Rainwater will cause some dissolution of minerals
and, during the periods of drying, the silica concentration may be higher so that smectite becomes stable.
As a result we will usually get illite or smectite
formed as authigenic minerals because they are stable
in the presence of high K
+
/H
+ ratios in the porewater.
Smectite (montmorillonite) is thus a common clay
mineral in desert areas, and its ability to swell when
wet renders sediments very plastic during floods. This
expansion of smectite also lowers its permeability and
may be the reason why water can flow over the surface
for a long time before sinking into the ground. In
addition, capillary forces will prevent rapid percolation of water through dry soil. On the ocean floor near
desert regions we find that illite and smectite are
typical minerals, brought there by aeolian transport.
In tropical areas where precipitation is relatively
high, the rate of weathering will be very rapid. This is
not only because weathering processes accelerate with
temperature, but also because vegetation produces
large amounts of organic acids (humic acids) which
are very effective in breaking down silicate minerals.
Microbiological organisms such as fungi and bacteria
also help in the breakdown process by producing CO 2 .
Gibbsite (A1 2 O 3 •3H 2 O) and iron oxides (haematite, goethite, Fe 2 O 3 •3H 2 O) are constituents of the
laterite which we find only in tropical areas with
rapid weathering and slow erosion. Whereas iron
oxides are also found at higher latitudes, gibbsite
occurs almost exclusively in humid tropical areas.
Laterisation is a very slow process and takes
millions of years, even in tropical regions with rapid
weathering. It is therefore primarily in tropical areas
that we find bauxite for the aluminum industry. Ironrich laterites may have an iron content of over 50%,
and in some areas (e.g. India) have been exploited as
iron ore. Laterite forms a very hard cement-like crust
over the weathering profile and is also virtually devoid
of nutrients, so crop cultivation is impossible. In East
Africa (especially Uganda), however, erosion has
incised through a layer of Tertiary laterites. While
the laterite cover remains on flat elevated surfaces,
fresher, more fertile, rocks and weathering material
are exposed in the valley sides. The vegetation in
some tropical areas is more abundant, even if the
soils are very poor in nutrients, because the vegetation
recycles those nutrients which are available. If the
vegetation is removed and organic material is no longer produced, oxidation and the absence of humic
acids (increased pH) will lead to precipitation of
oxides and hydroxides which make the soil hard and
uncultivable.
Volcanic ash consisting of glass and unstable volcanic mineral assemblages may alter to smectite on
land or on the seafloor. In deep sea sediments zeolites
like phillipsite are common.
Areas with volcanic rocks, particularly amorphous
material (volcanic glass), will often form zeolites.
These require a high concentration of both silica and
alkaline ions in the water, which is the situation when
glass dissolves. Zeolites, particularly phillipsite, are
formed authigenically on the Pacific Ocean bed and
are also found in lakes (e.g. in East Africa).
In summary we can say that the factors which
determine which types of clay minerals are “produced” in the various areas are:
1. The rocks which are eroded/weathered (source
rocks)
2. Rate of erosion
3. Temperature
4. Precipitation
5. Vegetation
6. Permeability of source rocks and sediments (percolation of water).
Typical distribution of various minerals:
1. Chlorite and biotite – high latitudes (cold climate) –
rapid erosion.
2. Kaolinite – humid temperate and humid tropical
regions – good drainage.
3. Smectite (montmorillonite) – low precipitation or
poor drainage. Typical of desert environments, but
also formed in impervious, e.g. basaltic, rocks in
more humid environments. Typically formed from
volcanic rocks.
4. Gibbsite – tropical humid climate – long
weathering period.
5. Zeolites – formed in areas with volcanic material
and restricted porewater circulation. Require a high
concentration of silica and alkali ions.
3.8
Geochemical Processes in the
Ocean
The ocean can be regarded as a reservoir of chemicals
dissolved in water. It looks as though the composition
of seawater has not altered radically throughout the
geological ages from the early Palaeozoic until the
3 Sedimentary Geochemistry
111
and, during the periods of drying, the silica concentration may be higher so that smectite becomes stable.
