510
Climatic Geomorphology
a pH lower than 3, whereas ferrous hydroxide is soluble with a pH of up to 8. Hence, the
value of the redox potential is essential to the mobility of iron. The main factors affecting
Eh in weathering environments are the availability of atmospheric oxygen and the presence
or absence of organic matter (Loughnan, 1969). Oxidation is an exothermic reaction that
takes place above the saturation zone. Below the water table a reducing environment
without oxygen prevails. For this reason, in these environments iron appears in its ferrous
state and is related to the gley horizon, generated in humid areas without any drainage. The
organic matter is a powerful reducing agent due to its rapid oxidation which produces
carbon dioxide. Potassium is a very soluble cation, although it is usually retained in the
structure of clay minerals of the illite type. Nevertheless, under conditions of high
precipitation it is dissolved, without leaving any remains in the system.
Chelation consists in the joining of a cation and a complexing agent (chelate), such as
the EDTA. As a result an organometallic compound is generated, bearing a ring-shaped
structure with the cation inside (Bland and Rolls, 1998). The cations fixed by these
chelates can be divalent or trivalent and the organometallic complexes are quite soluble. In
nature they are produced by the generation of humic and fulvic acids associated by the
decomposition of plants and root secretion (Keller, 1957).
Ion exchange is the most important process in chemical weathering (Carroll, 1970).
It consists of the reaction between the ions of a solution and the ones existing in minerals.
Clay minerals are the main agent responsible for the exchange capacity in rock weathering.
If the mineral is not electrically neutral, a cation adheres to the clay surface, neutralizing
the negative charge. A frequent replacement is Si 4+ by A13+. Each clay mineral has a
different ion exchange capacity and minerals of the montmorillonite group reach the
highest values.
Hydration is the addition of water molecules to a mineral structure. When hydrated,
iron oxides transform into iron oxihydroxides. Water is easily introduced into the
crystalline structure of expansive clays. The transformation of anhydrite (CaSO4) into
gypsum (CaSO4.2H20) also involves a considerable increase in volume.
The most important factor controlling the disintegration speed of crystalline structures
and resulting products is the quantity of solution water. This water removes the soluble
cations resulting from hydrolysis processes. The quantity of dissolution depends on the
water volume in contact with the mineral surface and on its solubility. When precipitation is important and takes place over long periods, the most resistant minerals,
such as quartz, can disappear by dissolution. The Weipa bauxites of North Queensland,
Australia derive from the leaching of a substratum constituted by kaolinitic sandstones
with 90% quartz and 10% kaolinite. Monsoonal precipitation give rise to bauxites with
5% quartz and small quantities of kaolinite (Loughnan and Bayliss, 1961).
Several investigations have been made for determining the order by which the most
common cations are lost. Polynov (1937) compared the composition percentage of the
solved matter in rivers, for drainage basins on igneous rocks with the composition of
the latter, obtaining the following losing order: Ca 2+ > Na + > Mg 2+ > K + > SiO2 >
Fe203 > A1203. Other studies indicate that this order can vary depending on the rock type
(Reiche, 1950; Carroll, 1970).
Crystalline structure and atomic bond strengths play an important role in mineral
weathering. During magmatic crystallization the minerals that first form are the ones with
the highest melting point and the largest number of weaker ionic bonds. The sequence is
Climatic Geomorphology
a pH lower than 3, whereas ferrous hydroxide is soluble with a pH of up to 8. Hence, the
value of the redox potential is essential to the mobility of iron. The main factors affecting
Eh in weathering environments are the availability of atmospheric oxygen and the presence
or absence of organic matter (Loughnan, 1969). Oxidation is an exothermic reaction that
takes place above the saturation zone. Below the water table a reducing environment
without oxygen prevails. For this reason, in these environments iron appears in its ferrous
state and is related to the gley horizon, generated in humid areas without any drainage. The
organic matter is a powerful reducing agent due to its rapid oxidation which produces
carbon dioxide. Potassium is a very soluble cation, although it is usually retained in the
structure of clay minerals of the illite type. Nevertheless, under conditions of high
precipitation it is dissolved, without leaving any remains in the system.
Chelation consists in the joining of a cation and a complexing agent (chelate), such as
the EDTA. As a result an organometallic compound is generated, bearing a ring-shaped
structure with the cation inside (Bland and Rolls, 1998). The cations fixed by these
chelates can be divalent or trivalent and the organometallic complexes are quite soluble. In
nature they are produced by the generation of humic and fulvic acids associated by the
decomposition of plants and root secretion (Keller, 1957).
Ion exchange is the most important process in chemical weathering (Carroll, 1970).
It consists of the reaction between the ions of a solution and the ones existing in minerals.
Clay minerals are the main agent responsible for the exchange capacity in rock weathering.
If the mineral is not electrically neutral, a cation adheres to the clay surface, neutralizing
the negative charge. A frequent replacement is Si 4+ by A13+. Each clay mineral has a
different ion exchange capacity and minerals of the montmorillonite group reach the
highest values.
Hydration is the addition of water molecules to a mineral structure. When hydrated,
iron oxides transform into iron oxihydroxides. Water is easily introduced into the
crystalline structure of expansive clays. The transformation of anhydrite (CaSO4) into
gypsum (CaSO4.2H20) also involves a considerable increase in volume.
The most important factor controlling the disintegration speed of crystalline structures
and resulting products is the quantity of solution water. This water removes the soluble
cations resulting from hydrolysis processes. The quantity of dissolution depends on the
water volume in contact with the mineral surface and on its solubility. When precipitation is important and takes place over long periods, the most resistant minerals,
such as quartz, can disappear by dissolution. The Weipa bauxites of North Queensland,
Australia derive from the leaching of a substratum constituted by kaolinitic sandstones
with 90% quartz and 10% kaolinite. Monsoonal precipitation give rise to bauxites with
5% quartz and small quantities of kaolinite (Loughnan and Bayliss, 1961).
Several investigations have been made for determining the order by which the most
common cations are lost. Polynov (1937) compared the composition percentage of the
solved matter in rivers, for drainage basins on igneous rocks with the composition of
the latter, obtaining the following losing order: Ca 2+ > Na + > Mg 2+ > K + > SiO2 >
Fe203 > A1203. Other studies indicate that this order can vary depending on the rock type
(Reiche, 1950; Carroll, 1970).
Crystalline structure and atomic bond strengths play an important role in mineral
weathering. During magmatic crystallization the minerals that first form are the ones with
the highest melting point and the largest number of weaker ionic bonds. The sequence is