Quartz and silica

Silica is one of the most common substances in the earth’s crust and mantle. It is composed of silicon dioxide tetrahedra that bond according to unique atomic characteristics to form crystalline structures. Quartz is a crystalline form of silica that forms under conditions of high temperatures and pressures in liquid environments. Silica and quartz are used for a variety of industrial applications, including chemistry and construction, and in the development of fiber-optic technologies.

Silicon Dioxide

Silicon dioxide (SiO₂), also called silica, is an abundant chemical substance in hundreds of different types of minerals and rocks. It forms a majority of Earth’s lithosphere.

The upper layers of the earth consist of the rocky crust, which comprises the outer 30 to 90 kilometers (19 to 56 miles) of the terrestrial environment and 5 to 8 km (3 to 5 mi) of the ocean floor. Below the crust is the mantle, a layer of ductile rock that reaches from the outer core of the earth to the bottom of the crust and comprises more than two-thirds of Earth’s mass. Silica accounts for more than 80 percent of the crust and approximately 50 percent of the mineral contained within the mantle, making it the most abundant single mineral compound on the planet.

Within the earth’s crust, silica usually appears as quartz, a crystalline form of silicon dioxide. Quartz is the second most abundant mineral in the earth’s crust, second only to feldspar, which is another mineral composed almost entirely of silica and included minerals. Quartz is composed of silica molecules organized into a tetrahedral pattern, which lends quartz its distinct six-sided crystalline shape, topped with a six-sided pyramid at the leading end of each crystal. Sand found on beaches is composed almost entirely of fragmented quartz crystals, blended with other elements in smaller quantities.

Both silica and quartz have been used in a variety of industrial and technological applications. Silica is a component of many building materials and has been used in the manufacture of microelectronic components. Quartz crystals are used in the manufacture of watches. Some of the more attractive varieties of quartz are prized for their aesthetic properties and are used in the manufacture of jewelry.

Silica is also melted and polished to make a type of glass called fused quartz or fused silica, which is used in the manufacture of eyeglasses and other optical lenses. Fused silica is also used in the construction of optical fibers for telecommunications applications. Optical fibers are superior to metal fibers at transmitting electrical signals because of the chemical properties of silica, which make the material largely immune to the effects of electromagnetic interference. Fiber optics have been used in a variety of applications, from lighting and telecommunications to computing. Quartz also is the ore used in the development of silicon computer chips.

Chemistry of Silica and Quartz

Silicon dioxide forms spontaneously at a variety of temperatures and pressures when silicon atoms encounter atomic oxygen. Oxygen atoms form covalent bonds to a silicon atom such that four atoms of oxygen are organized around each silicon atom in a tetrahedron or pyramid-shaped array.

The connections between silicon and oxygen and between molecules of silicon dioxide create flexible bonds that allow silicon dioxide to organize into a variety of shapes. By controlling the temperature and pressures at which silicon and oxygen interact, it is possible to develop silicate forms with a variety of structures.

Each tetrahedral molecule of silica then bonds to other silica molecules to form a three-dimensional structure. The nature of the crystal structure depends on the angle at which the silica molecules bond. Typically, molecules bond at an angle of between 140 to 160 degrees. Quartz crystals form when silica molecules bond at angles between 143.6 and 153 degrees.

Once formed, silica is highly stable and will resist further chemical reactions because of the strength of the covalent silicon-oxygen bonds within the crystal structure. Silica glass, which is a pure form of quartz glass containing more than 99 percent silica, is therefore used for a number of chemical applications, providing an inert environment that can contain chemical substances without becoming reactive.

Crystal Structure and Formation

Quartz comes in two basic varieties, based on the size of the crystals within a deposit. Macrocrystalline quartz contains crystals that can be viewed with the naked eye, while crypto- or microcrystaline quartz are a form in which the constituent crystals can be viewed only with a microscopic viewing aid.

