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Iron Mineral Formation (Chamosite Iron Silicate (Why rare now?…
Iron Mineral Formation
Iron Oxides
Where?
Oxidising environments
How?
Rock weathering
Concentration Fe oxides (often near source)
Examples
Water in structure - likely limolite/goethite
Solids - haematite / magnetite
'alluvial iron formation'
Chamosite
Iron Silicate
Where ?
Shallow marine conditions
i.e. near deltas
When ?
Rare in modern
No ooids found
Just gelatinous bodies
Important in Mesozoic (Jurassic)
Why rare now?
Conditions uncommon at present
Diagenesis requires time
Structure reorganised by deep burial to form ooid
Ooid structure developed via diagenesis
Formation
requires:
1. Hot tropical envmt
2. Very high Fe input
Conditions rare in modern
Only ever find precursory gelatinous body in modern
Glauconite
Iron Silicate
What ?
Iron silicate with Fe, Al, K in structure
Where ?
Continental shelf environments
(10s - 1000s metres)
Unconformity surfaces in TST
Fe2+ & Fe3+
in structure
∴ sensitive to oxidising/reducing conditions
Transitional envmt
Forms just beneath sediment surface
Water above = oxic
Sed below = anoxic
Transgressive systems tract
Sealevel rises due to land subsidence / RSL rise
Glauconite common on unconformity surfaces
∴ shoreline moves landward
Siderite
Fe Carbonate
Where?
Reducing environments only
Freshwater / brackish environments i.e. laggonal/swamp
Less common in fully marine environments
What ?
Calcite with Fe2+ replacing Ca2+ essentially -- Fe2+CO3 2-
Why less common
in marine envmts ?
pyrite preferentially forms instead
Calcite alters to siderite - rusty edges & veins
Pyrite
FeS2
Diagenetic
mineral
Doesn't form in modern world - takes 100 kyr - 1Myr
Forms from FeS
FeS deeply buried
Transforms to FeS2 after 100 kyr - 1 Myr
FeS = stinking black mud
Requires reducing conditions
Requires lotsa sulphur
1. Bacterial reduction sulphate
Seawater = rich in sulphate (SO4)
Bacterial reduction by microbes produces S
Sulphur combines with FeS to form FeS2
2. Sulphur from decaying organic matter
Organic matter decomposition releases S
Local release sulphur