Geology Reference
In-Depth Information
is known as
salt weathering
(Wellman and Wilson
1965). When salt crystals formed within pores are heated,
or saturated with water, they expand and exert pressure
against the confining pore walls; this produces ther-
mal stress or hydration stress respectively, both of which
contribute to salt weathering.
water molecules on their edges and surfaces, or, for simple
salts, in their crystal lattices, without otherwise changing
the chemical composition of the original material. For
instance, if water is added to anhydrite, which is calcium
sulphate (CaSO
4
), gypsum (CaSO
4
.2H
2
O) is produced.
The water in the crystal lattice leads to an increase of
volume, which may cause hydration folding in gyp-
sum sandwiched between other beds. Under humid
mid-latitude climates, brownish to yellowish soil colours
are caused by the hydration of the reddish iron oxide
hematite to rust-coloured goethite. The taking up of
water by clay particles is also a form of hydration. It leads
to the clay's swelling when wet. Hydration assists other
weathering processes by placing water molecules deep
inside crystal structures.
Chemical weathering
Weathering involves a huge number of chemical reac-
tions acting together upon many different types of
rock under the full gamut of climatic conditions. Six
main chemical reactions are engaged in rock decom-
position: solution, hydration, oxidation and reduction,
carbonation, and hydrolysis.
Solution
Oxidation and reduction
Mineral salts may dissolve in water, which is a very
effective solvent. The process, which is called
solu-
tion
or
dissolution
, involves the dissociation of the
molecules into their anions and cations and each
ion becomes surrounded by water. It is a mechanical
rather than a chemical process, but is normally dis-
cussed with chemical weathering as it occurs in partner-
ship with other chemical weathering processes. Solution
is readily reversed - when the solution becomes saturated
some of the dissolved material precipitates. The satura-
tion level is defined by the equilibrium solubility, that is,
the amount of a substance that can dissolve in water.
It is expressed as parts per million (ppm) by volume
or milligrams per litre (mg/l). Once a solution is satu-
rated, no more of the substance can dissolve. Minerals
vary in their solubility. The most soluble natural miner-
als are chlorides of the alkali metals: rock salt or halite
(NaCl) and potash salt (KCl). These are found only in
very arid climates. Gypsum (CaSO
4
.2H
2
O) is also fairly
soluble, as is limestone. Quartz has a very low solubility.
The solubility of many minerals depends upon the num-
ber of free hydrogen ions in the water, which may be
measured as the pH value (Box 3.2).
Oxidation occurs when an atom or an ion loses an
electron, increasing its positive charge or decreasing
its negative charge. It involves oxygen combining with
a substance. Oxygen dissolved in water is a preva-
lent oxidizing agent in the environment.
Oxidation
weathering
chiefly affects minerals containing iron,
though such elements as manganese, sulphur, and tita-
nium may also be oxidized. The reaction for iron,
which occurs mainly when oxygen dissolved in water
comes
into
contact
with
iron-containing
minerals,
is written:
4Fe
2
+
+
]
Alternatively, the ferrous iron, Fe
2
+
, which occurs in
most rock-forming minerals, may be converted to its
ferric form, Fe
3
+
, upsetting the neutral charge of the
crystal lattice, sometimes causing it to collapse and
making the mineral more prone to chemical attack.
If soil or rock becomes saturated with stagnant water,
it becomes oxygen-deficient and, with the aid of
anaero-
bic bacteria
, reduction occurs.
Reduction
is the opposite
of oxidation, and the changes it promotes are called
gleying. In colour, gley soil horizons are commonly a
shade of grey.
The propensity for oxidation or reduction to occur
is shown by the redox potential, Eh. This is measured
in units of millivolts (mV), positive values registering
3O
2
+
2
e
→
2Fe
2
O
3
[
e
=
electron
Hydration
Hydration
is transitional between chemical and
mechanical weathering. It occurs when minerals absorb
