chemical precipitates

• Various elements are dispersed throughout sea water in small amounts. 
• Evaporation of sea water concentrates the salts in the water and they crystalise. 
• Calcite 

  Gypsum 

  Anhydrite 

  Halite 

  Potassium
• Banded iron formations occur in Precambrian rocks. 
• Iron released by chemical weathering of mafic minerals did not oxidise but stayed in solution, so was transported by rivers to the sea. 
• Bacteria photosynthesised to produce oxygen and the oxygen would oxidise the iron to form insoluble iron oxides. 
• Some BIF extend for hundreds of kilometres and can be up to 500 meters thick, some suggest finest banding may be diurnal. 
• Iron ores include haematite and goethite.