Brick is not technically considered a rock, but rather a man-made product made from clay. However, the process of firing the clay to create bricks involves some interesting chemistry that mimics the transformation of clay minerals into a metamorphic rock.
Clay minerals are composed of layered sheets of atoms, which give clay its characteristic properties such as plasticity and ability to retain water. During the firing process, these clay minerals undergo structural changes due to the application of heat. The first major change that occurs is the release of chemically bound water.
Clay minerals contain water molecules that are held within their crystal structure. These water molecules are known as “structural water” and play a crucial role in the plasticity of clay. As the temperature increases during firing, the heat causes the structural water to be released from the clay minerals. This water vaporizes and escapes from the clay matrix.
As the structural water is driven off, the clay minerals begin to undergo mineralogical changes. One of the minerals that forms during this process is quartz. Quartz is a common mineral composed of silicon and oxygen atoms arranged in a crystal lattice. It is known for its hardness and resistance to weathering.
The formation of quartz occurs as a result of the breakdown of certain clay minerals, such as kaolinite. Kaolinite is a clay mineral that consists of stacked layers of aluminum silicate sheets with water molecules in between. As the clay is heated, the water is driven off and the aluminum silicate sheets recombine to form quartz.
Another mineral that forms during the firing process is mullite. Mullite is a mineral composed of aluminum and silicon oxides. It is formed through the reaction of aluminum silicate minerals, such as kaolinite, with silica (SiO2) that is present in the clay. Mullite is known for its high melting point and thermal stability, making it an ideal mineral for brick production.
Interestingly, the quartz that forms during firing remains in a glassy state. This means that it does not fully crystallize into a well-defined crystal structure. Instead, it retains some of the amorphous characteristics of glass. This glassy quartz contributes to the strength and durability of the fired clay, as it fills in the gaps between the mullite crystals.
The firing of brick clay involves the transformation of clay minerals into a mixture of quartz and mullite. The release of chemically bound water and the recombination of aluminum silicate sheets result in the formation of these minerals. The glassy state of the quartz contributes to the strength and durability of the fired clay. While brick may not be classified as a natural rock, the firing process simulates the chemical changes that occur during the metamorphism of clay minerals into a metamorphic rock.