| Sedimentary rocks formed at least 1800 million years ago (during the Proterozoic and Archean eons), composed of alternating layers of grey magnetite and red hematite,containing folds caused by soft sediment deformation and/or later folding events, to make an attractive, and very dense, rock. The red mineral is often thought to be chert, a form of silica, but the simple streak plate test shows a red streak, indiciating that it is definately not chert. It is also to dense to be silica. There are however small amounts of chert within the rock.
Banded Iron Formation Facts:
Banded iron formations are a great source of iron because they contain both magnetite (Fe3O4) and hematite (Fe2O3), which both have high iron to oxygen ratios.
Maximum deposition was reached between 2.5 and 2 billion years ago when about half a million (metric) tonnes of iron were deposited per year.
Banded iron formations form thousands of meters of rocks in some places, and account for most of the iron ore deposits on Earth.
The Hamersley Iron Province in Australia contains 20 million tonnes of iron ore, formed by banded iron formation. It contains 55% iron.
Most steel industries are based on iron ore deposited by this process.
The layers are sometimes found millions of times in a microscopic area. They can be traced for kilometers.
These rocks were only formed in the archean and early proterozoic times, when the Earth was much different that it is today, making any conceptual analysis harder.
How did the Banded Iron Formations Form?
The atmosphere on early Earth had little oxygen, and therefore little oxygen was dissolved in the ocean water. This allowed for iron to dissolve in the water readily, but iron requires an oxidating chemical reaction to precipitate. This creates a problem...
How was the iron oxidized?
Organic Theory:
Photosynthesizing bacteria "grabbed" an iron atom (presumably with the oxygen molecule they release), which then started to form a crystal, growing inorganically and dropping to the ocean floor. There was a discovery in 1993 of purple bacteria (cyanobacteria) that use photosynthesis to break up iron carbonate dissolved in water and precipitate oxidized iron as a by-product. This is strong evidence for this theory. Also see below, banded iron formations and stromatolites.
Inorganic Theory:
UV radiation gave the energy to form an iron ore compound in the upper regions of the oceans, which then fell to the ocean floor. This happenned slowly over the archean and early proterozoic as photosynthesizing organisms increased the atmospheric (and therefore dissolved) oxygen content.
Problems with Organic/Inorganic Theories:
Organic theory is hard to demonstrate since the iron formed grow inorganically, it can't be differentiated from inorganically formed iron ore.
Organic theory can't explain how the earliest banded iron formations were formed, when (it is speculated) there were no photosynthesizing bacteria.
Neither theory explains why there are alternating layers of the two forms of iron oxide.
In both theories there would be more iron forming in the summer months (when UV is at maximum), leaving a layer with more chert in the cooler months (since the precipitation of silica does not necessarily depend on sunlight or organic processes) . But this is not the case, it is generally a regular pattern.
Impact of Organic Theory of Iron-Ore Deposits:
Since the deposits are found all over the ocean floor, the bacteria would have to have been floating all over the ocean surface. Not just in shallow pools of primordial mud, as some theorized. These bacteria would have been responsible for creating the oxygen rich atmosphere we enjoy today.
Banded Iron Formations and Stromatolites:
Banded iron formations are often found in combination with stromatolites, which is strong evidence for the organic theory of iron ore precipitation. The cyanobacteria in the stromatolites must have oxidized iron in combination with capturing minerals so that they would be found together. However, the stromatolites are only found in low energy, shallow environments, whereas banded iron is found where there was deep and/or turbulant water as well. | 
