The majority of clay minerals occur when rocks come into contact with water, air, or steam. Weathering boulders on a slope, sediments on sea or lake bottoms, deeply buried sediments holding pore water, and rocks in contact with water heated by magma are examples of these conditions (molten rock). As water passes through the porous rock, it dissolves some of the mineral components, leaving behind solid particles of clay.
Clays form when this process continues over many years. The rate of formation depends on several factors: the type of rock that is being weathered; the temperature and pressure of the environment; and the amount of water available for dissolution-reformation cycles.
For example, clays often form in sedimentary rocks which have been compressed over time under great weight. This causes small pores to form between grains of sand or gravel. If enough water flows through these rocks, then it will dissolve some of the minerals and leave behind solid particles of clay.
Another example is volcanic eruptions. When lava cools, it becomes glass mixed with some metal oxides and silica. If water is present during this process, it will dissolve some of the silica and alumina, leaving behind solid particles of clay.
Yet another example is the action of wind and water on the exposed surface of a rock. Grit blown from a field into nearby streams can become packed with sand or clay particles.
Today, the major way that clay and clayminerals originate at the Earth's surface is through weathering of rocks and soil. The original kind of rock, the water-to-rock ratio, the temperature, the presence of organisms and organic material, and the period of time all have a role in rockweathering and soil formation. Over time, this process produces clays and other minerals that can be found in sediments or within the rock itself.
Clay minerals are often white or colorless, but they can also be red, blue, green, or black. They are made up of layers of atoms called tetrahedrons (four corners) with oxygen molecules (groups) between each layer. These layers can be flat or rolled, but they usually have a constant thickness of about 1 nanometer (10-9 m).
The amount of clay present in any particular sample is measured by performing a test called a "dipstick" analysis. In this method, a small amount of sediment is placed on a piece of paper with a light yellow background. If any clay particles are present, a clear band will form around the edge of the paper when it is dipped into a jar full of water. This indicates that there is enough clay for use in scientific studies.
When rocks are exposed to the elements, they may be covered in dust or sand that has been weathered from deeper within the earth. This is called windblown or desert dust.
Clay minerals are a broad category of fine-grained and crystalline minerals that arise as a result of water modification of parent igneous minerals at or near the Earth's surface. They have a layered structure that is often made up of repeated sheets of Si tetrahedra and Al octahedra. These layers can be very thin, with only a few atoms thick, but they also may be quite thick, such as in shale which contains fibers (laminae) formed from aggregates of these layers.
Layered structures are common in nature and are important for many reasons including storing energy through compression or tension, preventing stress fractures by distributing load across large areas, and allowing organisms to grow into their environment. Humans use layered materials in construction projects to create strong, durable, and attractive objects.
One of the most familiar examples of a layered material is paper, which is mostly made up of cellulose fibers that are bonded together by gluey substances called resins. The fibers themselves are thick and hollow, making them efficient at storing information while being light weight. They are also flexible and moldable, allowing us to write on both sides of a sheet of paper and fold it up into a book without damaging the page.
Clays come in many different forms; some are hard and smooth while others are soft and powdery. All clays contain silicon dioxide and aluminum oxide molecules that stack together to form layers.