Silicates

Silicate Units, Silicate Chains, Silicate Sheets

The basic unit of silicate, [SiO4]4- tetrahedron, can form single and double chains and sheets.

Learning Objectives

Describe the various structures of silicates.

Key Takeaways

Key Points

  • The basic building block of all silicate minerals is the [SiO4]4− tetrahedron.
  • Silicate minerals containing chains are termed inosilicates. In single chains (SiO32−)n, the silicon to oxygen ratio is 1:3, while in double chains (Si4O116−)n, the silicon to oxygen ratio is 4:11.
  • The formula of silicate sheet is (Si2O52−)n. Silicate minerals containing sheets are termed phyllosilicates.

Key Terms

  • Inosilicates: Inosilicates, or chain silicates, have interlocking chains of silicate tetrahedrons.
  • Silicate: A silicate (SiO44-) is a compound containing a silicon-bearing anion.
  • Phyllosilicates: Sheet silicate minerals, formed by parallel sheets of silicate tetrahedrons Si2O52-.

Silicate Tetrahedrons

The basic building block of all silicate minerals is the [SiO4]4− tetrahedron. There are four covalent Si−O bonds. Each oxygen atom forms one vertex of the tetrahedron. The silicon to oxygen atom ratio is 1:4.

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Silicate Tetrahedron: Ball-and-stick model of the silicate tetrahedron; red represents oxygen atoms and gray represents the silicon atom in the center.

Silicate minerals containing isolated [SiO4]4− tetrahedrons are called nesosilicates or orthosilicates.

Corner-Sharing Tetrahedrons

If two [SiO4]4− tetrahedrons share an oxygen atom at one common vertex, an [Si2O7]6− ion is formed. The silicon to oxygen ratio is 2:7. Silicate minerals containing isolated [Si2O7]6− double tetrahedrons are called sorosilicates.

Silicate Chains

Silicate minerals containing chains are termed inosilicates. They consist of single chains (SiO32−)n, in which the silicon to oxygen atom ratio is 1:3, and double chains (Si4O116−)n, in which the silicon to oxygen atom ratio is 4:11.

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Double Chain: Ball-and-stick model of silicate double chains. Red balls correspond to oxygen, and gray to silicon atoms.

Asbestos

Asbestos (from Greek ἅ, unquenchable) is a group of fibrous silicate minerals containing double chains. Prolonged exposure to dust containing fibres from certain types of asbestos is now known to cause scarring of the lungs, lung cancer, and a particularly aggressive cancer called mesothelioma. Mesothelioma is almost always fatal, with a median survival time of 11 months. Due to the exceptional danger posed by some absestos, certain counties now require all work involving asbestos to be done by specialist companies. The vast majority of asbestos is the so-called white form, which is not known to pose any real danger.

Silicate Sheets

SiO4 tetrahedrons can be arranged to form sheets. The formula of such a sheet can be written (Si2O52−)n. Silicate minerals containing sheets are termed phyllosilicates.

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Silicate Sheet: The formula of silicate sheet is (Si2O52−)n.

Three-dimensional Frameworks

Perhaps the most structurally complicated silicates are those based on networks of Si and O that extend in all three dimensions. Examples of such minerals include quartz, zeolites, and feldspars. Silicate minerals containing three-dimensional frameworks are termed tectosilicates.

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Three-dimensional structure of zeolite: In the mineral zeolite, silica and oxygen atoms are bonded layers of sheets.

Properties of Quartz and Glass

Glass is a non-crystalline solid material made of silica, while quartz is a crystalline silicate mineral with piezoelectric properties.

Learning Objectives

Discuss the properties of glass and quartz.

Key Takeaways

Key Points

  • Glass is a non- crystalline, often brittle, transparent solid material made of silica (SiO2) and other minor additives.
  • Glass has the ability to refract, reflect, and transmit light according to the principles of geometrical optics.
  • Color in glass may be obtained by adding electrically charged ions that are homogeneously distributed, or by precipitating finely dispersed particles.
  • Quartz is an abundant mineral made up of a continuous framework of SiO4 tetrahedra.
  • Quartz crystals have piezoelectric properties: they develop an electric potential with the application of mechanical stress. Today, a crystal oscillator is a common piezoelectric use for quartz.
  • Pure quartz is colorless and transparent or translucent. Microcrystal varieties are mostly opaque, while macrocrystals tend to be transparent.

Key Terms

  • glass: A solid, transparent substance made by melting sand with a mixture of soda, potash, and lime.
  • quartz: The most abundant mineral on the earth’s surface. Its chemical composition is silicon dioxide, SiO2. It occurs in a variety of forms, both crystalline and amorphous. It is found in every environment.
  • piezoelectricity: The ability of certain crystals to generate a voltage in response to applied mechanical stress.

Glass

Glass is an amorphous (non-crystalline) solid material. Glasses are typically brittle and optically transparent. The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica (SiO2) with the addition of sodium oxide (Na2O) from soda ash, lime (CaO), and several minor additives.

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Natural Glass: Moldavite, a natural glass formed by meteorite impact, from Besednice, Bohemia.

Optical properties

Glass is in widespread use largely due to the production of glass compositions that are transparent to visible wavelengths of light. In contrast, polycrystalline materials do not in general transmit visible light. The individual crystallites may be transparent, but their facets (grain boundaries) reflect or scatter light, resulting in diffuse reflection. Glass does not contain the internal subdivisions associated with grain boundaries in polycrystals, so it does not scatter light in the same manner as a polycrystalline material. The surface of a glass is often smooth—during glass formation, the molecules of the supercooled liquid are not forced to dispose in rigid crystal geometries. The molecules can follow surface tension, which imposes a microscopically smooth surface. These properties, which give glass its clearness, can be retained even if glass is partially light-absorbing or colored.

