Why Don’t Iron Meteorites Have Diamonds? A Cosmic Mystery

Meteorites can tell us incredible stories about the history of our solar system, and some even contain diamonds! But while diamonds in ureilite meteorites formed under extreme shock-induced p–T conditions, iron meteorites rarely contain diamonds, although they should have seen similar conditions. Why is that?

In this study, Christ and co-authors analyzed graphite from three IAB iron meteorites—Canyon Diablo, Campo del Cielo, and Yardymly—using micro-Raman spectroscopy. They found classic graphite signals, including the G-band (showing ordered graphite) and the D-band (indicating structural disorder). Temperature estimates suggested the graphite had been heated to over 1100°C, similar to diamond-bearing ureilites.

So where are the diamonds? The answer lies in the physical properties of iron meteorites. Unlike the brittle matrix of ureilites, which efficiently transmits shock energy to create diamonds, iron meteorites are ductile—meaning they absorb shock energy through plastic deformation rather than generating the extreme pressures needed for diamond formation.

This means that for diamonds to form in iron meteorites, an impact would need to be incredibly powerful or graphite must be positioned very close to the impact site, where it can experience the highest energies before they dissipate.

This research deepens our understanding of planetary collisions and meteorite formation, reminding us that even space rocks have fascinating differences!