The world's primary economic source of iron ore, iron formations, are ancient sedimentary rocks that appear as solid as rocks can be. How did they get to be like that? According to researchers, it has to do with "green rust" that formed in the water and sank to the bottom of the ocean billions of years ago.

In a study published in the journal Nature Geoscience, Itay Halevy and colleagues suggested that green rust was the foundation for today's iron formations. It might be rare now, but green rust used to be highly abundant.

Halevy got the idea that one of the insoluble compounds that formed banded iron was a rusty green material when he was attempting to recreate environmental conditions on early Mars. He didn't recognize the green stuff he got at first, but a few more experiments revealed that he was looking at a mineral known as green rust.

Iron Formation And The Early Earth

It has been established that dissolved iron was present in the early oceans, which is a strong indicator of exceedingly low concentrations of free oxygen on Earth. Otherwise, iron would have caused a reaction with oxygen, forming iron oxides, or the rust that people know so well now.

Today, iron makes its way to oceans in the form of small insoluble particles of oxide in rivers. However, this was a process that only occurred when free oxygen started accumulating in the planet's atmosphere some 2.5 billion years ago. With nearly zero oxygen in the air, oceans were rich in iron, although that didn't mean that it stayed indefinitely dissolved in the seas' water. Eventually, iron in the water combined with other elements to create insoluble compounds, settling on the seafloor to become the banded iron formations known today.

Halevy and colleagues found evidence for their ideas in Sulawesi, Indonesia, where Lake Matano is located. Green rust is formed in the lake today, thanks to its oxygen-poor but iron-rich environment, which was thought to be similar to seawater present during Earth's early history.

Recreating Early Oceans

The researchers tested out their ideas by setting up experiments aimed at recreating the ancient Precambrian ocean. Based on their observations, they not only saw green rust form under oxygen-free conditions but also that when it was left to age, the material transformed into the same composition making up Precambrian iron formations, which combined silicates, carbonates, and iron-containing oxides.

Could have green rust functioned as the main avenue for settling iron out of the seawater?

The researchers then developed models depicting the early oceans' iron cycle, including the possibility of green-colored rust forming and competing with other minerals capable of shuttling iron to the bottom of the sea. Their results suggest that the mineral was probably a major component in the iron cycle, with iron within the mineral transforming into what can be readily observed today in geological records.

"As far as we can tell, green rust should have delivered a substantial proportion of iron to the very early ocean sediments," said Halevy, although he acknowledged that there could have been several means of iron deposition in the past.