1.1.1: What Are Minerals?

Minerals are naturally occurring crystals. Since we consider physical science to be separate from biological science, minerals are generally crystals of nonbiological, or inorganic, origins. Crystals are solids possessing the characteristics of having a well-defined chemical composition and an ordered, repetitive atomic arrangement. Minerals are orderly assemblies of atoms, and rocks are random aggregates of minerals. In other words, minerals make possible the solid planet and everything on it, including life.

Consider the mineral calcite. Calcite (Figure \(\PageIndex{1}\)) is its mineral name; mineral names often end with the suffix "-ite". Its chemical name is calcium carbonate, which tells us about its chemical composition, the compositional makeup of atomic elements. Its chemical formula is CaCO₃, which again tells us about its chemical composition, but in formulaic form. In this formula, Ca represents the positively electrically charged calcium atomic ion, labeled as Ca²⁺; CO₃ represents the negatively electrically charged carbonate molecular ion, labeled as CO₃^2-. The opposite electronic charges on these two ionic components provide the electrostatic attractive force that brings them together. It is the chemical order of the electronic charges that gives rise to the well-defined chemical composition. But coming together is only the first step. One Ca²⁺ ion and one CO₃^2- ion make only a single CaCO₃ formula unit. To become a mineral—a macroscopic crystalline solid—the unit must be able to grow by replicating the assembly.

Figure \(\PageIndex{1}\): Calcite native to Hardin County, in southern Illinois. The shinny faces are the cleavage planes of the crystal. (James St. John via Wikimedia Commons; CC BY.)

It is the spatial order of geometry that gives rise to the ordered, repetitive atomic arrangement. A formula unit can grow only if its constituent components—the Ca²⁺ atomic ion and the CO₃^2- molecular ion—find an orderly fit in geometric arrangement. This is analogous to laying a floor by piecing together a large number of floor tiles; the tiles must fit orderly with neighboring tiles for the floor to expand. The layout itself needs adjustment relative to the shape of the tiles; for example, square-shaped tiles are arranged at 90° angles, whereas hexagonal-shaped tiles are arranged at 60° angles. The same principle applies to ions in the creation of a crystal (Figure \(\PageIndex{2}\)). This is why a mineral specimen often has specific angles at which its faces intersect. Through the criteria of a well-defined chemical composition and an ordered, repetitive atomic arrangement, microscopic-scale entities like atoms and ions assemble into macroscopic-scale solid materials or geologic formations, such as rocky mountains.

Figure \(\PageIndex{2}\): Atomic force microscope image showing a \(7\times 7 \mu {\rm m}^2\) area of the calcite cleavage plane (about 2% of the cross sectional area of the human hair). Lines are steps of atomic layers. Like tiles are assembled to make a floor, atoms are assembled to make a crystal. If atoms were the size of a 1-inch tile, this imaged area would be the area of four city blocks. (Bedzyk and Cheng, Reviews in Mineralogy and Geochemistry, 2002. Copyright 2002 by Mineralogical Society of America.)

Why are macroscopic solid materials desirable? The answer is simple: function. Macroscopic solid materials can support weight, transport cargos, conduct electricity, provide shelter, and serve many other purposes. For calcite, both the natural world and human civilizations have come up with a variety of ways to use it for specific functions. To begin, when life started on Earth, it started in the tropical seas. When the early marine mollusks needed to build seashells to protect themselves from ocean turbulence, calcite was the construction material of choice. The Ca²⁺ and CO₃^2- ions were readily recruited from the water to make the shell. Modern humans also use calcite for buildings. In Chicago, many of the city's landmark buildings have facades of limestone—a rock made of calcite minerals—including the Water Tower, the Art Institute, and the Chicago Cultural Center (Figure \(\PageIndex{3}\)). Most limestones formed in the sea from the accumulation of partially dissolved shells and skeletons of dead marine species on the seafloor; these marine layers were later uplifted to the surface by geologic processes. The formation of calcite limestone in itself has served a monumental function in the geologic history of Earth. It is this process that helped remove CO₂ from the atmosphere and store it as calcite and other carbonate minerals in the rocks; without this carbon sink Earth's atmospheric temperatures would have been too warm to sustain life. Not all uses of calcite are monumental. Tiny calcite crystals are used by industry as a texturizer in bubble gums, providing the desired crunchiness of chewing which in time dissolves into harmless carbonate water in the mouth. Nature is just as clever; evolution has placed tiny calcite crystals in the mammalian inner ear, including human's, to help organisms sense gravity and maintain body balance when tilted. These are only a few of the many functions of one important mineral. There are over 4,000 minerals on Earth.

Figure \(\PageIndex{3}\): Chicago Cultural Center. The building's exterior is made of calcite limestone, prized by architects for its softness for ornament carving and its grey color that blends into natural and urban landscapes. (Ajay Suresh via Wikimedia Commons; CC BY.)