epsilon iron (e-Fe) are synonyms for the hexagonal close-packed (HCP) phase of iron that is stable only at extremely high pressure. Takahashi and Bassett at the University of Rochester mixed 99.8% pure a-iron powder with sodium chloride, and pressed a 0.5-mm diameter pellet between the flat faces of two diamond anvils. The deformation of the NaCl lattice, as measured by x-ray diffraction (XRD), served as a pressure indicator. At a pressure of 130 kbar (13 GPa) and room temperature, the body-centered cubic (BCC) ferrite powder transformed to the HCP phase in Figure 1. When the pressure was lowered, e-Fe transformed back to ferrite (a-Fe) rapidly. A specific volume change of -0.20 cm³/mole ± 0.03 was measured. Hexaferrum, much like austenite, is more dense than ferrite at the phase boundary. A shock wave experiment confirmed the diamond anvil results. Epsilon was chosen for the new phase to correspond with the HCP form of cobalt.