weathering, the concentration of insoluble iron oxides
(Fe(OH) 3 ) and manganese (Mn(OH) 4 ) will increase,
along with aluminium (Al(OH) 3 ), and constitute a
major part of laterite.
Bogs, such as we find in the Scandinavian
highlands, have conditions ideal for such precipitating
iron (“bog iron”). Water seeping through bogs will
have low Eh and pH, and iron-containing minerals
from underlying rocks will be dissolved by humic
acids, transported in the reduced state and precipitated
through oxidation where the water flows out of the bog.
Porewater in sediments and sedimentary rocks is
normally reducing. This is because most sediments
contain reducing agents, particularly organic material.
Only the upper 0–20 cm below the sea (lake) floor are
oxidised. The depth to which the groundwater will be
oxidising depends on the supply of oxygen, i.e. the
oxygen content and rate of flow of groundwater compared to the consumption of oxygen due to oxidation.
Desert sediments contain little organic material which
can function as a reducing agent, and the groundwater
will therefore remain oxidising longer. This can
explain the red colouration of desert sediments which
is due to trivalent iron.
The redox boundary, which normally lies just
below the sediment/water boundary on the ocean bottom, represents an important geochemical trap. The
concentration of elements on each side of this boundary is very different because of the different solubility
of elements in the two separate chemical
environments. A high concentration gradient of Fe
2+
and Mn
2+ on the reducing side of the redox boundary
leads to a diffusion and precipitation of oxidised iron
and manganese immediately above the boundary. At a
depth where there is little oxygen in the porewater,
sulphate-reducing bacteria consume the oxygen in the
sulphate ions to form H 2 S. The sulphate concentration
in the porewater declines rapidly downwards causing a
downwards diffusion. H 2 S is an acid which can release
(leach) iron bound in various clastic minerals to form
FeS (machinawite), which is black and easily
oxidised, and then pyrite (FeS 2 ) which is a little
more stable. If the sediments have a high silica content, chamosite may also be formed. Glauconite must
be formed right at the redox boundary, since it
contains both bivalent (Fe
2+ ) and trivalent (Fe
3+ )
iron. Uranium and vanadium, which have low solubility in the reduced state and are more soluble when
oxidised, will be able to diffuse downwards and be
precipitated in the reducing zone below the redox
boundary.
Manganese will not be as easily trapped in the
sulphate-reducing zone, as Mn
2+ does not form such
stable sulphides as Fe
2+ , and it will therefore have a
greater tendency to be precipitated in the oxidised
zone.
During breaks in sedimentation, or slow sedimentation, porewater expelled by compaction will cross
the redox boundary and this may cause the precipitation of iron and manganese on the seafloor. Manganese nodules are concretions of manganese and iron
hydroxides and oxides found on the seafloor, particularly in the ocean basins (South Atlantic and Pacific).
They also contain relatively high concentrations of
metals like Ni, Cu, Zn and Co and there have been
plans to mine these deposits. The nodules grow by
very slow concentric accretion in a pelagic ooze.
Plankton is capable of accumulating very high
concentrations of metals from seawater and when the
organisms dissolve, the planktonic ooze becomes very
rich in these metals, which will be precipitated
together with the manganese hydroxides in the
concretions.
Iron and manganese are virtually insoluble in
oxygenated sea water, and consequently cannot be
transported in ordinary solution. However, rivers can
carry a good deal of iron and manganese adsorbed
onto organic particles or as clay-sized iron oxides
which in many cases may produce a red colour.
Iron can be precipitated not only by oxidation, but
also in the reduced state in a basic environment. Thin
carbonate laminations in shales represent a local highpH environment, where reduced iron which is normally soluble can be precipitated:
Fe
2þ
þ CaCO 3 ¼ FeCO 3 þ Ca
2þ
This reaction occurs at relatively low Fe
2þ
=Ca
2þ
ratios because iron carbonate (siderite) is less soluble
than calcite. If there is sulphur present, iron will first
form the sulphide, however, and iron carbonate will
not be stable. Siderite is therefore most typically
formed in freshwater basins, and not directly in the
sulphate-reducing zone in marine sediments. Siderite
can also be formed below the sulphate-reducing zone,
for example by reaction between aragonite and iron in
the sediment.
