Interactions with inorganic and organic compounds
- Related Topics:
- montmorillonite
- vermiculite
- sepiolite
- chlorite
- kaolinite
- On the Web:
- Tulane University - Weathering and Clay Minerals (Feb. 17, 2025)
Smectite, vermiculite, and other expansible clay minerals can accommodate relatively large, inorganic cations between the layers. Because of this multivalency, the interlayer space is only partially occupied by such inorganic cations that are distributed in the space like islands. Hydroxy polymers of aluminum, iron, chromium, zinc, and titanium are known examples of the interlayering materials. Most of these are thermally stable and hold as pillars to allow a porous structure in the interlayer space. The resulting complexes, often called pillared clays, exhibit attractive properties as catalysts—namely, large surface area, high porosity, regulated pore size, and high solid acidity.
Cationic organic molecules, such as certain aliphatic and aromatic amines, pyridines, and methylene blue, may replace inorganic exchangeable cations present in the interlayer of expansible minerals. Polar organic molecules may replace adsorbed water on external surfaces and in interlayer positions. Ethylene glycol and glycerol are known to form stable specific complexes with smectites and vermiculites. The formation of such complexes is frequently utilized for identifying these minerals. As organic molecules coat the surface of a clay mineral, the surface of its constituent particles changes from hydrophilic to hydrophobic, thereby losing its tendency to bind water. Consequently, the affinity of the material for oil increases, so that it can react with additional organic molecules. As a result, the surface of such clay materials can accumulate organic materials. Some of the clay minerals can serve as catalysts for reactions in which one organic substance is transformed to another on the mineral’s surface. Some of these organic reactions develop particular colours that may be of diagnostic value in identifying specific clay minerals. Organically clad clay minerals are used extensively in paints, inks, and plastics.
Physical properties
Clay mineral particles are commonly too small for measuring precise optical properties. Reported refractive indices of clay minerals generally fall within a relatively narrow range from 1.47 to 1.68. In general, iron-rich mineral species show high refractive indices, whereas the water-rich porous species have lower ones. Specific gravities of most clay minerals are within the range from 2 to 3.3. Their hardness generally falls below 21/2, except for antigorite, whose hardness is reported to be 21/2–31/2.
Size and shape
These two properties of clay minerals have been determined by electron micrographs. Well-crystallized kaolinite occurs as well-formed, six-sided flakes, frequently with a prominent elongation in one direction. Halloysite commonly occurs as tubular units with an outside diameter ranging from 0.04 to 0.15 micrometre.
Electron micrographs of smectite often show broad undulating mosaic sheets. In some cases the flake-shaped units are discernible, but frequently they are too small or too thin to be seen individually without special attention.
Illite occurs in poorly defined flakes commonly grouped together in irregular aggregates. Although their sizes vary more widely, vermiculite, chlorite, pyrophyllite, talc, and serpentine minerals except for chrysotile are similar in character to the illites. Chrysotile occurs in slender tube-shaped fibres having an outer diameter of 100–300 Å. Their lengths commonly reach several micrometres. Electron micrographs show that palygorskite occurs as elongated laths, singly or in bundles. Frequently the individual laths are many micrometres in length and 50 to 100 Å in width. Sepiolite occurs in similar lath-shaped units. As mentioned above, allophane occurs in very small spherical particles (30–50 Å in diameter), individually or in aggregated forms, whereas imogolite occurs in long (several micrometres in length) threadlike tubes.
High-temperature reactions
When heated at temperatures beyond dehydroxylation, the clay mineral structure may be destroyed or simply modified, depending on the composition and structure of the substance. In the presence of fluxes, such as iron or potassium, fusion may rapidly follow dehydroxylation. In the absence of such components, particularly for aluminous dioctahedral minerals, a succession of new phases may be formed at increasing temperatures prior to fusion. Information concerning high-temperature reactions is important for ceramic science and industry.
Solubility
The solubility of the clay minerals in acids varies with the nature of the acid and its concentration, the acid-to-clay ratio, the temperature, the duration of treatment, and the chemical composition of the clay mineral attacked. In general, ferromagnesian clay minerals are more soluble in acids than their aluminian counterparts. Incongruent dissolutions may result from reactions in a low-acid-concentration medium where the acid first attacks the adsorbed or interlayer cations and then the components of the octahedral sheet of the clay mineral structure. When an acid of higher concentration is used, such stepwise reactions may not be recognizable, and the dissolution appears to be congruent. One of the important factors controlling the rate of dissolution is the concentration in the aquatic medium of the elements extracted from the clay mineral. Higher concentration of an element in the solution hinders to a greater degree the extractions of the element.
