Corundum
Introduction
Corundum is a rock-forming mineral known for its hardness and its varied color range. It is a crystalline form of aluminium oxide (Al2O3) typically containing traces of iron, titanium, vanadium, and chromium. It is one of the naturally transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red and padparadscha if pink-orange. All other colors are called sapphire, e.g., "green sapphire" for a green specimen.
Properties
Corundum has a hardness of 9 on the Mohs scale, making it among the hardest minerals on Earth. Only moissanite and diamond are harder. This hardness is what makes corundum such a powerful abrasive. It is also what makes it so useful in applications like sandpaper and grinding wheels. Corundum's hardness is primarily due to the strong and short oxygen-aluminium bonds. These bonds pull the oxygen and aluminium ions close together, making the crystal lattice very strong.
Occurrence and Formation
Corundum is commonly found in metamorphic rocks such as marble, gneiss, and schist. It is also found in igneous rocks like syenite and nepheline syenite. Corundum crystals can form in the cavities of these rocks or in the surrounding hydrothermal fluids. The formation of corundum involves the crystallization of aluminium oxide from these fluids, often in the presence of common rock-forming minerals such as feldspar and quartz.
Varieties
The red variety of corundum is known as ruby. The color in ruby is caused by a small amount of chromium replacing aluminium in the crystal structure. The blue variety of corundum is known as sapphire. The color in sapphire is caused by a small amount of iron and titanium replacing aluminium in the crystal structure. Other colors of corundum are also classified as sapphires, and these are often referred to by color, such as green sapphire or yellow sapphire. The rare pink-orange variety of corundum is known as padparadscha.
Uses
Corundum has many applications due to its hardness and heat resistance. It is used as an abrasive in applications such as sandpaper and grinding wheels. High-quality corundum is also used in the manufacture of scratch-resistant windows for electronic devices. In addition, corundum is used in the making of certain types of lasers, and as a gemstone in jewelry.