Conditions of Formation of the Pentlandite (Fe,Ni1 – x)9S8 and Heazlewoodite Ni3S2 in the Pallasite Seymchan and Dronino Iron Meteorite

Fe–Ni sulfides and metal–sulfide segregations in shock melt veins (pallasite Seymchan) and in a nodule (iron meteorite Dronino) were studied by scanning electron microscopy (SEM). It was found that sulfide assemblages contain not only troilite (FeS) but also Fe–Ni sulfides, namely pentlandite (FexNi1–x)9S8, and heazlewoodite Ni3S2, which form rims around primary troilite. The metal–sulfide segregations, in contrast to sulfide ones, consist of troilite, pentlandite and metallic nickel and have an unusual reticulate texture. The reticulate texture consists of isolated troilite grains surrounded by fibrous intergrowths of troilite, pentlandite, and metallic nickel. The Fe–Ni sulfides and metal–sulfide assemblages correspond to the solid–phase equilibrium in the Fe–Ni–S system in the low-temperature region (T < 875°C). Meanwhile, the shock melt veins in Seymchan demonstrate a liquid immiscibility of phosphate and metal–sulfide melts, which indicates the local shock melting of the vein material at temperatures T > 1500°C. In accordance with the FeNi–FeS phase diagram, the solidification of metal–sulfide melts begins at a temperature of 988°C via the formation of FeNi + FeS eutectic intergrowths. The disagreement of the observed phase composition and microtexture of metal–sulfide intergrowths in Seymchan and Dronino with those of eutectic FeNi + FeS intergrowths indicates the modification of the former metal–sulfide eutectic microtexture with the appearance of a low-temperature association FeS + pentlandite (FexNi1 – x)9S8 + Ni. On the basis of the data obtained, it was proposed that troilite + pentlandite + heazlewoodite and troilite + pentlandite + Ni(metal) assemblages in Seymchan and Dronino meteorites were formed due to long-term low-temperature interaction between coexisting FeNi metal and troilite, which proceeds with participation of groundwater on the Earth surface. Electrochemical reactions between meteorite material and partially dissociated aqueous solutions circulating along fractures are suggested to explain the formation of secondary sulfide phases, pentlandite (FexNi1 – x)9S8 and heazlewoodite Ni3S2, in pallasites and iron meteorites. The model of the supergene origin of the troilite + pentlandite + heazlewoodite and troilite + pentlandite + Ni (metal) associations is consistent with the intense corrosion of the Seymchan and Dronino meteorites, which is expressed by the extensive development of Fe-oxide/Fe-hydroxide rims and especially by the appearance of secondary hydrated minerals of supergene origin in Dronino.