Corundum is a mineral that occurs naturally in a variety of colors, including blue, red, pink, yellow, and green. Sapphires, one of the most popular gemstones in the world, are made of corundum. Understanding the chemistry of corundum crystals can reveal the secrets of their color, structure, and formation, including the important role of trace elements.

Sapphires are highly valued for their rich blue color, which can range from pale sky blue to a deep navy. But what gives sapphires their color? The answer lies in the trace elements that are present during the formation of the corundum crystals that make up sapphires.

Chemical Makeup of Corundum

Corundum, with the chemical formula Al2O3, is a mineral that plays an important role in the gemstone industry. It is the mineral that makes up sapphires and rubies, two of the most prized gemstones in the world. The chemical makeup of corundum is relatively simple, but recent research has revealed new insights into the properties of this mineral.

The crystal structure of corundum is composed of oxygen atoms arranged in a hexagonal close-packed lattice, with aluminum atoms occupying two-thirds of the octahedral sites. This arrangement gives corundum its exceptional hardness and durability. It is one of the hardest minerals on Earth, with a Mohs hardness scale of 9. This is due to the strength of the covalent bonds between the aluminum and oxygen atoms, as well as the stability of the crystal lattice structure.

Formation of Corundum Crystals

Corundum crystals are formed under high temperature and pressure conditions deep within the earth's crust. They are often found in metamorphic rocks, such as schist or gneiss, or in igneous rocks, such as basalt. The exact process of corundum formation is still not fully understood, but it is believed to involve the interaction of aluminum-rich fluids with pre-existing minerals.

The Role of Trace Elements in Sapphire Colors

One of the fascinating aspects of corundum chemistry is the role of trace elements in the formation of sapphires and rubies. These trace elements can have a significant impact on the color and appearance of the gemstones. For example, the blue color of sapphires is due to the presence of iron and titanium, which give rise to specific energy levels and absorption spectra. The amount and type of trace elements present can vary, resulting in a wide range of blue hues. Other trace elements, such as chromium, can result in different colored sapphires, such as pink or red.

Treatment of Corundum Crystals

Corundum crystals, including sapphires and rubies, are often treated to enhance their color or clarity. One common treatment is heat treatment, which involves heating the gemstone to high temperatures to improve its color or remove inclusions. Another treatment is diffusion, which involves introducing trace elements into the surface of the gemstone to alter its color. These treatments can result in chemical changes in the corundum crystals and may affect their value and durability.

Industrial Use of Sapphires

Corundum crystals are not only of interest to gemologists but also to materials scientists. Corundum has a wide range of industrial applications, such as cutting tools, grinding wheels, and abrasives. Understanding the chemistry of corundum, including the role of trace elements, can lead to the development of new materials and applications.

Conclusion

Understanding the chemistry of corundum crystals, including the role of trace elements, is essential for gemologists, jewelers, and anyone interested in the science of gemstones. By understanding the chemical makeup, formation, and trace elements of corundum crystals, we can unlock the secrets of their beauty and durability, and appreciate the natural wonders of sapphires and rubies.