Raman spectroscopic study of inclusions of ferropericlase and other minerals from super-deep diamonds

Twenty-two inclusions of ferropericlase from thirteen lower-mantle diamonds, collected in the Rio Sorriso placer deposit in the Juina area, Mato Grosso State, Brazil, were studied using Raman spectroscopy. Pure periclase (MgO) and pure wüstite (FeO) with NaCl-type structure are inactive in first-order Raman scattering. However, natural ferropericlase are characterized by broad compositional variations in terms of the contents of Fe 2+ , Fe 3+ , Ni 2+ and other minor elements, so that their Raman spectra may become informative. Most spectra exhibited prominent and intense magnetite and hematite peaks resulting from laser-induced modification of the ferropericlase during analysis. The probable errors in ferropericlase spectra interpretation are discussed. Besides oxidation of ferropericlase under laser, one more likely explanation of Raman spectra appearance is that ferropericlase might contain nanoinclusions, formed during multistage transformations of iron oxides under variable scenario of changing pressure, temperature and oxygen fugacity. In addition, the formation of nanoinclusions and other transformations may cause structural defects that contribute to artifacts in Raman spectra. Within the host diamonds, rutile and anatase were identified. Anatase may be a retrograde phase of rutile formed during the ascent of the host diamond to the surface.