⚱️ Clay | Formation, Types, and Properties
⛏️ Formation of Clay
Clay is formed from the decomposition of parent rocks, particularly feldspathic rocks that have disintegrated to give rise to the soil.
This fragmentation occurred over millions of years due to erosion, pressure, volcanic activity, and other factors, creating very small particles, smaller than 2 µm (micrometer, the thousandth part of a millimeter).
Therefore, clay is a sedimentary mineral. These extremely light particles eventually get carried by water currents and deposited in places called clay deposits or beds.
Clays formed from the transport of the parent rock are called secondary or sedimentary clays:
- They were transported away from the parent rock by water.
- They mixed with various organic materials along the way.
- They are more superficial formations, less pure, and less clean.
- Impurities provide greater plasticity, making these clays very fine and plastic.
- Usually, they have a high iron oxide content or dark tones, as in the case of red clay, very common in Portugal.
Primary or residual clays are formed at the same site as the parent rock:
- They are rarely affected by atmospheric agents.
- They remain in the same location, without contamination from external elements.
- They have coarser particles (therefore, they are not very plastic).
- They present a lighter coloration because they are purer.
- They have a very high melting point.
Kaolin is an example of this type of clay.
⛏️ The Soil and Its Layers
The first layer is the fertile layer, where plants find some mineral salts and water. The second layer is the mineral salts layer.
To inspect if an area is clayey, holes 8 to 10 meters deep are dug with excavating machines.
As we have seen, parent rocks basically decompose into limestone, clay, and sand. We can say that clay is a natural material that, when mixed with water, becomes a plastic paste and, when subjected to high temperatures, acquires a high degree of hardness and strength.
Chemically, clay is an aluminum silicate, because its crystals are mostly made up of water, aluminum, and silica. It also contains other variable components, such as quartz, micas, feldspars (the rock from which clay originates is feldspathic rock), iron oxide, and other clay minerals, as well as other organic matters and metals, which together determine its properties.
⛏️ Collecting Clay
Although clay does not form on the surface, it can be easily found, mainly due to erosion caused by physical and chemical agents over time. It can be collected near rivers, in vertical cuts in cliffs, and in various surprising places where the soil exposes the layer of clay earth. In a typical example, rain opens paths as it descends a mound of clay, dragging the lightest and finest particles and forming a puddle on the ground. This puddle is an excellent deposit of fine and, therefore, plastic clay. By losing some water, it will become a moldable paste.
Clay "shrinks" as it loses water; in this drying process, it contracts and begins to show cracks due to shrinkage. Thus, clay can be detected through the cracks it presents on the surface, although this feature is not always present. A simple way to confirm if earth is clayey is to crumble a bit into powder in the palm of your hand (removing stones or small leaves), add a few drops of water (or saliva), and mix well until it results in a homogeneous paste.
Depending on the different mixtures of oxides and other components that occurred over millions of years, we can find clays of many different colors and textures in nature.