The mystery of the green rust

Scientists from the Weizmann Institute speculate that the rust that sank to the bottom of the oceans billions of years ago was "green rust" - an iron-based mineral that may have been more common in the past. 

The sedimentary rocks on Earth may be hard as stone, but the iron found in them was once part of the seawater solution. How did the iron turn from a molten substance into a hard ore? Dr. Itai Halevi, and his research group in the Department of Earth and Planetary Sciences at the Weizmann Institute of Science, hypothesize that the "rust" present in the ocean waters, which sank to the bottom of the sea billions of years ago, was "green rust" - an iron-based mineral, which is now quite rare – but may have been more common in the past.

It is known that the waters of the ancient oceans contained dissolved iron, indicating very low concentrations of free oxygen (O₂) on Earth in those eras. Otherwise, the iron and oxygen would react with each other, and would form together iron oxides - reddish deposits that everyone who has left a pair of bicycles in the rain knows very well. Today, says Dr. Halevi, iron passes from the land to the ocean waters as tiny, insoluble oxide particles, through river water. But the formation of sediments in this way could only begin to occur after enough free oxygen had accumulated in the atmosphere, which happened about 2.5 billion years ago. Before that, when the presence of oxygen in the environment was very poor, the oceans were rich in iron. However, over time the iron dissolved in the water did not remain, but formed insoluble compounds with other chemical elements, and sank to the bottom of the ocean in a gradual process that results in the formation of the iron ores we know today.

The possibility that these insoluble compounds were the mineral known as "green rust" occurred to Dr. Halevi while he was working on his doctoral research, in which he tried to recreate in the laboratory the conditions that prevailed on the planet Mars in its early stages, including the characteristic red-rusty iron deposits His. "In the system we created, a green substance was formed that we didn't recognize at first, and following its exposure to the air, it quickly changed its color to orange," he says. In further experiments, he discovered that the substance is "green rust", a very rare mineral on Earth today, due to its strong affinity for oxygen: under the conditions that exist on Earth today, the mineral quickly turns into the familiar red rust. On the other hand, Dr. Halevi hypothesized, in an oxygen-poor environment in ancient times, it could have been a central means of transporting the dissolved iron and forming solid compounds that sank to the bottom of the sea.

Support for this belief comes from the Sulawesi region of Indonesia, where "green rust" is currently forming in Lake Matano, the waters of which are rich in iron and poor in oxygen - thus they are similar to the waters of the oceans of the ancient Earth. In order to examine the possibilities that emerged from the experiments and to emphasize their importance, Dr. Halevi and the members of his group created a system that simulates as much as possible the conditions that prevailed in the oxygen-free ocean of the Precambrian era. Their experiments showed that not only did the mineral "green rust" form under these conditions, but also that if allowed to, it may turn over time into minerals known from iron formations from the Precambrian era, which are iron-bearing oxides, carbons (carbonates) and silicates.

Could the mineral "green rust" have been the main mechanism of fixing iron and removing it from the Precambrian seawater solution? Dr. Halevy and his group members developed models to describe the iron cycle in the Earth's ancient oceans, including the possibility of the formation of "green rust" and competition with other iron-carrying minerals to the seabed. Analysis of the results of the model raises the possibility that, indeed, this mineral played a central role in the iron cycle on the ancient Earth, and the iron in the "green rust" became over time the minerals that can be found in geological layers. "Of course, this was one of several ways to create iron ores, just as today there are several processes involved in the creation of sediments in the oceans," says Dr. Halevi. "But to the best of our understanding, 'green rust' apparently transferred a considerable amount of iron to the earliest sediments that formed on the bottom of the oceans."

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Until about 2.4 billion years ago, when oxygen appeared in the Earth's atmosphere, more than half of the iron that entered the ocean probably sank as green rust.