Quartz

Quartz is a hard mineral composed of silica, also known as silicon dioxide. It is a vital component of the Earth's lithosphere, making up about 12% of its mass. Structurally, quartz is classified as a framework silicate mineral. Compositionally, it is classified as an oxide mineral. Its atoms are linked in a continuous framework of SiO4 silicon–oxygen tetrahedra. In this structure, each oxygen atom is shared between two tetrahedra. This specific arrangement gives quartz its chemical formula of SiO2.

Quartz exists in two distinct forms called polymorphs. These are α-quartz (alpha-quartz) and β-quartz (beta-quartz). Alpha-quartz is the stable form at room temperature. It crystallizes in the trigonal crystal system. Beta-quartz is the high-temperature form. It possesses greater symmetry and crystallizes in the hexagonal crystal system. The transformation from alpha to beta occurs abruptly at a specific temperature threshold. This transition involves a minor rotation of the tetrahedra. However, it also causes a significant change in volume. This volume shift can induce microfracturing in rocks or ceramics.

Both forms of quartz are chiral. This means they have a specific handedness in their structure. They are composed of achiral SiO4 building blocks. The two forms can exist in different space groups based on their chirality. Above the transition temperature, alpha-quartz in space group 152 becomes beta-quartz in space group 181. Similarly, alpha-quartz in space group 154 becomes beta-quartz in space group 180. Because beta-quartz is unstable at room temperature, all quartz found at Earth's surface is alpha-quartz.

Human interest in quartz dates back to antiquity. The Ancient Greeks called it krystallos, meaning "crystal." This came from the word for "icy cold." Many early philosophers believed quartz was a form of supercooled ice. The Roman naturalist Pliny the Elder shared this belief. He thought quartz was ice permanently frozen over long periods. He noted that quartz was often found near Alpine glaciers. This idea remained common until at least the 17th century. Later, Nicolas Steno's research helped create the field of modern crystallography. He discovered the law of constancy of interfacial angles. He proved that quartz prism faces always meet at a 60° angle.

Quartz is defined by its hardness on the Mohs scale. It holds a value of 7 on this qualitative scratch method. This makes it a very durable mineral. Because of this durability, people in Europe and Asia have used it for jewelry and carvings since ancient times. The mineral can appear in many shapes. The ideal shape is a six-sided prism with pyramid-like rhombohedrons at the ends. In nature, crystals may be twinned or distorted. They can also form as a druse, which is a layer of crystals lining a void. Geodes are famous examples of these crystal layers.

Many beautiful varieties of quartz exist due to chemical impurities. Clear quartz, or rock crystal, is colorless and transparent. Other colors come from electronic transitions caused by impurities. Amethyst is a purple variety caused by iron impurities and natural irradiation. Citrine is a yellow variety, though much commercial citrine is heat-treated amethyst. Rose quartz is a pink variety containing microscopic inclusions. Other examples include smoky quartz and milky quartz. Some varieties are categorized by the size of their crystals. Macrocrystalline quartz has crystals visible to the eye. Microcrystalline and cryptocrystalline varieties are often grouped as chalcedony.

Chalcedony is a specific form of silica consisting of fine intergrowths. It contains quartz and a polymorph called moganite. This group includes many famous stones. Agate is a banded, multicolored variety of chalcedony. Onyx is a monochromatic variety containing carbon impurities. Carnelian is an orange to red variety of chalcedony. Jasper is an opaque, microcrystalline variety that is often red or brown. Some rare forms even exist, such as cotterite. Cotterite is an extremely rare, silvery metallic quartz found in Ireland. These diverse forms show how small changes in chemistry create vast variety.