154-5 Coordinated Analyses of Iron Nitride Minerals in Asteroid Samples

Session: Asteroid Observations, Return Missions, and Meteoritics: Interweaving Perspectives and Data (Posters)

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Recent sample return missions from asteroids Ryugu and Bennu by JAXA and NASA respectively, have provided new insights into the formation and evolution of these primitive bodies. Our efforts have focused on understanding the mineralogical and compositional underpinnings of the UV-visible reflectance spectral characteristics of these objects measured from orbit. The reflectance spectra from these objects are modified by space weathering processes that are dominated by the effects of micrometeoroid impacts and solar wind irradiation. For Bennu in particular, recent work has shown that the accumulation of impact-derived melt deposits on regolith particle surfaces is a major driver of the spectral changes.

Here we present a coordinated analysis of one of these impact melt deposits that hosts at least two unusual iron nitride minerals, roaldite (Fe₄N) and siderazot (Fe₃N_1.33). Siderazot is a rare mineral known from terrestrial volcanic fumarole deposits. In the Bennu sample, the nitride minerals occur in a thin 10 nm polycrystalline layer on the uppermost surface of a melt deposit consisting of recrystallized FeNi sulfides (pyrrhotite and pentlandite) and FeNi metal on sample OREX-501017-101. A focused ion beam (FIB) instrument was used to extract an electron transparent cross section through the deposit and was analyzed using transmission electron microscopy (TEM). High resolution imaging and electron diffraction confirmed the presence of roaldite and siderazot. Electron energy loss spectroscopy (EELS) data from the layer shows a strong N k-edge consistent with nitride and the Fe L_2,3 edge fine structure is consistent with metallic Fe.

The formation of the Fe nitride layer likely originated from the surface of the FeNi sulfide-FeNi metal melt deposit that was irradiated by the solar wind resulting in the preferential loss of sulfur from the surface and the formation of a thin metallic Fe layer. The Fe metal was subsequently altered by gas phase reaction with a N-bearing volatile phase, most likely indigenous ammonia that is known to be present in the Bennu samples. Roaldite was first reported in Ryugu samples and was proposed to have formed by reaction of Fe metal with N-rich ices. The discovery of Fe nitrides in both Ryugu and Bennu samples suggests a possibly common process involving interactions between ammonia and the space-weathered regolith particles has occurred on carbonaceous asteroids.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8811

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