A rusty green early ocean? (2017)

How did that iron go from a dissolved state to banded iron formations? Dr. Itay Halevy and his group in the Weizmann Institute of Science's Department of Earth and Planetary Sciences suggest that, billions of years ago, the "Rust" that formed in the seawater and sank to the ocean bed was green - an iron-based mineral that is rare on Earth today but might once have been relatively common. Otherwise, the iron would have reacted with oxygen to form iron oxides, which are the rusty red deposits familiar to anyone who's left a bike out in the rain. The idea that one of those insoluble compounds could be a rusty green mineral occurred to him during his doctoral research, says Dr. Halevy, when he was trying to recreate the conditions on early Mars, including its rusty-red iron sediments. "I got some green stuff I didn't recognize at first, which quickly turned orange when I exposed it to air. With a little more careful experimentation, I found that this was a mineral called green rust, which is extremely rare on Earth today, owing to its affinity for oxygen." Today, green rust quickly transforms into the familiar red rust, but with not much free oxygen around, Dr. Halevy reasoned, it could have been an important way for dissolved iron to form solid compounds and settle to the sea floor. Could green rust have been a main vehicle for settling iron out of seawater? Dr. Halevy and his team developed models to depict the iron cycle in Earth's early oceans, including the possibility of green rust formation and competition with other mineral shuttles of iron to the seafloor. The iron in the green rust later transformed into the minerals we can now observe in the geologic record. "But as far as we can tell, green rust should have delivered a substantial proportion of iron to the very early ocean sediments."