Banded iron formations

What are banded iron formations?

a rock made up of layers of iron oxides and chert

What are the most common iron oxides in banded iron formations?

• Magnetite and hematite (Iron oxide with different chemical formula)-grey layers 

• have >15% iron

What is required to make the layers?

repeating motion

What is chert?

• primarily composed of silica 

  • Rock not very crystalline 

  • Direct from precipitate from the ocean 

Where are BIF’s formed?

in ocean basins

How does iron get in the ocean basin for BIF’s to form?

• Iron dissolved in rivers and stuff and goes to the ocean 

• Submarine volcanoes-magmas contain iron 

• Most of iron in the rock is from the volcanoes-tell using chemistry of iron 

How does iron build up high concentrations in water?

• Depends on oxidation state 

• Ferrous iron is soluble (Fe2+-lost 2 electrons) 

• We want the iron to be in Fe3+ form–reduced form of iron 

  • Ferric Iron (3+) oxidized form of iron 

• Ocean basins need to be oxygen deficient, otherwise iron will become iron 2+

• Want bottom waters of the basin to anoxic to build up iron concentration

  • Don't see much today, depends on atmosphere 

  • Too much oxygen=too much oxygen in ocean 

Why cant BIF’s form anymore?

• Back in earth's history-time period- atmosphere must have been oxygen deficient= so waters were good to build (anoxic) 

• After 1.8 billion years ago, atmosphere too much oxygen, cant form

How do layers form?

• Ocean upwelling 

  • Take these iron 2+ water to shallow regions 

• Photosynthesising organisms in shallow waters for sun

  • Produce-oxygen 

• Iron water interact with oxygen=iron 2+ converts to iron 3+ (insoluble) so precipitates 

• Deposition of layers occurs 

• No upwelling=silica and precipitated as chert