Iron carbide inclusions in lower-mantle diamond from juina, brazil

Iron carbides in association with native iron, graphite, and magnetite were identified in a crystal of diamond from the Juina area, Brazil, that contains a series of other, deep-mantle mineral inclusions. Among the iron carbides, Fe₃C, Fe₂C ("chalypite"), and Fe ₂₃C₆ (haxonite) are present; the two latter phases are identified in the terrestrial environment for the first time. Some of the analyzed iron carbide grains contain 7.3-9.1 at.% N and are, in fact, nitrocarbide. We suggest, on the basis of the high-pressure mineral parageneses previously observed in the diamond and experimental data on the system Fe-C, that "chalypite" crystallized within a pressure interval of 50-130 GPa from an iron-carbon melt rich in nitrogen. Following crystallization, iron carbides and native iron were partially oxidized to magnetite, and encapsulated in diamond along with other high-pressure minerals. The finds of various iron carbides, some of which are rich in nitrogen, in lower-mantle diamond confirm a significant role of carbides and nitrogen in the Earth's interior.