In alkaline solutions, a cation-exchange reaction first takes place, and then the silica part of the structure is attacked. The reaction depends on the same variables as those stated for acid reactions.
Occurrence
Soils
All types of clay minerals have been reported in soils. Allophane, imogolite, hydrated halloysite, and halloysite are dominant components in ando soils, which are the soils developed on volcanic ash. Smectite is usually the sole dominant component in vertisols, which are clayey soils. Smectite and illite, with occasional small amounts of kaolinite, occur in mollisols, which are prairie chernozem soils. Illite, vermiculite, smectite, chlorite, and interstratified clay minerals are found in podzolic soils. Sepiolite and palygorskite have been reported in some aridisols (desert soils), and kaolinite is the dominant component in oxisols (lateritic soils). Clay minerals other than those mentioned above usually occur in various soils as minor components inherited from the parent materials of those soils.
Soils composed of illite and chlorite are better suited for agricultural use than kaolinitic soils because of their relatively high ion-exchange properties and hence their capacity to hold plant nutrients. Moderate amounts of smectite, allophane, and imogolite in soils are advantageous for the same reason, but when present in large amounts these clay minerals are detrimental because they are impervious and have too great a water-holding capacity.
Recent sediments
Sediment accumulating under nonmarine conditions may have any clay mineral composition. In the Mississippi River system, for example, smectite, illite, and kaolinite are the major components in the upper Mississippi and Arkansas rivers, whereas chlorite, kaolinite, and illite are the major components in the Ohio and Tennessee rivers. Hence, in the sediments at the Gulf of Mexico, as a weighted average, smectite, illite, and kaolinite are found to be the major components in the clay mineral composition. Although kaolinite, illite, chlorite, and smectite are the principal clay mineral components of deep-sea sediments, their compositions vary from place to place. In general, illite is the dominant clay mineral in the North Atlantic Ocean (greater than 50 percent), while smectite is the major component in the South Pacific and Indian oceans. In some limited regions, these compositions are significantly altered by other factors such as airborne effects, in which sediments are transported by winds and deposited when the carrying force subsides. The high kaolinite concentration off the west coast of Africa near the Equator reflects this effect.
Under highly saline conditions in desert areas, as in soils, palygorskite and sepiolite also form in lakes and estuaries (perimarine environments).
Ancient sediments
Analyses of numerous ancient sediments in many parts of the world indicate that smectite is much less abundant in sediments formed prior to the Mesozoic Era (from 251 million to 65.5 million years ago) with the exception of those of the Permian Period (from 299 million to 251 million years ago) and the Carboniferous Period (359.2 million to 299 million years ago), in which it is relatively abundant.
The available data also suggest that kaolinite is less abundant in very ancient sediments than in those deposited after the Devonian Period (416 million to 359.2 million years ago). Stated another way, the very old argillaceous (clay-rich) sediments called physilites are composed largely of illite and chlorite. Palygorskite and sepiolite have not been reported in sediments older than early Cenozoic age—i.e., those more than about 65.5 million years old.
Kaolinite and illite have been reported in various coals. Bentonite generally is defined as a clay composed largely of smectite that occurs in sediments of pyroclastic materials as the result of devitrification of volcanic ash in situ.
Sediments affected by diagenesis
As temperature and pressure increase with the progression of diagenesis, clay minerals in sediments under these circumstances change to those stable under given conditions. Therefore, certain sensitive clay minerals may serve as indicators for various stages of diagenesis. Typical examples are the crystallinity of illite, the polytypes of illite and chlorite, and the conversion of smectite to illite. Data indicate that smectite was transformed into illite through interstratified illite-smectite mineral phases as diagenetic processes advanced. Much detailed work has been devoted to the study of the conversion of smectite to illite in lower Cenozoic-Mesozoic sediments because such conversion appears to be closely related to oil-producing processes.
Hydrothermal deposits
All the clay minerals, except palygorskite and sepiolite, have been found as alteration products associated with hot springs and geysers and as aureoles around metalliferous deposits. In many cases, there is a zonal arrangement of the clay minerals around the source of the alteration, a process which involves changes in the composition of rocks caused by hydrothermal solutions. The zonal arrangement varies with the type of parent rock and the nature of the hydrothermal solution. An extended kaolinite zone occurs around the tin-tungsten mine in Cornwall-Devon, Eng. Mica (sericite), chlorite, tosudite, smectite, and mica-smectite interstratifications are contained in an extensive clay zone formed in a close association with kuroko (black ore) deposits. Smectites are known to occur as alteration products of tuff and rhyolite. Pottery stones consisting of kaolinite, illite, and pyrophyllite occur as alteration products of acidic volcanic rocks, shales, and mudstone.
