Ala Lombolo
- One of Sweden's most polluted lakes

The word clay is referring to the size of a rock fragment which is smaller than 0.0002 mm (Burden & Sims, 1999). Despite the small size, there are several various types of clay particles with different properties and structures.

 

Commonly, clay particles are built up by sheets of tetrahedrons, comprised by silica cations (Si4+) surrounded by four oxygen atoms, and octahedron sheets, where aluminium cations (Al3+) are surrounded by eight oxygen atoms. These sheets constitute base units in either tetrahedral - octahedral layers (1:1 arrangement) or tetrahedral - octahedral - tetrahedral layers (2:1 arrangement). The layers are bond together by sharing oxygen atoms. In the 1:2 arrangement, where a base unit has octahedrons on both sides, a multi layered structure will need other molecules or ions to glue the base units together. The arrangement of these different base units can result in a variety of clay types, see the illustration below. (Burden & Sims, 1999)

 

The globally most abundant clay type is Kaolinite. It is a 1:1 type of clay which does not swell. It is the least reactive clay and it does not bind cations or pollutants to a large extent. The smectite clays have a 2:1 arrangement which make them prone to swell in contact with water. When swelling, a larger surface area is exposed and the sorption of cations will increase. These clays are therefore more apt to absorb pollutants. Illite or mica is an other type of clay which exhibit the 2:1 arrangement. This type of clay is the most common in colder climates such as in Sweden. These clays differ from the smectites by having potassium as a glue in between the layers, binding the base units together. The potassium neutralises the charge of the clay particle and also prevents the structure from swelling in contact with water. This results in a lower interaction with pollutants in the ground. (Burden & Sims, 1999)

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To establish the mineralogy of a clay, detailed studies of the sediment have to be conducted. Since the clay fraction is very small it is not possible to do this by ocular investigations. Information about the type of clays which are present in the Ala Lombolo lake sediments sre not known, but it is reasonable to assume that they to a large extent could consist of illites due to the climate in which the lake is located. Despite the already high degree of pollution, if the clays would have been smectites even greater concentrations could be possible to find in the lake if the source of the contaminant was large enough.