Microcrystaline quartz can further be broken down into two basic types, fibrous and grainy, based on the appearance of thin layers of material viewed through a polarizing microscope. The difference in structure between macro- and microcrystalline quartz is related to the environment in which the crystals form.

Most macrocrystalline quartz forms either in heated, silica-rich water solutions or within igneous rock deposits from silica-rich magma. Macrocrystalline quartz is a primary component of many rock types, meaning that it was one of the basic components present during the formation of the rock. Individual crystals of quartz are generally considered a secondary product, forming within quartz rocks or within other rocks that contain quartz as part of their overall structure.

Macrocrystalline quartz grows in hydrothermal environments, where water temperature is between 100 and 450 degrees Celsius (212 to 842 degrees Fahrenheit) with correspondingly high pressure. In this environment, rocks containing silica dissolve to produce orthosilicic acid (H₄SiO₄), which reacts to form layers of crystalline silica, giving off water in the process. If quartz grains are already present in the solution, these new layers of crystallized silica will add to existing layers, building into larger crystals in the process.

If no preformed crystals are present, then crystalline silica can bond together to form new crystals, though this occurs only in environments in which the temperature and pressure slowly decrease. This is largely because silica tetrahedra are continually being both added and removed from the aggregating crystals. For crystals to form, there must be sufficient dissolved silica in the environment, but macrocrystalline structures will not form if silica saturation rises above a certain critical level. At this point, crystal aggregation occurs so frequently throughout the entire medium that only tiny, microcrystalline structures will form.

In environments closer to the surface of the earth, the medium will cool more rapidly, and fewer crystals will form. In some situations, crystal-forming aqueous environments develop within a solid host rock, which may lead to large crystals as the heat gradually escapes, buffered by the surrounding rock layer. Extremely large quartz deposits result from tectonic collisions, which can drive sediment from deep within the mantle or crust toward the surface through millions of years. In this situation, temperatures and pressures slowly change, and pockets of heated water may be able to generate quartz crystals for thousands of years before crystal formation ceases.

In molten rock, silica does not exist as an individual tetrahedral but forms instead into long chains of tetrahedra that move together through the molten rock. Silica chains of this type cause the viscosity of flowing magma and lava, and magma with higher proportions of silica will be more viscous. Whether cooling magma will form quartz crystals depends on the chemical composition of the magma and the conditions under which the magma cools.

Magma that erupts onto the surface and cools rapidly will not form crystals because insufficient time exists for the bonds between tetrahedra in the silica chains to break and then re-form into crystallized structures. Lava therefore gives rise to either porous volcanic rock or, when the lava cools extremely rapidly, glassy structures like obsidian. Similarly, high-viscosity magma, which is rich in silica chains, will not generally form into crystals and usually gives rise to other types of silicate mineral. Alternatively, when molten magma cools beneath the surface, cooling occurs gradually, forming into a variety of silica-based rock types. In granite, for instance, mica, feldspar, and quartz are the primary types of silicate crystals that form as the granitic magma cools. Quartz is usually the last type of crystal to form in a sample of granite and therefore occurs in pockets within the stone.

Microcrystalline structures, sometimes called chalcedony, form in aqueous environments through a similar mechanism as macrocrystalline quartz, but they tend to form under different basic conditions. Higher saturation of silica tetrahedra favors the development of microcrystalline structures. Microcrystalline quartz also tends to form in environments where the overall temperature is below 150 degrees Celsius (320 degrees Fahrenheit); microcrystal growth is highly inhibited at temperatures above 200 degrees Celsius (395 degrees Fahrenheit).

In addition, microcrystalline structures tend to grow only in aqueous environments and are inhibited by environments in which there is insufficient water for the formation of dissolved silica tetrahedra. For this reason, microcrystalline structures are largely absent from magmatic rocks and other types of igneous rock deposits.

Varieties of Quartz

There are many varieties of quartz, all based on the size and shape of the crystals and on the inclusion of other minerals and elements that cause variations in color and texture. Common types of macrocrystalline quartz include rock crystal, amethyst, milky quartz, and smoky quartz.