As a result we will usually get illite or smectite
formed as authigenic minerals because they are stable
in the presence of high K
+
/H
+ ratios in the porewater.
Smectite (montmorillonite) is thus a common clay
mineral in desert areas, and its ability to swell when
wet renders sediments very plastic during floods. This
expansion of smectite also lowers its permeability and
may be the reason why water can flow over the surface
for a long time before sinking into the ground. In
addition, capillary forces will prevent rapid percolation of water through dry soil. On the ocean floor near
desert regions we find that illite and smectite are
typical minerals, brought there by aeolian transport.
In tropical areas where precipitation is relatively
high, the rate of weathering will be very rapid. This is
not only because weathering processes accelerate with
temperature, but also because vegetation produces
large amounts of organic acids (humic acids) which
are very effective in breaking down silicate minerals.
Microbiological organisms such as fungi and bacteria
also help in the breakdown process by producing CO 2 .
Gibbsite (A1 2 O 3 •3H 2 O) and iron oxides (haematite, goethite, Fe 2 O 3 •3H 2 O) are constituents of the
laterite which we find only in tropical areas with
rapid weathering and slow erosion. Whereas iron
oxides are also found at higher latitudes, gibbsite
occurs almost exclusively in humid tropical areas.
Laterisation is a very slow process and takes
millions of years, even in tropical regions with rapid
weathering. It is therefore primarily in tropical areas
that we find bauxite for the aluminum industry. Ironrich laterites may have an iron content of over 50%,
and in some areas (e.g. India) have been exploited as
iron ore. Laterite forms a very hard cement-like crust
over the weathering profile and is also virtually devoid
of nutrients, so crop cultivation is impossible. In East
Africa (especially Uganda), however, erosion has
incised through a layer of Tertiary laterites. While
the laterite cover remains on flat elevated surfaces,
fresher, more fertile, rocks and weathering material
are exposed in the valley sides. The vegetation in
some tropical areas is more abundant, even if the
soils are very poor in nutrients, because the vegetation
recycles those nutrients which are available. If the
vegetation is removed and organic material is no longer produced, oxidation and the absence of humic
acids (increased pH) will lead to precipitation of
oxides and hydroxides which make the soil hard and
uncultivable.
Volcanic ash consisting of glass and unstable volcanic mineral assemblages may alter to smectite on
land or on the seafloor. In deep sea sediments zeolites
like phillipsite are common.
Areas with volcanic rocks, particularly amorphous
material (volcanic glass), will often form zeolites.
These require a high concentration of both silica and
alkaline ions in the water, which is the situation when
glass dissolves. Zeolites, particularly phillipsite, are
formed authigenically on the Pacific Ocean bed and
are also found in lakes (e.g. in East Africa).
In summary we can say that the factors which
determine which types of clay minerals are “produced” in the various areas are:
1. The rocks which are eroded/weathered (source
rocks)
2. Rate of erosion
3. Temperature
4. Precipitation
5. Vegetation
6. Permeability of source rocks and sediments (percolation of water).
Typical distribution of various minerals:
1. Chlorite and biotite – high latitudes (cold climate) –
rapid erosion.
2. Kaolinite – humid temperate and humid tropical
regions – good drainage.
3. Smectite (montmorillonite) – low precipitation or
poor drainage. Typical of desert environments, but
also formed in impervious, e.g. basaltic, rocks in
more humid environments. Typically formed from
volcanic rocks.
4. Gibbsite – tropical humid climate – long
weathering period.
5. Zeolites – formed in areas with volcanic material
and restricted porewater circulation. Require a high
concentration of silica and alkali ions.
3.8
Geochemical Processes in the
Ocean
The ocean can be regarded as a reservoir of chemicals
dissolved in water. It looks as though the composition
of seawater has not altered radically throughout the
geological ages from the early Palaeozoic until the
3 Sedimentary Geochemistry
111