Lexikon

clay is a naturally occurring material composed primarily of fine-grained minerals, which show plasticity through a variable range of water content, and which can be hardened when dried and/or fired. Clay deposits are mostly composed of clay minerals phyllosilicate minerals, minerals which impart plasticity and harden when fired and/or dried, and variable amounts of water trapped in the mineral structure by polar attraction.

Clay minerals are typically formed over long periods of time by the gradual chemical weathering of silicate rocks.

Clays are distinguished from other fine-grained soils by their grain-size. The grain size of clay-minerals are close to the colloidal size-range. Geologists and soil scientists usually consider 2 µm and under, sedimentologists often use 4-5 μm and under, and colloid chemists use 1 μm border line. Geotechnical engineers measure the plasticity properties of the soil to indentify clay-content and its geotechnical consequences Atterberg limits. See also soil texture.

clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths and other cations. Clay minerals are common weathering products including weathering of feldspar and low temperature hydrothermal alteration products. Clay minerals are very common in fine grained sedimentary rocks such as shale, mudstone and siltstone and in fine grained metamorphic slate and phyllite.

Clays are ultra fine 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 such as Mossbauer spectroscopy, infrared spectroscopy, and EDS or energy dispersive spectroscopy. These methods should always augment standard polarized light microscopy, a technique which is sometimes overlooked but often where fundamental occurrences or petrologic relationships are established.

Clays are fundamentally built of tetrahedral sheets and octahedral sheets. A 1:1 clay would consist of one tetrahedral sheet and one octahedral sheet, and examples would be kaolinite and serpentine. A 2:1 clay consists of an octahedral sheet sandwiched between two tetrahedral sheets, and examples are illite, smectite, attapulgite, and chlorite although chlorite has an external octahedral sheet often referred to as "brucite".

Clay minerals include the following groups:

• Kaolin group which includes the minerals kaolinite, dickite, halloysite and nacrite. Some sources include the serpentine group due to structural similarities Bailey 1980.
• Smectite group which includes dioctahedral smectites such as montmorillonite and nontronite and trioctahedral smectites for example saponite.
• Illite group which includes the clay-micas. Illite is the only common mineral.
• Chlorite group includes a wide variety of similar minerals with considerable chemical variation.
• Other 2:1 clay types exist such as sepiolite or attapulgite, clays with long water channels internal to their structure.

Mixed layer clay variations exist for most of the above groups. Clay minerals in soil have an important role in bonding both nutrients and pollutants.

Sorce: Wikipedia, http://en.wikipedia.org/wiki/Clay_minerals