Unique Mineral Association and the First Finding of Extraterrestrial Ferrodimolybdenite in the Kunya-Urgench H5 Chondrite

Abstract

The mineral ferrodimolybdenite (FeMo₂S₄) and the associated mineral assemblage were identified for the first time in an extraterrestrial environment: in a sulfide–metal veinlet of the Kunya-Urgench (H5) ordinary chondrite. They were studied using optical microscopy, SEM, EPMA, and EBSD. Ferrodimolybdenite was found as an inclusion in troilite in terrestrial pyrometamorphic rocks in 2023. Its synthetic analogue has been known as a semiconductor since 1960. Experimental data and properties of the natural mineral assemblage suggest that ferrodimolybdenite should have crystallized from troilite melt at a temperature close to 1100–1000°C. The quenching of metal–sulfide melt enriched in Mo, Cu, and Mn probably formed the metastable phase FeMo₂S₄ in association with native copper, alabandite, and mercury sulfides. The presence of alabandite can indicate strongly reducing conditions (log fO₂ < –4 IW), which are atypical of the impact melting of ordinary chondrites. The fact that this phenomenon occurs locally suggests that a reducing agent may have been locally involved, which was probably a carbon phase contained in the groundmass of the chondrite or brought from the meteoroid that initiated the impact event with the formation of the veinlet. The anomalously high concentrations of Mo (2 × 10² CI), Mn, Cu, and Hg in the Fe–S melt could not have been reached either during the fractional crystallization of large volumes of Fe–FeS melt or during the recurrent partial melting of metal sulfide and silicates during impact events. The ferrodimolybdenite and associated mineral phases were most likely formed during the impact melting of an foreign sulfide–metal aggregate that had been formed under conditions different from those characteristic of the formation of the chondrite matrix in which carbonaceous chondrites were presumably formed. An alternative explanation is hydrothermal activity on the parent body of H chondrites. Although prerequisites for this activity have been identified, its P–T boundary parameters remain uncertain.