A microcrystalline polymineral aggregate was identified as an inclusion in a diamond from the Rio Soriso area, Mato Grosso State, Brazil. It is composed of iron carbides, Fe-rich periclase (with exsolved magnesioferrite), and two orthorhombic, postspinel phases — Mg-Cr-Fe and Ca-Cr oxides, which are new mineral phases. This association was formed during several stages and may be considered as a rock microfragment from the lower mantle. In addition to the carbide-oxide fragment, other phases were identified as inclusions in the diamond, such as parascandolaite KMgF3 (that is stable at pressures of up to 50 GPa) in association with orthorhombic MgO, and porous dolomite occurring together with nanoinclusions of calcite, apatite, spinel, Fe-sulfide, and an assemblage of periclase plus wüstite. These minerals belong to the carbonatitic association and, along with syngenetic nanoinclusions of fluid nitrogen, represent the media of diamond formation. The assemblage of periclase and wüstite within the carbonatitic matrix points to the origin of diamond from a carbonatitic environment within the lower mantle under pressure conditions of ≥ 85–86 GPa (~1,900 km depth).