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Further Reading:
Birkeland P.W. 1984. Soils and Geomorphology. New York, Oxford University Press.
Nahon D.B., 1991. Introduction to the Petrology of Soils and Chemical Weathering. John Wiley & Sons, Inc., New York.
Robinson D.A., and Williams R.B.G., 1994. Rock Weathering and Landform Evolution. John Wiley & Sons, Inc., New York.
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6) Silicates
Weathering in general refers to a group of processes by which surface rock disintegrates into smaller particles or dissolve into water due to the impact of the atmosphere and hydrosphere. The weathering processes often are slow (hundred to thousands of years). The amount of time that rocks and minerals have been exposed at the earth's surface will influence the degree to which they have weathered.
Weathering processes are divided into three categories:
physical weathering
chemical weathering
biological weathering
Primary minerals and rocks are splitted in fragments due to physical weathering. This leads to environmental conditions (e.g. a higher surface area) that favor chemical weathering. There are several forms of physical weathering:
Abrasion: Water carrying
suspended rock fragments has a scouring action on surfaces. Examples
are the grinding action of glaciers, gravel, pebbles and boulders
moved along and constantly abraded by fast-flowing streams. Particles
carried by wind also have a 'sand-blasting effect'.
Wetting and drying: Water
penetrates into rocks and reacts with their constituent minerals.
Freezing and thawing: When
water is trapped in the rock (or in cracks) repeatedly freezing and
thawing results in forces of expansion and contraction (when water
freezes, the increase in its volume is about 9 %).
Thermal expansion and
contraction of minerals: Rocks are composed of different kind of
minerals. When heated up by solar radiation each different mineral
will expand and contract a different amount at a different rate with
surface-temperature fluctuations. With time, the stresses produced
are sufficient to weaken the bonds along grain boundaries, and thus
flaking of fragments.
For instance, the difference in temperature in desert environments or mountain regions may range from 30 - 50 degrees C between day and night. Rocks are heated and cooled from the outside by change in solar radiation, which results in high temperature gradients inside and outside of the rocks (the heat conductivity of rocks is very low).
Pressure unloading or
pressure-release jointing: There is a reduction in pressure on a
rock due to removal of overlying material. This allows rocks to split
along planes of weakness, called joints.
Crystallization: In arid
environments, water evaporates at the surface of rocks and crystals
form from dissolved minerals. Over time, the crystals grow (They
expand their volume) and exert a force great enough to separate
mineral grains and break up rocks.
Action of organisms: They
aid in the physical disintegration of rocks.
Plant roots: They aid in the
physical disintegration of rocks. Pressures exerted by roots during
growth are able to rupture rocks.
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The difference between physical and chemical weathering is that with the latter one the mineral composition of the mineral or rock is changed. The larger the surface area, i.e, the smaller the fragments, the better for chemical weathering. Water is the dominant agent because it initiates chemical weathering. In the following there is a brief description of the most important chemical weathering processes:
Hydration: Ions have the
tendency to hydrate when H2O is
present and dissociate. This kind of weathering happens in arid
environments where salts are present. For example, chlorides and
sulfates weather due to hydration. In general, ions with the same
charge but smaller ion radius have a larger layer of H2O ions and therefore do not tend to adsorb
tight. The small Li+ ion tends to
remain hydrated at the surface, whereas the large Al3+ ion tends to dehydrate and become tightly
adsorbed. The strength of adsorption increases in the following
sequence:
Li+x
x Na+ xx K+ xx Mg2+ xx Ca2+ x xAl3+
Hydrolysis: Water molecules
at the mineral surface dissociate into H+ and OH-
and the mobile H+ ions (actually
H3O+) penetrate the crystal lattice, creating a
charge imbalance, which causes cations such as Ca2+, Mg2+,
K+ and Na+ to diffuse out. For example, the feldspar
orthoclase hydrolyses to produce a weak acid (silicic acid), a strong
base (KOH), and leaves a residue of clay mineral illite, which is a
secondary mineral:
3KAl4 + Si3O8 + 14H2O <- -> K(AlSi3)4Al24O10(OH)2 + 6Si(OH)4 + 2KOH
In hydrolysis reactions it has to be taken into account the important role played by dissolved CO2. This is shown in the hydrolysis of Mg-olivine:
Mg2SiO4 + 4CO2 + 4H2O <- -> 2Mg2+ + 4HCO3- + H4SiO4
This reaction uses an acid (carbonic acid - H2CO3)and therefore the solution becomes increasingly alkaline during completion of hydrolysis reactions.
