Banded Iron Formations (BIFs)
theory (see here)
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 = Fe3O4,
= Fe2O3, and siderite = Fe(CO)2) and silicate
minerals like chert (SiO2).
Questions and answers about BIFs
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.
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
Once iron is depleted in oceans, oxygen level builds up in oceans; but
early photosynthesizers were poisoned by the overabundance of oxygen in
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.
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.
redbeds from when hematite (Fe2O3) is produced during
weathering of iron-rich rocks in an oxidizing atmosphere
Questions and answers about redbeds
Questions and answers about uraninite:
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)
Under what atmospheric conditions would uraninite form in sedimentary rocks?