Clay Minerals

CLAY MINERALS

WRITTEN REPORT

CLAY MINERALS

Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations. Clay minerals are typically formed over long periods of time by the gradual chemical weathering of rocks, usually silicate-bearing, by low concentrations of carbonic acid and other diluted solvents. These solvents, usually acidic, migrate through the weathering rock after leaching through upper weathered layers. In addition to the weathering process, some clay minerals are formed by hydrothermal activity. Clay deposits may be formed in place as residual deposits in soil, but thick deposits usually are formed as the result of a secondary sedimentary deposition process after they have been eroded and transported from their original location of formation. Clay deposits are typically associated with very low energy depositional environments such as large lakes and marine deposits.

“The contact of rocks and water produces clays, either at or near the surface of the earth”

Rock +Water ® Clay

For example,

The CO2 gas can dissolve in water and form carbonic acid, which will become hydrogen ions H+ and bicarbonate ions, and make water slightly acidic.

CO2+H2O ® H2CO3 ®H+ +HCO3-

The acidic water will react with the rock surfaces and tend to dissolve the K ion and silica from the feldspar. Finally, the feldspar is transformed into kaolinite.

Feldspar + hydrogen ions+water ® clay (kaolinite) + cations, dissolved silica

2KAlSi3O8+2H+ +H2O ® Al2Si2O5(OH)4 + 2K+ +4SiO2

• Note that the hydrogen ion displaces the cations.

Clays are ultrafine-grained (normally considered to be less than 2 micrometres in size on standard particle size classifications) and so require special analytical techniques. Standards include x-ray diffraction, electron diffraction methods, various spectroscopic methods and infrared spectroscopy. These methods can be augment polarized light...