Oxidation-Reduction: Several
primary minerals contain Fe2+ and
Mn2+. If there are oxidizing
environmental conditions the Fe2+
is oxidized to Fe3+ (precipitates
as an insoluble oxyhydroxide, usually either ferrihydrite or the
stable mineral goethite) and Mn2+
to Mn3+ or Mn4+ partly inside the minerals, which results
in a positive charge and the mineral becomes unstable. This charge
imbalance is neutralized by a loss of some oxidized iron and
manganese ions and/or some cations dissociate from the mineral. The
precipitate may form a coating over the mineral surface, which slows
down the subsequent rate of hydrolysis. Note that the oxidation of
Fe2+ to Fe3+ according to:
Fe2+ + 2H2O + 1/2O2 < - -> Fe(OH)3 + H+
is an acidifying reaction (acid solution weathering). The H+ ions produced by this reaction will generally accelerate the rate of hydrolysis.
Complexation: Metals
released from primary minerals such as Fe, Mn, and Al, build
complexes with organic components, such as fulvic acids and humic
acids, which are very stable. Important referring to chemical
weathering is the loss of the cations out of the active system,
therefore causing an imbalance between cations and anions.
Figure 5.2.1. Chemical weathering processes.
Summary
Weathering of primary minerals produce secondary minerals. Elements released from primary minerals are prone to leaching if they do not form complexes. The area of weathering is depleted first by Na+, Ca2+, and Mg2+.
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Lichens play an important part in weathering, because they are rich in chelating agents, which trap the elements of the decomposing rock in organo-metallic complexes. Some of the lichens being epilithic (i.e. living on the rock surface), some endolithic (actively boring into the rock surface), and some chasmolithic (living in hollows or fissures within the rock). Evidence for the operation of these processes comes mainly from detailed microscopic and microchemical analyses of the lichen : rock interface. The mechanisms and results of their actions is summarized in Table 5.3.1.
Table 5.3.1. Lichen weathering mechanisms and forms produced (Robinson et al., 1994).
|
Mechanisms |
Results of weathering |
|
Chemical mechanisms
|
Results of chemical action
|
|
Physical mechanisms
|
Results of physical action
|
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The resistance to weathering, i.e. the mineral stability of parent material depends on:
Types of mineral present
Surface area of rock exposed
Porosity of rocks
Weathering is not only dependent on the mineral composition but also on the porosity of the rock. Rocks consisting of coarse fragments (e.g. granite) easily weather physically but do not weather chemically fast. In contrast, rocks consisting of fine fragments (e.g. basalt) chemical weathering is higher than physical weathering. The weathering of stratified sedimentary rocks is dependent on the orientation of the stratification and the cementation.
In general, the resistance of a primary mineral to weathering increases with the degree of sharing of oxygens between adjacent Si tetrahedra in the crystal lattice. The SI-O bond has the highest energy of formation, followed by the Al-O bond, and the even weaker bonds formed between O and the metal cations (e.g. Na+, Ca2+). In Figure 5.4.1 the ranking of some primary minerals in order of increasing stability is shown. Olivine weathers rapidly because the silicon tetrahedra are only held together by O-metal cations. In contrast quartz is very resistant because it consits entirely of linked silicon tetrahedra. In the chain (amphiboles and pyroxenes) and sheet (phyllosilicates) structures, the weakest points are the O-metal cation structures. Isomosphous substitution of Al3+ for Si4+ also contributes to instability because the proportion of Al-O to Si-O bonds increases and more O-metal cations bonds are necessary. This accounts for the decrease in stability of the calcium feldspars when compared with the sodium and potassium feldspars.
weak stabilityxxOlivine, Ca2+-Plagioclasex
xxxxxxxxxxxxxxxxxxxxxxPyroxenex
xxxxxxxxxxxxxxxxxxxxxxxxxxxxAmphibolex
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxNa+-Plagioclasex
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxK+-Plagioclasex
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxMica
(Muscovite)x
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxQuartzx x x
xhigh stability
Figure 5.4.1. Stability of some primary minerals.
The rate of weathering is influenced by:
Temperature
Rate of water percolation
Oxidation status of the weathering
zone
Weathering is dependent on the climate, i.e., the temperature and the mean annual precipitation rates resulting in different soil moisture contents. The mean lifetime of one millimeter of different rocks into a kaolinitic saprolite is shown in Table 5.4.1. These numbers exhibit that in cold, temperate, or tropical humid zones, the climate (temperature and precipitation) controls the rate of weathering.
Table 5.4.1. Mean lifetime of one millimeter of fresh rock (Nahon, 1991).
|
Rock Type |
Climate |
Lifetime (years) |
|
Acid rocks |
tropical semi-arid tropical humid temperate humid cold humid |
65 to 200 20 to 70 41 to 250 35 |
|
Metamorphic rocks |
temperate humid |
33 |
|
Basic rocks |
temperate humid tropical humid |
68 40 |
The oxidation status influences the degree of chemical weathering processes. An oxidizing environment favors the oxidation of ions such as Fe2+ and Mn2+. Water is the agent forcing the processes of hydration and hydrolysis. High water contents mean also reducing (anaerobic) environmental conditions, which decrease the rate of oxidation.
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