Corundum is a naturally occurring solid defined by its specific chemical formula and ordered internal structure. This substance is a crystalline form of aluminum oxide (\(\text{Al}_2\text{O}_3\)). The mineral is most widely known through its gem-quality varieties, which include the vibrant ruby and the diverse sapphire. It is recognized for its remarkable hardness, which is second only to that of diamond among naturally occurring substances.
What Makes a Substance a Mineral?
The geological definition of a mineral is based on five specific criteria that a substance must meet. These requirements ensure the substance is naturally occurring, inorganic, and possesses a definite chemical composition.
- It must be naturally occurring, formed by natural geological processes and not manufactured.
- It must be inorganic, excluding materials formed from the processes of living organisms, such as coal or pearl.
- It must be a solid under normal conditions, which excludes liquids or gases (though ice is considered a mineral).
- It must possess a definite chemical composition, which can be expressed by a chemical formula, such as the \(\text{SiO}_2\) of quartz.
- It must have an ordered internal structure, meaning its atoms are arranged in a repeating, three-dimensional pattern called a crystal lattice.
The final requirement of an ordered internal structure means atoms are arranged in a repeating, three-dimensional pattern. This crystalline arrangement distinguishes true minerals from amorphous solids like glass, which lack long-range atomic order.
Corundum’s Distinct Chemical and Structural Identity
Corundum meets all the established criteria for mineral classification, starting with its precise chemical makeup of aluminum oxide (\(\text{Al}_2\text{O}_3\)). This formula shows a ratio of two aluminum ions to every three oxygen ions, fulfilling the requirement for a definite chemical composition. Naturally occurring corundum forms in silica-poor, aluminum-rich environments, typically found in metamorphic or certain igneous rocks.
The internal architecture of corundum is a highly ordered, dense crystal lattice with trigonal symmetry. Aluminum ions occupy specific sites between the oxygen ions, which creates a very stable and tightly bonded structure.
The extreme stability of this crystalline structure gives corundum its defining physical property: exceptional hardness. It registers a 9 on the Mohs scale of mineral hardness, making it the hardest mineral after diamond (10). This characteristic is a direct result of the strong, short bonds between the aluminum and oxygen ions in the repeating crystal lattice.
The Gemstone Variations: Ruby and Sapphire
The gem varieties of corundum are distinguished purely by their color, not their base chemical structure. Ruby is the term used exclusively for corundum that exhibits a red to reddish-pink color. This specific hue is caused by the presence of trace amounts of the element chromium substituting for aluminum within the crystal lattice.
Sapphire is the name given to all other colors of gem-quality corundum, including blue, yellow, pink, green, and purple. Blue sapphire, the most famous color, is the result of trace impurities of both iron and titanium. These elements interact within the corundum structure to selectively absorb light, producing the characteristic deep blue color.
The wide spectrum of sapphire colors is caused by other trace elements. For instance, pink sapphire owes its color to lower concentrations of chromium than those found in ruby. The color variations demonstrate how minute substitutions of impurity atoms in the rigid aluminum oxide structure dramatically affect how the mineral interacts with light.