Clays
Introduction
Clay is a sedimentary rock composed of aggregates of hydrated aluminum silicates from the decomposition of rocks that contain feldspar, such as granite. It has various colors depending on the impurities it contains, from orange-red to white.
Physically it is considered a colloid, with extremely small particles and a smooth surface. The diameter of the clay particles is less than 0.0039 mm. In the textural fraction clay there may be non-mineral particles, phytoliths. Chemically it is a hydrated alumina silicate, whose formula is: AlO · 2SiO · 2HO.
It is characterized by acquiring plasticity "Plasticity (solid mechanics)") when mixed with water, and also sonority and hardness when heated above 800 °C. Fire-hardened clay was the first ceramic made by humans, and it is still one of the cheapest and most widely used materials. Bricks, kitchen utensils, art objects and even musical instruments like the ocarina are made from clay. It is also used in many industrial processes, such as paper making, ceramic coatings, cement production and chemical processes.
Classification
Clays can be classified according to several factors. Thus, depending on the geological process that originated them and the location of the deposit in which they are found, they can be classified as:
If we look at the structure of its components, lamellar clays and fibrous clays are distinguished.
Clays can also be distinguished according to their plasticity. Thus, there are plastic clays (such as kaolinitic clay) and less plastic clays (such as smectic clay, which absorbs fats).
Finally, there are also calcareous or marly clays (with a high proportion of limestone), clay with blocks or tillites (clay, gravel and rock blocks from moraines), decalcification clay and claystones (lithified clays).
Chemistry and structure
These minerals are made up of sheets of tetrahedra with a general chemical composition of SiO, where each tetrahedron (SiO) is joined by its vertices to three others, forming a hexagonal network. Aluminum and iron atoms can partially replace silicon in the structure.[1] The oxygen atoms located at the apices of the tetrahedra of these sheets can, at the same time, form part of another parallel sheet composed of octahedra. These octahedra are usually coordinated by cations of Al, Mg, Fe and Fe, that is, the atoms of these elements occupy the central position of the octahedron. More infrequently, atoms of Li, V, Cr, Mn, Ni, Cu or Zn occupy said coordination site.[1].