Rock crystal is the name for basic, colorless macrocrystalline quartz, consisting of 99.5 percent or more silica. Rock crystal quartz can be found around the world and in many types of geological environments.

A valuable and highly prized type of quartz is amethyst, a variety of macrocrystalline quartz with included pockets of purple, red, or violet formed by the presence of embedded iron molecules. When these iron molecules are exposed to radiation during formation, the oxygen that is bonded to silicon oxidizes in such a way to produce the distinctive color. Amethyst remains highly sensitive to radiation; the violet hues will fade in amethyst exposed to sunlight and other forms of radiation. The color in most amethyst is concentrated at the pyramidal heads of the crystals, though in some cases iron can form into waves of color called phantoms within the body of the crystals.

Iron present in quartz also can produce ametrine, a type that varies in colors and often includes both yellow and violet sections within a single crystal. The yellow areas reflect yellow light because of the higher concentration of iron atoms within the lattice.

Smoky quartz is named for its gray to brown color formed by the irradiation of aluminum molecules within the lattice. The color in smoky quartz can occur in isolated areas or can affect the entire crystal structure. The color in smoky quartz forms long after the initial formation of the crystal, after extended exposure to irradiation chemically transforms the included aluminum ions through oxidation, thereby altering their structure and interaction with light. Smoky quartz can be artificially created by exposing quartz with included aluminum to radiation.

Milky quartz has a translucent white color that forms from the inclusion of numerous microscopic chambers containing liquid or gas molecules that alter the way the crystal reacts to light. Milky quartz has been discovered in a variety of environments and ranges in color from slightly translucent to specimens that are nearly solid white and opaque. Because inclusions tend to distort crystal growth, milky quartz exhibits unusual morphological shapes, with crystals often shifting in their growth patterns as the crystal develops.

One of the most common types of microcrystalline quartz is agate, which is a translucent, multicolored rock that exhibits bands of different colors organized parallel to the surface of the rock. Agate does not appear to have a crystalline structure unless examined with a high-powered electron or other type of microscope. Agate is not composed of pure silica, and the colored bands result from inclusions of various types of minerals and other rock types. Agates can have a variety of colors, from greens and reds to shades of brown and gray.

One of the most common types of microcrystalline quartz is flint, which is translucent to opaque and exhibits no obvious crystalline structure. Flint is usually dark brown, gray, or tan. Color varieties are caused by included minerals and oxides, which typically include iron, aluminum, and other metals.

One of the most unusual properties of flint is that it exhibits conchoidal fracturing, which is fracturing that does not follow a standard plane of separation. Conchoidal fractures and flint fractures, which are formed similar to glass fractures, have made flint useful in a variety of applications. Because it is fine grained and can be sharpened because of its fracturing pattern, flint was used to manufacture arrowheads and other types of weaponry, making it one of the most anthropologically important rocks on Earth.

Principal Terms

amethyst: a variety of quartz noted for its violet to purple color produced by irradiated molecules of iron contained within its crystalline structure

chalcedony: a form of cryptocrystalline quartz formed from microcrystals of quartz and other included minerals

conchoidal fracture: fracturing of the type exhibited by materials that do not break along planes of separation

feldspar: a silicate rock that forms in igneous environments and is the most common mineral in the earth’s crust

fiber optics: flexible transparent fiber made of fused quartz or silica glass; used in computer and telecommunications applications

flint: a type of microcrystalline quartz that exists in large quantities within the earth’s crust and is notable for its glass-like characteristics

igneous rock: rocks formed from magma that cools either on the earth’s surface or in pockets within the mineral layers of the earth’s crust

quartz: crystalline form of silica that develops when silica is heated in a liquid or semiliquid environment and then slowly cooled

silicates: compounds originating from tetrahedra of silicon dioxide

tetrahedron: a polyhedron composed of four triangular sides organized into pyramidal shape

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