Physical Weathering
Reactions at the Earth's surface tend to make large particles smaller. A decrease in particle size increases the total surface area of the particles. Consider a cube of rock that is 1" on a side. The total surface area is 6 square inches. If this cube is broken down into 8 cubes each 0.5 " on a side the total surface area increases to 12 square inches (confirm!). Continued reduction in size leads to an increasing total surface area. This is important because chemical reactions generally begin at the surface of an object. The greater the surface area the greater the reactivity of the particles.
A number of process are capable of reducing particle size. A particularly effective process involves the repeated freezing and thawing of water in cracks in rocks. When water freezes there is a 9% increase in volume (recall that water has a higher density than ice so that the ice will occupy a larger volume than the water it formed from). The expansion exerts a force on the sizes of the crack which may cause the crack to widen. Repeated freezing and thawing can reduce a large exposure to millions of individual particles. In mountainous regions these particles may accumulate at the bottom of steep slopes forming
talus
piles
When erosion exposes an igneous body at the surface the pressure acting on the rock is reduced considerably. The rock tends to expand by the development of
joints
(breaks in the rock) roughly parallel to the surface of the rock body. Eventually large sheets of rock may "slide" off the rock creating a rounded upper surface. These
exfoliation
domes are quite common.
The loose material released by physical weathering is termed
sediment
. The size of sediment is described using the following terms:
gravel - > 2 mm in diameter
sand - 1/16 mm to 2 mm in diameter
silt - 1/256 mm to 1/16 mm in diameter
clay - < 1/256 mm in diameter
When someone describes sediment as sand note that this describes the size of the particles but says nothing about the composition of the particles.
Chemical Weathering
The reactive components in the Earth's atmosphere include:
Oxygen
Carbon Dioxide - CO
Water - H
Nitrogen, the most abundant consistent in the atmosphere has little effect on the chemical weathering of rocks and minerals.
Oxidation
involves the loss of electrons. Oxygen is a particularly good agent of oxidation but it is not unique. Recall that oxygen has an affinity for two electrons which will result in a filled s and p set of orbitals. This gives the oxygen anion a net negative charge of -2. Iron (Fe) has a total of 26 electrons. Some of its valence electrons can be removed by an oxidizing agent. Two common cations of iron are Fe 3+ (ferric) and Fe 2+ (ferrous) which have lost 3 and 2 electrons respectively. These cations are very common coloration (pigmentation) agents and a small quantity of ferric iron produces a red color whereas ferrous iron produces a green color. A mixture of ferric and ferrous iron imparts a buff (yellow) color to the material.
Another common pigmentation agent is organic matter which produces a black color. Oxidation is enhanced when sediment is close to the atmosphere. The oxygen content of shallow moving water is higher than that of deeper, quiet water. In the later case, material may not be oxidized.
Reduction
is the "opposite" of oxidation and involves a gain of electrons. If an electron is added to Fe3+, Fe2+ is produced; that is, ferric iron has been reduced to ferrous iron. Organic matter is easily oxidized. A reducing environment is required to preserve organic matter in sediments. Coal and hydrocarbons (gas and oil) are derived from organic matter.
Acid/Base
The acidity of a solution is measured by the amount of the hydrogen ion (H+) in the solution. At 25
C water dissociates into H+ and (OH-). Note that there is one H+ and one (OH-) for every one H
O. At this temperature there are 10
-7
units of H+ and 10
-7
units of (OH-) at equilibrium. Rather than deal with such a small number, chemists take the negative logarithm of the concentration of the H+ content as the
pH
of the solution. Therefore, at 25
C a neutral solution has a pH of 7. If the pH is less than 7 the solution is an acid and if it is greater than 7 is a base. At the surface of the Earth natural solutions range from about 9 to 4:
pH = 8 : basic water - ocean water
pH = 6.5: rain water
pH = 5: acid water - stream draining a coal mine
Carbonate minerals (calcite and dolomite) are soluble in acid solutions whereas quartz is soluble in more basic solutions. Thus, carbonates will begin to dissolve in rain water whereas quartz will not.
Water and Carbon Dioxide react to form carbonic acid H
CO
which dissociates to form H+ and HCO
. This is the source of the H+ in rain water, for example. Caves are underground openings usually created in carbonate rocks which have interacted with slightly acidic waters.
Water is a remarkable compound. A good picture of a water molecule is given by an ellipse with a positive charge at one end and a negative charge at the other. Many material are soluble in water and the "polar" nature of the water molecule plays an important role. Negatively charged ions are surrounded by the positive end of the water molecules and vice versa.
Chemical Alteration of Minerals
Sam Goldick (a TAMU geology professor) noted that the minerals at the high temperature end of Bowen's Reaction Series are more reactive at the surface than those minerals which crystallize out of a melt at lower temperatures. In general, olivine breaks down before pyroxene and plagioclase breaks down before alkali feldspar. For all practical purposes quartz does not react chemically at the surface of the Earth. (The solubility of quartz in surface water is a few parts per million). The rest of the common rock forming minerals will eventually produce
clay minerals
or a mixture of various hydrated oxides - including limonite ("rust"). The formation of clays is an important part of the soil forming processes. There are different types of clay minerals (all are phyllosilicates/sheet silicates) and starting material, climate, and time have a great influence on what type of clay mineral forms.
Kaolinite - an aluminum silicate with (OH-)
Expandable clays
Illite
Kaolinite forms when the climate is warm to tropical and when there is sufficient time for the reactions to go to completion. All of the major cations except Al and Si must be leached from the parent material to form kaolinite. If the area is geologically stable the probability that there is sufficient time is increased. When the environment is oxidizing these reactions are accelerated with the result that red soils (ferric iron) usually contains kaolinite. Kaolinite is an excellent starting material for making ceramics and is also used in making Kaopectate.
Expandable clays can form from a variety of different parent materials including volcanic glass. These clays can swell in the presence of water (or other polar liquids) and shrink in the absence of water. They pose a real challenge to engineers and home owners.
Illite is similar to the mineral Muscovite except that some of the Potassium (K) has been leached from the parent material. Illite thus carries a + charge (partial loss of K+). Illite often forms by the alteration of feldspars in temperate climates when there is not sufficient time to completely remove the potassium.
It should be noted that clay minerals are not necessarily clay sized. However, it is true that clay minerals usually are in the fine silt to clay sized range.
Erosion
, a future topic, involves weathering plus transportation of the particles both as solids (clastic) and as ions in solution (chemical).
Soils
Loose rock material at the surface of the Earth is referred to as
regolith
. Much of the unconsolidated material returned from the moon's surface is referred to as the
lunar regolith
. Regolith becomes soil when a surface layer that can support plant life has developed due to a combination of climatic and weathering factors.
A long time ago I read that candy bar companies used to put large quantities of expandable clay in their products. Why do you think they might do that?
_________________________________________________________________________________________________
jbutler@uh.edu
ClassListserv
Textbook Home Page
Glossary of Geologic Terms
Search These Pages
Other Courses
Resources
Grade Book
_________________________________________________________________________________________________
Copyright by John C. Butler, July 29, 1995
Return