Updating the Urey-Craig diagram: The iron redox states in the building blocks of the outer solar system

This idea is illustrated in an iconic plot in planetary science, the so-called Urey-Craig diagram (UC diagram), where the amount of iron accommodated in sulfides and metallic alloys is plotted against that found in oxides and silicates. Hydrous carbonaceous chondrites (CI and CM) are usually considered the oxidized end-member despite the minor presence of sulfides.

Here, we reevaluate the position of CIs inspired by the recent space missions that sampled CI-like materials and from a fresh CI meteorite (OC 002). The same approach was undertaken for C2-ungrouped, CM, and CR chondrites with varying proportions of phyllosilicates based on X-ray diffraction and Mössbauer spectroscopy. As a result, all fresh CI materials contain a considerable amount of iron sulfides and therefore plotted in the central part of the UC diagram.