0.29 Mohs hardness 4.0 Vickers hardness 608 MPa Brinell hardness 490 MPa CAS registry number 7439-89-6 Selected isotopes Main article: Isotopes of iron iso NA half-life DM DE ( MeV ) DP 54 Fe 5.8% >3.1×10 22 y 2ε capture  ? 54 Cr 55 Fe syn 2.73 y ε capture 0.231