Geoscience Reference
In-Depth Information
occurring in aerobic or anaerobic environments and
changing valency in a positive or negative direction
respectively. Oxidation promotes weathering in mafic
minerals, changing ferrous iron oxide (FeO) to its ferric
form (Fe
2
O
3
), destabilizing the crystal lattice and requiring
the compensating loss of another cation. Finally, chemical
weathering is also enhanced by certain biological
processes. Decomposition inflates soil concentrations of
CO
2
, potentially increasing solution rates, and secretes
biochemical
chelating agents
which aid the solution of
otherwise stable cations.
The chief products of chemical weathering are stable
quartz - the principal ingredient of siliciclastic sediments
- clay minerals and dissolved products. Clay minerals are
capable of further weathering which enhances silica
removal, particularly in warm, humid environments.
Residual or redeposited minerals create economically
viable sources of alumina (
bauxite
) and iron (
laterite
), etc.
Gibbsite
and to a lesser extent
kaolinite
, found in Wales and
Cornwall, are assumed to be relic Tertiary humid-tropical
weathering clay minerals.
soluble in very acid (pH
4·0) or alkaline (pH
9·0)
water.
Solution potential is enhanced through carbonation -
the incorporation of dissolved atmospheric CO
2
in water
during or after precipitation - forming dilute carbonic
acid, H
2
CO
3
. This stage in the process provides a rare
instance of a chemical reaction rate inversely proportional
to temperature. Other dilute acids may form in association
with atmospheric constituents, including sulphuric acid
(H
2
SO
4
) as
acid rain
. Carbonation is enhanced by passing
through soil rich in biogenic carbon dioxide and is a
common form of chemical weathering, particularly of
limestone. When hydration equilibrium between H
2
O
and CO
2
has been reached, the carbonic acid is dissociated
to HCO
3
-
and H
+
ions. CaCO
3
(calcium carbonate), in
contact with the solution, is dissociated into Ca
++
and
CO
3
--
ions, combining with H
+
to form more HCO
3
-
ions.
This complex, multiphase reaction is often simplified to
the form:
CaCO
3
+ CO
2
+ H
2
O qCa(HCO
3
)
2
with calcium bicarbonate removed in solution. Each
process is reversible, and reprecipitation of calcium
carbonate may occur eventually as
tufa
or a more resistant
crystalline form (
travertine
). The carbonation of ortho-
clase feldspar creates a new clay mineral species,
illite
,
produces potassium and bicarbonate ions and removes
dissolved silicate:
Weathering landforms
Landforms of physical and chemical weathering range
from residual, less weathered or even non-weathered
bedrock to deep
in situ
weathered debris sheets. Survival
depends on the balance between weathering and removal
fine-grained or amorphous chemical weathering product
with no direct physical equivalent. General rock debris or
regolith
is non-specific, but
felsenmeer
or
blockfield
,
found on low-angled surfaces, is more closely associated
granite tors are often relics of intense weathering in late
Tertiary tropical climates, exhumed as climates changed
during the Quaternary (
Plate 13.9
).
The survival of thick
weathered profiles and core stones below glacigenic
sediments indicates that glacial erosion may not remove
all weathered products right down to the weathering front
or
etch front
.
On a grand scale, calcareous (limestone) solution-
carbonation weathering is so distinctive as to justify its
own subdiscipline of karst geomorphology. Limestone
regions provide a specific weathering case study. They are
not uniquely susceptible to solution but, given the right
conditions, develop a unique
karst
landsystem dominated
by the solution of carbonate rocks and progressive
development of underground drainage. Carbonate rocks
of clastic, biogenic and inorganic precipitate origins cover
15 per cent of continental land surfaces, in all climates.
6KAlSi
3
O
8
+ 4H
2
O + 4CO
2
qK
2
AL
4
(Si
6
Al
2
O
20
)
(OH
4
) + 12SiO
2
+ 4K
+
+ 4HCO
3
-
Hydrolysis
involves H
+
and OH
-
reactions with minerals
and is important, for example, in the decomposition of
granite containing plagioclase feldspar, with the clay
mineral
kaolinite
the principal product:
4NaAlSi
3
O
8
+ 6H
2
O
3
qAl
4
Si
4
O
10
(OH
8
) + 8SiO
2
+
4Na
+
+ 4OH
-
Hydration
occurs when minerals absorb water into their
crystal lattice and establish tensile stress in addition to
chemical alteration. Hydration of iron oxides to the form
limonite
, an important weathering process in mafic rocks,
illustrates the latter and is reversible:
2Fe
2
O
3
+ 3H
2
O
2Fe
2
O
3
.3H
2
O
Two sets of
redox
reactions involve oxygen either
by combination (
oxidation
) or removal (
reduction
),
