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Iron & Mobility (Oxidising waters e.g. (Rainwater, Stream / fresh…
Iron & Mobility
Oxidising
waters e.g.
Rainwater
Stream / fresh lake water
Peat bog
Aerated saline lakes
Seawater
Porewater in surficial seafloor sediments
Bauxite
Aluminium hydroxide
Al = very immobile element
Oxidised aluminium doesnt move in sedimentary environment
Aluminium builds up in position
--> forms bauxite (rock ore)
Controls on Fe Mineral Precipitation
Eh
pH
Activity of carbonate ions
effective conc of carbonate ions
measured as pCO2
Activity of sulphur
Reducing Envmts e.g.
Organic rich saline lakes
Euxinic marine water (anoxic)
Neutral to high pH
Low Eh (negative)
Geochemical Mobility
Can infer if elements likely to occur
near to sediment source
distally
Expected gradation of occurence as f(mobility)
Bauxite - local occurrence (nr source)
Iron - transitional envmts
Silica - deep sea (distally as v mobile)
Manganese - deep sea
True up to a point
cannot explain everything with this
tho useful consideration
Silica
Very soluble
May be mobilised
Travel far from source in solution i.e. reaches oceans
Manganese Ore Deposits
Mn4+
Immobile
Mn2+
Mobile
Lots of Mn 2+ as it is very easy to reduce Mn
Manganese nodules / crusts common in deep sea
since it is easy to reduce Mn
Eh
how oxidising or reducing a solution/envmt is
i.e. whether an element will tend to gain or lose electrons
positive = oxidising envmt
Iron mobility
Ferric Iron (Fe3+)
immobile
oxidised state (rust)
Ferrous Iron (Fe2+)
soluble & mobile
may move away in solution
Organic
Matter & Eh
Organic matter decomposition affects Eh of natural environments
Decomposition organic matter -->
Consumes oxygen -- creates reducing conditions
Oxidation
State of Fe
Eh & pH of envmt
dictate
oxidation state of iron
presence / form of iron