Clay mineral

Clay minerals are special kinds of rocks that form when certain types of 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 specific 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, and 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 very fine type of soil that becomes soft and moldable when it is wet, but hard when it dries or is heated. It is one of the oldest materials used by humans, who made pottery from it long ago. Clay can hold important nutrients like potassium and ammonium, which help plants grow.

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

Occurrence

Clay minerals are common in places where rocks break down and mix with water. They are often found in soil and in soft, fine-grained rocks like shale, mudstone, and siltstone. Because they need water to form, clay minerals are not often seen in places far from water, but they are found on Earth and in some places in space. Scientists have found clay minerals on Mars and other objects like 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 that clay could help simple molecules join together to form more complex ones.

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

Biomedical applications of clays

Clay minerals have unique shapes and compositions that make them useful in biology. Because of their flat, disc-like shapes and charged surfaces, clays can interact with many important molecules in the body, such as medicines, proteins, and DNA. This makes clays helpful for delivering medicines, building tissue structures, and creating printed biological materials.

Mortar applications

Clay minerals can be added to special building mixes to make them stronger. Scientists use special methods to create clay products that are very small and tightly packed, which 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 several tests to find out if clay minerals are present. These tests include looking at how clay absorbs certain substances, measuring its ability to swap out parts of its structure, and using special equipment to see its patterns. After checking for other materials like organic matter and carbonates, they use the results from these tests to estimate how much clay mineral is there. One type of clay called illite can be measured by looking at its potassium content, as it is the only clay mineral that contains potassium.

Argillaceous rocks

Argillaceous rocks are rocks that contain a lot of clay minerals. An example is argillaceous limestones, which are mostly made of calcium carbonate but also have 10 to 40 percent clay minerals. When these limestones are soft, they are often called marls. Another example is argillaceous sandstones, like greywacke. These sandstones are mainly made of quartz grains, with clay minerals filling the spaces between the grains.