high K
þ
=H
þ Na
þ
=H
þ
ð
Þ activity ratios and with
increasing temperature and K
+
/H
+ activity ratio. We
see that when the porewater is basic (equilibrium with
calcite), the potassium concentration may be lower
before smectite converts to illite. The stability of
smectite is also a function of the activity of silica.
When quartz precipitation starts at about 80
C, the
silica concentration in the porewater is reduced to a
low level corresponding to the solubility of quartz.
Vermiculite has a structure reminiscent of the
smectites, and also undergoes ion exchange and thus
charge deficit in the tetrahedral layer, so that the bonding between each layer is too strong for much swelling
to occur. Vermiculites are mostly trioctahedral,
containing mostly Mg or Fe in the octahedral layer.
Glauconite is a green mineral which forms on the
seabed. It is a potassium- and iron-bearing silicate
somewhat similar to illite and contains both di- and
trivalent iron. It is therefore formed right on the redox
boundary, and during periods with little or no clastic
sedimentation this can result in relatively pure beds of
glauconite.
Kaolinite consists just of a tetrahedral layer and an
octahedral layer and is very stable at low temperatures.
There are no positions in the structure where exchange
can procede easily, which gives kaolinite a much
lower ion exchange capacity than smectite. At higher
temperatures kaolinite becomes unstable and will convert to illite if K-feldspar or other sources of potassium
are available (at 130
C) or pyrophyllite
(Al 2 Si 4 O 10 (OH) 2 ) at higher temperatures.
Kaolinite is part of the kaolin mineral group, which
includes dickite which tends to form at slightly higher
temperatures (100
C).
Chlorite is a mineral which consists of two tetrahedral layers and two octahedral layers, totalling 14 A ˚ .
The octahedral layer is filled with Mg
++ and Fe
++ .
Magnesium-rich chlorites are typical of high temperature metamorphic rocks, while iron-rich ones may
form authigenically in sediments near the seafloor or
at shallow depth.
Chamosite is an iron-rich chlorite mineral which
often forms close to the sediment surface in reducing
conditions. Together with siderite, chamosite is an
important mineral in sedimentary ironstones which,
especially in England, earlier were used as iron ore.
Clay minerals have a number of properties which
distinguish them from most other minerals. Because of
their very large specific surface area they have a great
capacity for adsorbing ions, which is increased by the
fact that clay minerals have negatively charged edges
due to broken bonds. In water with a low electrolyte
content, clay minerals will therefore repel each other.
If cations are added, clay minerals will then accumulate a layer of positive ions (a double layer), and
repulsion between negatively charged clay minerals
declines as the strength of the electrolyte increases.
Van der Waal’s forces will therefore cause flocculation more easily in saltwater, where repulsion due to
the negative charge is reduced. This is why clays
transported by rivers flocculate into larger particles
when they enter seawater and sink more rapidly to
the seafloor.
A colloid solution of clay in water is called a sol,
which can be regarded as a Newtonian fluid.
Flocculated clay is called a gel and has thixotropic
properties. This means that the shear strength
decreases with increasing deformation so that it
changes from a gel to a sol which consists of dispersed
colloidal particles in water (hydrosol). After a time
without deformation, it regains its strength. This is
typical for smectitic clays.
Sediments which increase their volume when they
are deformed, are called dilatant. When the original
packing of the grains is destroyed, the new packing
may be less effective and the volume then increases.
Walking on a beach the deformation of the sand causes
increased porosity so that water is sucked into the sand
at the surface.
Norwegian clays deposited in the sea when the ice
sheet retreated about 10,000 years ago, consist of
crushed rock fragments and have approximately the
same composition as the parent rock. During deposition these clays acquired a markedly porous structure,
with the minerals stacked like a card house. Saltwater
helped to hold this structure together because it
neutralised the negative charges. When the clays are
elevated above the sea by isostatic recovery of the
land, they are exposed to meteoric water. Even if the
clay has a low permeability, freshwater will slowly
seep through it and remove the saline water. This
reduces the strength of the clay’s structure and hence
its stability and the clay becomes “quick”. This is
caused by overpressure and weak effective stress
between the grains and hence little friction. The card
house structure may then collapse, releasing the
3 Sedimentary Geochemistry
103
þ
=H
þ Na
þ
=H
þ
ð
Þ activity ratios and with
increasing temperature and K
+
/H
+ activity ratio. We
see that when the porewater is basic (equilibrium with
calcite), the potassium concentration may be lower
before smectite converts to illite. The stability of
smectite is also a function of the activity of silica.
