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.

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