In the Berzelius–Goldschmidt classification, siderophile elements alloy with metallic iron (sideros), most notably in planetary cores. Transition elements such as Ni, Co, Mn, Mo, Au, and the Platinum-Group Elements (Os, Pd, Pt, Re, Rh, Ru) are highly siderophilic. Under reducing conditions, carbon, silicon, and phosphorus become moderately siderophilic. Likewise, under high pressure, some lithophile elements tend to fractionate into the core and become siderophilic. The question of which elements behave in this way is hotly debated for K, Pb, and Nb for the implication it has on the evolution of planetary interiors.