When quartz precipitation starts at about 80
C, the
silica concentration in the porewater is reduced to a
low level corresponding to the solubility of quartz.
Vermiculite has a structure reminiscent of the
smectites, and also undergoes ion exchange and thus
charge deficit in the tetrahedral layer, so that the bonding between each layer is too strong for much swelling
to occur. Vermiculites are mostly trioctahedral,
containing mostly Mg or Fe in the octahedral layer.
Glauconite is a green mineral which forms on the
seabed. It is a potassium- and iron-bearing silicate
somewhat similar to illite and contains both di- and
trivalent iron. It is therefore formed right on the redox
boundary, and during periods with little or no clastic
sedimentation this can result in relatively pure beds of
glauconite.
Kaolinite consists just of a tetrahedral layer and an
octahedral layer and is very stable at low temperatures.
There are no positions in the structure where exchange
can procede easily, which gives kaolinite a much
lower ion exchange capacity than smectite. At higher
temperatures kaolinite becomes unstable and will convert to illite if K-feldspar or other sources of potassium
are available (at 130
C) or pyrophyllite
(Al 2 Si 4 O 10 (OH) 2 ) at higher temperatures.
Kaolinite is part of the kaolin mineral group, which
includes dickite which tends to form at slightly higher
temperatures (100
C).
Chlorite is a mineral which consists of two tetrahedral layers and two octahedral layers, totalling 14 A ˚ .
The octahedral layer is filled with Mg
++ and Fe
++ .
Magnesium-rich chlorites are typical of high temperature metamorphic rocks, while iron-rich ones may
form authigenically in sediments near the seafloor or
at shallow depth.
Chamosite is an iron-rich chlorite mineral which
often forms close to the sediment surface in reducing
conditions. Together with siderite, chamosite is an
important mineral in sedimentary ironstones which,
especially in England, earlier were used as iron ore.
Clay minerals have a number of properties which
distinguish them from most other minerals. Because of
their very large specific surface area they have a great
capacity for adsorbing ions, which is increased by the
fact that clay minerals have negatively charged edges
due to broken bonds. In water with a low electrolyte
content, clay minerals will therefore repel each other.
If cations are added, clay minerals will then accumulate a layer of positive ions (a double layer), and
repulsion between negatively charged clay minerals
declines as the strength of the electrolyte increases.
Van der Waal’s forces will therefore cause flocculation more easily in saltwater, where repulsion due to
the negative charge is reduced. This is why clays
transported by rivers flocculate into larger particles
when they enter seawater and sink more rapidly to
the seafloor.
A colloid solution of clay in water is called a sol,
which can be regarded as a Newtonian fluid.
Flocculated clay is called a gel and has thixotropic
properties. This means that the shear strength
decreases with increasing deformation so that it
changes from a gel to a sol which consists of dispersed
colloidal particles in water (hydrosol). After a time
without deformation, it regains its strength. This is
typical for smectitic clays.
Sediments which increase their volume when they
are deformed, are called dilatant. When the original
packing of the grains is destroyed, the new packing
may be less effective and the volume then increases.
Walking on a beach the deformation of the sand causes
increased porosity so that water is sucked into the sand
at the surface.
Norwegian clays deposited in the sea when the ice
sheet retreated about 10,000 years ago, consist of
crushed rock fragments and have approximately the
same composition as the parent rock. During deposition these clays acquired a markedly porous structure,
with the minerals stacked like a card house. Saltwater
helped to hold this structure together because it
neutralised the negative charges. When the clays are
elevated above the sea by isostatic recovery of the
land, they are exposed to meteoric water. Even if the
clay has a low permeability, freshwater will slowly
seep through it and remove the saline water. This
reduces the strength of the clay’s structure and hence
its stability and the clay becomes “quick”. This is
caused by overpressure and weak effective stress
between the grains and hence little friction. The card
house structure may then collapse, releasing the
3 Sedimentary Geochemistry
103