The issue of Earth and Planetary Science Letters for November (vol 669) contains:
Amano, K. Viennet, J-C. Beck, P. et al. Updating the Urey-Craig diagram: The iron redox states of the building blocks of the outer solar system Art 119587 .2025.119587
Once again, we are trying to piece together the formation of our Solar System… our common heritage. We do that by examining unmelted (“primitive”) meteorites, which have not lost their original minerals through magmatism (or outgassing) as larger bodies did.
The Urey-Craig diagram is a plot of meteorite groups, showing which ones group together as having more “free” iron (metallic iron grains) versus bound iron ore (iron oxide). On Earth, free (native) iron largely doesn’t exist; we have an oxygen atmosphere, plus available water, which turns iron into rust or other forms. So even though Earth, technically, has lots of iron, we need to smelt it from its ore forms when we want to make metal implements. Not so in space.
Since free space has negligible atmosphere (at best, the solar wind- hydrogen ions) and no mobile forms of water, iron can and does persist in free, metallic form. But that’s largely in the inner Solar System; the outer Solar System has less influence from the Sun (solar wind or otherwise), and it has (in the past and, to an extent, the present) a trickle of infalling material from interstellar space. This is embodied in meteorites: those meteorites from the inner Solar System (OC and E) have more metallic grains, while those from the outer Solar System (CCs) bind their iron into minerals. Amano et al. dive deeper into those CC meteorites, and their iron.