| A microinclusion of a rock fragment was identified within one of the diamonds from the
Rio Soriso placer deposit in the Juina area, Brazil. It is composed of iron carbide,
magnesiowüstite and two orthorhombic, postspinel phases, Fe-Mg-Cr and Ca-Cr oxides. All
mineral grains are closely intergrown, demonstrating their single-stage origin, except iron
carbide, which is partly resorped that implies its early crystallization.
Diffraction patterns of iron carbide grains reveal two mineral species, nitrogen-containing
analogues of chalypite Fe7C3 (or Fe2C) [1] and yarlongite Fe9C4 [2]. They contain up to 17.5
at.% of N in the cation part (N/(C+N) = 0.47) and should be considered as nitrocarbides. In
addition to N, admixtures of O and Si present in these mineral species. Magnesiowüstite has,
in its cation composition, 47.75-67.16 at.% Fe, 36.72-47.75 at.% Mg, and minor admixtures
of Mn and Cr. Among the two new oxides with orthorhombic structure, one is a polymorph of
chromite (Fe2+,Mg,Mn) (Cr,Fe3+,Al,V)2O4, which is new in terrestrial environment. It is
analogues to xieite, found in the Suizhou meteorite [3], but strongly enriched in Fe. The other
orthorhombic mineral fits the stoichiometry Ca(Cr,Fe3+)2O4. This compound is new in the
natural environment; the CaCr2O4 postspinel phase is known only as a synthetic compound
[4].
The studied association was formed within the lower mantle at pressure conditions
above 40 GPa (that correspond to the formation of CaTi2O4 [5]). The presence of
magnesiowüstite and chalypite suggests the rock origin in the lower part of the lower
mantle.
References:
Kaminsky, F.V., Wirth, R. (2011) Iron carbide inclusions in lower-mantle
diamond from Juina, Brazil. Canadian Mineralogist 49(2), p. 555-572.
Shi, N., Bai, W., Li, G., Xiong, M., Fang, Q., Yang, J., Ma, Z., Rong, H. (2009)
Yarlongite: A new metallic carbide mineral. Acta Geologica Sinica 83(1), 52-56.
Chen, M., Shu, J., Mao, H.-k. (2008) Xieite, a new mineral of high-pressure
FeCr2O4polymorph. Chinese Science Bulletin 53(21), 3341-3345.
Hörkner, W., Müller-Buschbaum, Hk. (1976) Einkristalluntersuchungen von
β-CaCr2O4. Zeitschrift für Naturforschung, Teil B, Anorganische Chemie,
Organische Chemie 31, 1710-1711.
Chen, M., Shu, J., Xie, X., Mao H.-k. (2003) Natural CaTi2O4-structured
FeCr2O4polymorph in the Suizhou meteorite and its significance in mantle
mineralogy. Geochimica et Cosmochimica Acta 67(20), 3937–3942. |