^{Banded Iron Formations (BIFs)}
• BIFs form when iron-rich chemicals precipitate out of the oceans during iron-rich phases of ocean history.  Such phases occured with regularity from 3.8 to 1.8 Ga and reoccurred from 0.8 to 0.6 Ga, with a formation peak around 2.5 Ga.
• BIFs are sedimentary, layered formations consisting of thin (mm to a few cm), alternating layers of iron-rich material (commonly magnetite = Fe₃O_4, hematite = Fe₂O₃, and siderite = Fe(CO)₂) and silicate minerals like chert (SiO₂).

1. without an oxygen rich atmosphere, eroded  iron enters oceans as ferrous ions (Fe²⁺).  Ferrous iron dissolves well in sea water.  Low oxygen content of oceans permits iron to remain in solution.  Meanwhile, silicate-rich sediments accumulate on ocean floor.
2. as cyanobacteria (primitive photosynthesizers that produce oxygen but do not thrive in oxygen-rich environments) begin to prosper, oxygen level of oceans begins to increase.  In oxygen-richer environment, iron precipiates out via reactions like:

4Fe²⁺ + 3O₂ --> 2Fe₂O₃. This forms iron-rich sediments and keeps oxygen from building up in atmosphere or ocean.
3. Once iron is depleted in oceans, oxygen level builds up in oceans; but early photosynthesizers were poisoned by the overabundance of oxygen in oceans.
4. With destruction of most of photosynthetic life, iron builds up in oceans, a silicate layer is deposited on the ocean floor, and the cycle repeats.
5. This cycle largely ended when most of the iron (that was continually released through tectonic processes) was locked away into BIFs.

• What are the three oxidation states of iron? elemental, Ferrous (Fe²⁺) and Ferric (Fe³⁺)
• Which is soluble in water? Ferrous
• What happens when Ferrous iron is exposed to oxygen? It loses electrons and becomes oxidized (Ferric)
• What iron-bearing minerals are found in BIFs? magnetite, hematite.
• Where in the ocean did BIFs form? Continental shelves.  If they had formed in the deep ocean, they would have been subducted and recycled. Also, only shallow waters would have been heavily oxygenated.
• How do we know the oceans were anoxic prior to about 1.9 BY ago? The Fe would have precipiated out any time oxygen was present.
• What would happen to eroded Fe if the atmosphere were oxygen poor?  It would be removed from the soil as  (Fe²⁺) since Ferrous iron is soluble and would wash into the oceans.
• What would happen to eroded Fe if the atmosphere were oxygen rich?  It would become  insoluble Ferric (Fe³⁺) and would stay in the local soil.

Questions and answers about redbeds

• What are redbeds? Redbeds are reddish-colored sandy and silty sediments.
• Where do they form? In arid regions such the American Southwest.
• What do redbeds indicate? Redbeds indicate oxidizing conditions at the time of their formation because the redbeds are

coated with hematite (Fe₂O₃) which is an oxidized form of iron.
 

• What is uraninite?   UO₂
• What ion forms it? U⁴⁺.
• What are the oxidized and oxidized states of uranium? U⁴⁺(unoxidized) and U⁶⁺(oxidized).
• Under what atmospheric conditions would uraninite form in sedimentary rocks? Low oxygen.