A comparative study of experimental and meteoritic metal-sulfide assemblages

Sulfide formation via a gas-solid reaction between iron-nickel metal and H_2/H_2S gas mixtures was studied experimentally. This reaction produces distinctive chemical fractionations in both metal and sulfide that can help identify pristine nebular sulfide condensates in meteorites. The resulting sulfide morphology consists of a troilite scale divided into two distinct layers : an inner layer containing small, randomly oriented crystals and an outer layer consisting of large, columnar crystals. A thin band of metal surrounding the unreacted metal core and small metal blebs located in the inner sulfide layer are significantly enriched in nickel relative to the starting metal composition. The stoichiometry of the sulfide is nearly ideal {(Fe+Ni+Co)/S=1} at the metal-sulfide interface but the sulfur content increases with distance from the metal. A significant amount of nickel is present in the sulfide layer and increases in concentration across the sulfide layer. The nickel concentration gradient results from diffusion of nickel ions in the sulfide being faster than that of iron ions. Microprobe analyses on metal-sulfide assemblages in the LL3 unequilibrated ordinary chondrite Allan Hills-764 (ALH-764) do not show these chemical fractionations. Instead, textural and chemical evidence suggests that these meteoritic sulfides were altered during a post accretion heating event