Iron Oxide and Iron Deposits
Banded Iron Formation
Whilst in Frankfurt a few days ago, a team member from Everything Dinosaur took the opportunity to photograph some of the amazing outdoor exhibits on display opposite the Senckenberg Naturmuseum (Frankfurt Natural History Museum). Amongst the stunning replicas of prehistoric plants and of course, the iconic, life-size model of Tyrannosaurus rex, our staff member spotted a beautiful example of a banded iron formation.
Not Too Difficult to Spot – A Huge Monolith (Banded Iron Formation)
An example of a banded iron formation.
Picture credit: Everything Dinosaur
A Record of a Changing Atmosphere on Planet Earth
Formed in marine environments at least 2.5 billion years ago, these deposits provide information about when the atmosphere of our planet and its oceans became oxygenated. Iron oxide is not soluble, therefore the iron must have been transported in a non-oxidised form. This could only have happened if there was almost no oxygen in the atmosphere or the oceans.
A Closer View of the Iron Banded Formation Showing the Deposition
The individual layers can be clearly seen in this banded iron deposit.
Picture credit: Everything Dinosaur
Banded Layers of Iron Oxide
The picture above shows a close-up view of the banded layers. Each layer is quite thin and these patterns are usually formed by alternating bands of iron-rich material (usually magnetite) and silica (chert). During the Archean Eon, the primitive Earth had little free oxygen. Rocks rich in iron were weathered at the surface and this iron remained largely unchanged as there was no free oxygen to combine with it and create iron oxide (rust). The iron ions entered the sea in an unaltered chemical state. They formed the band of iron-rich material seen in some of the layers.
However, primitive cyanobacteria (blue/green algae), were beginning to become more abundant in surface waters. As algae populations grew, there was a subsequent increase in photosynthesis. Oxygen is a bi-product of photosynthesis and this free oxygen began to combine with the iron ions in the water to form magnetite (Fe3O4), iron oxide.
A Banded Iron Formation
As more and more algae photosynthesised so the amount of oxygen available to combine with the iron increased, until a point was reached whereby the O2 production of the biomass in the marine ecosystem exceeded the amount of iron that was available to combine with. The oxygen was left in the ocean and this gas rose to toxic levels decimating the cyanobacteria. The algae population collapsed and led to the accumulation of a sedimentary layer on the seabed low in iron (the chert). Over time, something like 800,000 years, algal blooms and peaks and troughs of oxygen production via photosynthesis led to the banded formations seen today in rocks dating from the Archean.
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