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Plastic Clays

Re-plastified Palaeozoic/Mesozoic claystones as well as Cenozoic clay and kaolin are indispensable basic materials in the brick industry. They enable plastic molding and an effective sintering process.

Clays are clastic sediments with a grain size of d < 2 µm which form during chemical and physical weathering, erosion, transport and sedimentation. The type and amount of clay minerals determines the properties of plastic clay. Carbonates, colouring oxides like iron, manganese and titan, their quantity and oxidation state determine the firing colour of clay.

Clay minerals consist of planar crystals with platy structure and belong to the class of phyllosilicates. They are formed predominantly by intensive weathering of feldspar-rich rocks in tropical to subtropical zones. The main formational setting accordingly is continental climate, predominantly in equatorial forests. In these regions the weathered soil can be up to hundreds of meters thick.

Clay mineral zones and depth of weathering (modified after Brinkmann, R. 1980)
Clay mineral zones and depth of weathering (modified after Brinkmann, R. 1980)

High precipitation and low pH-values favour the hydrolytic dissolution of the source rock and therefore the formation of clay minerals as a product of chemical weathering. The process is characterized by depletion in silicic acid and potassium, enrichment in aluminium oxides and incorporation of hydroxyl groups.

The weathering coverage of continents is later eroded by wind, water and ice. Each year about 16 billion tons of particular sediment are deposited in the ocean. Additionally, about 4 billion tons of dissolved matter is transported by rivers into the oceans each year.  Finally, about 1.8 billion tons of dust are transported from desert regions into the oceans by aeolian transport.

Sediment discharge and suspension load on earth (modified after Gaupp, R. 2005)
Sediment discharge and suspension load on earth (modified after Gaupp, R. 2005)

After transportation clay minerals are deposited together with resistant weathering remains such as quartz, feldspar, micas and rock fragments. They can often be found as oversize material in clays. The sedimentary deposition of clay can be on-shore, in shelf areas and off-shore.

Correlation between kaolinite distribution and climate zones of the earth (modified after Füchtbauer, H. 1988)
Correlation between kaolinite distribution and climate zones of the earth (modified after Füchtbauer, H. 1988)

Arid climates are ideal for carbonate and sulfate precipitation, especially in lakes and shallow marine basins. This way chemical sediments are deposited in terrestrial clay muds. Glauconite and concretions of pyrite/ marcasite, calcite and siderite can form in fresh clay sediments. Organic carbon can be incorporated from terrestrial plants and oceanic microorganisms. Euxinian environments in still water conditions prevent organic decomposition.

 

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