Clay mineral

Clay minerals are special kinds of rocks that form when certain earth materials break down in water. They are made mostly of a metal called aluminium, along with silicon and oxygen, and they often have water in their structure too. Some common clay minerals include kaolin, which has a chemical makeup that scientists write as Al₂[Si]₂O₅([OH])₄.

These minerals are very important for life on Earth. Many ideas about how life first began, called theories of abiogenesis, include clay minerals as key players. They are also a big part of the soil we see everywhere. People have used them for thousands of years. Farmers use clay-rich soil to help plants grow, and many products we use every day, like pottery and bricks, are made from clay.

Properties

See also: Pottery

Clay is a fine type of soil that becomes soft and moldable when wet, but hard when it dries or is heated. It is one of the oldest materials humans have used, making pottery long ago. Clay can hold nutrients like potassium and ammonium, which help plants grow.

Because clay particles are very tiny, scientists needed special tools to study them. In the 1930s, they used x-ray diffraction to learn about clay's structure. They found that clay is made of thin, flat layers, like minerals called mica. Today, scientists use many tools, including electron diffraction, different types of spectroscopy, and SEM-EDS methods, to study clay.

Occurrence

Clay minerals are common where rocks break down and mix with water. You can find them in soil and in soft rocks like shale, mudstone, and siltstone. Since they need water to form, clay minerals are usually near water. They are found on Earth and also in space, such as on Mars, the dwarf planet Ceres, and the comet Tempel 1.

Structure

Clay minerals are made of flat, two-dimensional sheets. These sheets are built from small shapes called tetrahedra and octahedra. In these shapes, atoms like silicon, oxygen, and aluminum fit together in a pattern.

In clay, these flat sheets stick together in layers. Some clays have one flat sheet and one octahedral sheet in each layer, called 1:1 clays. Others have two flat sheets with an octahedral sheet in between, called 2:1 clays. These layers can hold water and other small atoms between them, which helps give clay its soft and bendable qualities.

Classification

Clay minerals can be grouped into two main types: 1:1 and 2:1.

A 1:1 clay has one layer of tetrahedral shapes and one layer of octahedral shapes. Examples include kaolinite and serpentine.

A 2:1 clay has an octahedral layer between two tetrahedral layers. Examples are talc, vermiculite, and montmorillonite.

Clay minerals include groups such as the Kaolin group, which has minerals like kaolinite, dickite, halloysite, and nacrite. The Smectite group includes minerals like montmorillonite, and tests by the Curiosity rover found these minerals on Mars. Other groups are the Illite group and the Chlorite group. There are also special types of clays with mixed layers, like R1 ordered illite-smectite.

Clay and the origins of life

Scientists have wondered how life first began. In 1985, a scientist named Graham Cairns-Smith suggested that life might have started with clay. He thought clay could help small molecules join together to form more complex ones.

Studies show that a special kind of clay called montmorillonite can help tiny parts called nucleotides link up to make RNA. RNA can carry instructions for making living things. Other ideas suggest that early life might have used clay to change carbon dioxide into simple acids. Later, life learned to use other materials to grow and change.

Biomedical applications of clays

Clay minerals have special shapes and compositions that make them useful in biology. Because of their flat, disc-like shapes and charged surfaces, clays can interact with important molecules in the body, such as medicines, proteins, and DNA. This helps clays deliver medicines, build tissue structures, and create printed biological materials.

Mortar applications

Clay minerals can be added to special building mixes to make them stronger. Scientists use special methods to create tiny clay products that are tightly packed. This helps make better ceramics and materials that can clean up harmful metals.

These clays can also help neutralize acidic soil, making it safer for plants to grow. This is important for taking care of the environment, especially in areas where mining has taken place. It helps both the local community and the wider region.

Tests which verify that clay minerals are present

Scientists use different tests to check if clay minerals are present. They look at how clay absorbs certain substances and measure its ability to swap parts of its structure. They also use special equipment to see patterns. After checking for other materials like organic matter and carbonates, they use these test results to estimate how much clay mineral is there. A type of clay called illite can be measured by looking at its potassium content, because it is the only clay mineral that contains potassium.

Argillaceous rocks

Argillaceous rocks are rocks that have a lot of clay minerals. One example is argillaceous limestones. These are mostly made of calcium carbonate but also contain clay minerals. When soft, they are often called marls. Another example is argillaceous sandstones, such as greywacke. These sandstones are mainly made of quartz grains, with clay minerals in the spaces between the grains.