6 Mudrocks, Shales, Silica Deposits and Evaporites
227
(Fe(OH) 3 ) and manganese (Mn(OH) 4 ) will increase,
along with aluminium (Al(OH) 3 ), and constitute a
major part of laterite.
Bogs, such as we find in the Scandinavian
highlands, have conditions ideal for such precipitating
iron (“bog iron”). Water seeping through bogs will
have low Eh and pH, and iron-containing minerals
from underlying rocks will be dissolved by humic
acids, transported in the reduced state and precipitated
through oxidation where the water flows out of the bog.
Porewater in sediments and sedimentary rocks is
normally reducing. This is because most sediments
contain reducing agents, particularly organic material.
Only the upper 0–20 cm below the sea (lake) floor are
oxidised. The depth to which the groundwater will be
oxidising depends on the supply of oxygen, i.e. the
oxygen content and rate of flow of groundwater compared to the consumption of oxygen due to oxidation.
Desert sediments contain little organic material which
can function as a reducing agent, and the groundwater
will therefore remain oxidising longer. This can
explain the red colouration of desert sediments which
is due to trivalent iron.
The redox boundary, which normally lies just
below the sediment/water boundary on the ocean bottom, represents an important geochemical trap. The
concentration of elements on each side of this boundary is very different because of the different solubility
of elements in the two separate chemical
environments. A high concentration gradient of Fe
2+
and Mn
2+ on the reducing side of the redox boundary
leads to a diffusion and precipitation of oxidised iron
and manganese immediately above the boundary. At a
depth where there is little oxygen in the porewater,
sulphate-reducing bacteria consume the oxygen in the
sulphate ions to form H 2 S. The sulphate concentration
in the porewater declines rapidly downwards causing a
downwards diffusion. H 2 S is an acid which can release
(leach) iron bound in various clastic minerals to form
FeS (machinawite), which is black and easily
oxidised, and then pyrite (FeS 2 ) which is a little
more stable. If the sediments have a high silica content, chamosite may also be formed. Glauconite must
be formed right at the redox boundary, since it
contains both bivalent (Fe
2+ ) and trivalent (Fe
3+ )
iron. Uranium and vanadium, which have low solubility in the reduced state and are more soluble when
oxidised, will be able to diffuse downwards and be
precipitated in the reducing zone below the redox
boundary.
Manganese will not be as easily trapped in the
sulphate-reducing zone, as Mn
2+ does not form such
stable sulphides as Fe
2+ , and it will therefore have a
greater tendency to be precipitated in the oxidised
zone.
During breaks in sedimentation, or slow sedimentation, porewater expelled by compaction will cross
the redox boundary and this may cause the precipitation of iron and manganese on the seafloor. Manganese nodules are concretions of manganese and iron
hydroxides and oxides found on the seafloor, particularly in the ocean basins (South Atlantic and Pacific).
They also contain relatively high concentrations of
metals like Ni, Cu, Zn and Co and there have been
plans to mine these deposits. The nodules grow by
very slow concentric accretion in a pelagic ooze.
Plankton is capable of accumulating very high
concentrations of metals from seawater and when the
organisms dissolve, the planktonic ooze becomes very
rich in these metals, which will be precipitated
together with the manganese hydroxides in the
concretions.
Iron and manganese are virtually insoluble in
oxygenated sea water, and consequently cannot be
transported in ordinary solution. However, rivers can
carry a good deal of iron and manganese adsorbed
onto organic particles or as clay-sized iron oxides
which in many cases may produce a red colour.
Iron can be precipitated not only by oxidation, but
also in the reduced state in a basic environment. Thin
carbonate laminations in shales represent a local highpH environment, where reduced iron which is normally soluble can be precipitated:
Fe
2þ
þ CaCO 3 ¼ FeCO 3 þ Ca
2þ
This reaction occurs at relatively low Fe
2þ
=Ca
2þ
ratios because iron carbonate (siderite) is less soluble
than calcite. If there is sulphur present, iron will first
form the sulphide, however, and iron carbonate will
not be stable. Siderite is therefore most typically
formed in freshwater basins, and not directly in the
sulphate-reducing zone in marine sediments. Siderite
can also be formed below the sulphate-reducing zone,
for example by reaction between aragonite and iron in
the sediment.
6 Mudrocks, Shales, Silica Deposits and Evaporites
227