Chemistry in Pictures: Billion-year-old oxygen sensor

The people at the Smithsonian Institution National Museum of Natural History were nice enough to give a couple members of the Chemistry in Pictures team a behind-the-scenes look at their mineral collection. For the next couple weeks, we’ll bring you photos of some of the minerals that wowed us, some of which are billions of years old.

Within the red and blue bands of this mineral lies the story of an oxidizing Earth. Taken from Jasper Knob in the Upper Peninsula of Michigan, the banded iron formation is about 2.5 billion years old. According to Elizabeth Cottrell, chair of the department of mineral sciences at the National Museum of Natural History, the alternating layers of the two mineral types likely reflect cyclic variations in oxygen levels. The dominant theory goes: as waves of cyanobacteria—a type of photosynthesizing, oxygen-generating microbe—first arrived in Earth’s bodies of water, they created enough oxygen to convert the iron dissolved in the water to insoluble iron oxides. It’s possible that the cyanobacteria periodically accumulated in oxygen-rich biofilms on growing rocks, and those biofilms and their minerals then became embedded in the rock structure. The blue-gray bands reflect these oxygen-rich periods and contain insoluble hematite and magnetite. Meanwhile, the red bands reflect periods that saw less oxygen and contain silica with a bit of iron mingled in.

This specimen fits on a tabletop, but the museum has a truly huge slab from this formation mounted on a wall near their labs (below).

Credit: Chip Clark/Smithsonian Institution National Museum of Natural History (tabletop specimen), Manny Morone/C&EN (wall)

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