Aqueous Environmental Geochemistry. Lindsay, W. Chemical Equilibria in Soils. Caldwell, NJ: Blackburn Press, pp. Millot, G. Geologie des Argiles, Alterations, Sedimentologie, Geochimie.
Paris: Masson, pp. Niederbudde, E. Bodenk , 2 : — Norrish, K. Factors in the weathering of mica to vermiculite. Madrid , 1 : — Robert, M. Incipient weathering: some new concepts on weathering, clay formation and organization. Amsterdam: Elsevier, pp. Singer, A. Illite in the hot aridic soil environment. Pedogenic palygorskite occurrences in Australia. Tazaki, K. Denver , 53— Velbel, M. Weaver, C.
Clays, Muds and Shales. Yaalon, D. Another important property of clay minerals, the ability to exchange ions, relates to the charged surface of clay minerals. Ions can be attracted to the surface of a clay particle or taken up within the structure of these minerals. The property of clay minerals that causes ions in solution to be fixed on clay surfaces or within internal sites applies to all types of ions, including organic molecules like pesticides.
Clays can be an important vehicle for transporting and widely dispersing contaminants from one area to another. Clays and clay minerals occur under a fairly limited range of geologic conditions. The environments of formation include soil horizons, continental and marine sediments, geothermal fields, volcanic deposits, and weathering rock formations.
Most clay minerals form where rocks are in contact with water, air, or steam. Examples of these situations include weathering boulders on a hillside, sediments on sea or lake bottoms, deeply buried sediments containing pore water, and rocks in contact with water heated by magma molten rock. All of these environments may cause the formation of clay minerals from preexisting minerals. The transport and deposition of clays and clay minerals produced by eroding older continental and marine rocks and soils are important parts of the cycle that forms sedimentary rocks.
The ancient sedimentary rock record is composed of about 70 percent mudstones which contain about 50 percent clay-sized fragments and shales which are coarser than mudstones but which may contain clay-sized particles Blatt and others, Today, sedimentary environments that contain muds cover about 60 percent of marine continental shelves and 40 percent of deep ocean basins; continental aquatic environments such as lakes, rivers, estuaries, and deltas also contain high proportions of fine-grained sediments Hillier, , p.
Clearly, clays and clay minerals are critical components of both ancient and modern sedimentary environments. Diagenesis is the in-place alteration of a mineral to more stable forms, excluding surficial alteration which is weathering ; diagenesis occurs, for example, when minerals stable in one depositional environment are exposed to another by burial and compaction.
Common silicate materials such as quartz, feldspars, and volcanic glasses, as well as carbonates, noncrystalline iron oxides, and primary clay minerals, are transformed during diagenesis into more stable clay minerals mainly by dissolution and recrystallization. The formation of bentonite beds containing smectite-group clay minerals including montmorillonite and fuller's earth a type of clay mineral deposit that has high capacity to absorb water may occur primarily by diagenesis, although some deposits may also form by hydrothermal processes.
Bentonite beds usually form from altered volcanic ash, but other types of rock may also serve as sources. The absorptive properties of bentonites and fuller's earth make them ideal for such diverse uses as drilling mud; foundry-sand bond; binder for pelletizing iron ore and bleaching liquids; absorbents for oil, grease, and animal waste; and carriers for pesticides and fertilizers.
Bentonite is also used as a soil liner for environmental containment applications and with polyacrylamide for making paper. Weathering of rocks and soil is the primary way that clays and clay minerals form at the Earth's surface today. The weathering process involves physical disaggregation and chemical decomposition that change original minerals to clay minerals; weathering is uneven, and many stages of breakdown may be found in the same clay sample. Factors governing rock weathering and soil formation include the initial type of rock, the ratio of water to rock, the temperature, the presence of organisms and organic material, and the amount of time.
Additionally, most clay formations occur when clay minerals are in contact with air, water or steam. Clay soil is essentially composed of several minerals that deposit together and, over time, form a hardened clay deposit. Silicates, mica, iron and aluminum hydrous-oxide minerals are the most common minerals found in clay deposits.
However, other minerals, such as quartz and carbonate, are also present in clay soils. In order to form clay soil, the particles that compose the soil need to come from somewhere. Erosion is one source of particles for clay soils and it occurs when water rushes over the surface of rock. However, the largest source of clay particles is from weathering of rocks and soil.
During weathering, both physical and chemical changes take place that create the small particles required to form clay soil. Lastly, diagenesis--the process that occurs when minerals that are stable in one environment destabilize because of compaction or burial--is another source of clay particles. As discussed, clay soil is formed through the deposition of a variety of particles. Therefore, clay soils differ in their composition, usually based on the geological process that created the particles erosion, weathering or diagenesis.
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