Heterogeneous histories of Ni-bearing pyrrhotite and pentlandite grains in the CI chondrites Orgueil and Alais

Compositional and structural analyses of CI chondrite iron–nickel sulfide grains reveal heterogeneity both across and within the Orgueil and Alais meteorites. Orgueil grains with the 4C monoclinic pyrrhotite structure have variable metal-to-sulfur ratios and nickel contents. These range from the nominal ratio of 0.875 for Fe₇S₈ with <1 atom% nickel to a high metal-to-sulfur ratio of 0.97 with 15 atom% nickel. These data reveal a previously unrecognized low-temperature solid solution between Fe₇S₈ and Fe₅Ni₃S₈. We have also identified 6C monoclinic pyrrhotite among the Orgueil iron–nickel sulfides. The occurrence of pentlandite in Orgueil is confirmed for the first time crystallographically. In contrast, sulfide grains in Alais do not show the same spread in composition and structure; rather they represent the endmembers: low-Ni 4C monoclinic pyrrhotite and pentlandite. We investigate possible formation/alteration scenarios: crystallization from a melt, solid-state diffusion and/or exsolution, oxidation of pre-existing sulfides, and precipitation from a fluid. Sulfide grains are sensitive to alteration conditions; these data suggest that the structures and compositions of the sulfide assemblages in Orgueil and Alais were established by late-stage parent body aqueous alteration, followed in some cases by low-temperature solid-state processes. The samples record different alteration histories, with Orgueil experiencing lower equilibration temperatures (25 °C) than Alais (100–135 °C). We conclude that millimeter-scale heterogeneity existed in alteration conditions (e.g., temperature, pH, oxygen fugacity, sulfur fugacity, duration of alteration) on the parent body. This variability is evidenced by the diversity among sulfide grains located within millimeters of one another.