How do pyrite and uraninite grains provide evidence for an early oxygen-free atmosphere?

Pyrite and uraninite serve as critical indicators of an early oxygen-free atmosphere because they are chemically unstable in the presence of oxygen. In an oxygenated environment, these minerals would be rapidly destroyed by oxidation during the processes of weathering, erosion, and transport.123

However, the geological record contains numerous well-rounded, detrital grains of these minerals in sedimentary deposits older than 2.2 billion years. Their physical state—showing signs of tumbling and transport in ancient rivers or beaches without evidence of chemical attack—demonstrates that they were not exposed to significant levels of atmospheric oxygen during their deposition. The preservation of these minerals in their original, unoxidized form provides strong evidence that the Earth's atmosphere was anoxygenic during the Archean Eon.123