Glass has the ability to refract, reflect, and transmit light according to the principles of geometrical optics. Common glass has a refractive index of 1.5. According to the Fresnel equations, the reflectivity (the amount of light that gets reflected off the air-glass interface) of a sheet of glass is about 4% per surface (at normal incidence in air). This means the amount of light that gets transmitted through a glass surface (the transmissivity) is 96%. The transmissivity of a glass element with two surfaces is about 92%.

Glass also finds application in optoelectronics for light-transmitting optical fibers.

Color

Color in glass may be obtained by adding electrically charged ions that are homogeneously distributed, or by precipitating finely dispersed particles (such as in photochromic glasses). Ordinary soda-lime glass appears colorless to the naked eye when it is thin, although iron (II) oxide (FeO) impurities of up to 0.1 % by weight produce a green tint. This is seen in thick pieces or with the aid of scientific instruments. Manganese dioxide can be added in small amounts to remove the green tint given by iron(II) oxide. FeO and Cr2O3 additions may be used for the production of green bottles. Sulfur, together with carbon and iron salts, is used to form iron polysulfides and produce amber glass ranging from yellowish to almost black. A glass melt can also acquire an amber color from a reducing combustion atmosphere.

When used in art glass or studio glass, glass is colored using closely guarded recipes that involve specific combinations of metal oxides, melting temperatures, and ‘cook’ times. Most colored glass used in the art market is manufactured in volume by vendors, although there are some glass makers with the ability to make their own color from raw materials.

Quartz

Quartz is an abundant mineral in the Earth’s continental crust. It is made up of a continuous framework of SiO4 silicon– oxygen tetrahedra. Each oxygen atom is shared between two tetrahedra, giving an overall formula of SiO2. There are many different varieties of quartz, several of which are semi-precious gemstones.

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Quartz: This diagram shows the crystal structure of quartz. Silicon atoms are grey, and oxygen atoms are red.

Physical properties

Owing to its abundance and high thermal and chemical stability, quartz is widely used in many large-scale applications—abrasives, foundry materials, ceramics, and cements. Quartz crystals have piezoelectric properties. Piezoelectricity is the ability to develop an electric potential upon the application of mechanical stress. An early use of this property of quartz crystals was in phonograph pickups, where the mechanical movement of the stylus in the groove generates a proportional electrical voltage by creating stress within a crystal.

Today, a crystal oscillator is a common piezoelectric use for quartz: the vibration frequency of the crystal is used to generate an electrical signal of very precise frequency. This is employed in many modern electronic devices (wristwatches, clocks, radios, computers, cellphones) to keep track of time or provide a stable clock signal for digital circuits.

Color

Pure quartz, traditionally called rock crystal (sometimes called clear quartz), is colorless and transparent or translucent. Common colored varieties include citrine, rose quartz, amethyst, smoky quartz, and milky quartz.

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Rose Quartz Crystal: Rose quartz crystals, Minas Gerais.

The cryptocrystalline (crystals barely visible under microscope) varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline (large crystals identified by sight). Chalcedony is a cryptocrystalline form of silica consisting of fine intergrowths of quartz and its monoclinic polymorph, moganite. Other opaque gemstone varieties of quartz—or mixed rocks including quartz—often include contrasting bands or patterns of color. These include agate, onyx, carnelian, and jasper.

Aluminosilicates

Aluminosilicate minerals are composed of aluminum, silicon, and oxygen.

Learning Objectives

Identify composition of aluminosilicates and differentiate their polymorphs.

Key Takeaways

Key Points

  • Andalusite, kyanite, and sillimanite are naturally occurring aluminosilicate minerals that have the composition Al2SiO5. Each of these minerals occur under different temperature – pressure regimes, and can thus be used to identify the pressure-temperature paths of their host rocks.
  • Hydrated aluminosilicate minerals are referred to as zeolites. They are porous structures that are naturally occurring materials.
  • Calcium aluminosilicate and sodium aluminosilicate are common food additives.

Key Terms

  • Aluminosilicate: Mineral composed of aluminum, silicon, and oxygen, plus countercations.
  • Zeolite: Microporous aluminosilicate mineral commonly used as commercial absorbent.

Aluminosilicate minerals are composed of aluminum, silicon, oxygen, and countercations. They are a major component of kaolin and other clay minerals.

Andalusite, Kyanite, and Sillimanite

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Andalusite: Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.

Andalusite, kyanite, and sillimanite are all naturally occurring aluminosilicate minerals that have the composition Al2SiO5. The triple point of these three polymorphs is located at a temperature of 500 °C and a pressure of 0.4 GPa. These three minerals are commonly used as index minerals in metamorphic rocks. Each of these minerals occurs under different temperature-pressure regimes, and they are therefore rarely found together in the same rock. Because of this, the three minerals are a useful tool in identifying the pressure-temperature paths of the host rock in which they are found.

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Phase Diagram of Aluminosilicate Mineral: The phase diagram of Al2SiO5 showing its different forms (called “polymorphs”). (Note: What is referred to as “Cianite” in the figure corresponds to “kyanite” in the text.)

Zeolites

Hydrated aluminosilicate minerals are referred to as zeolites. They have porous structures and are naturally occurring materials. Zeolites are commonly used as commercial absorbents. Zeolites are used for a variety of tasks, including purifying water, catalyzing reactions, preparing certain advanced materials, and nuclear reprocessing. They are used to extract nitrogen from air, which then increases the general oxygen content for various industrial and medical purposes. They are used most frequently in the production of laundry detergents, but are also used in medicine and in agriculture.

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Zeolite: Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents.

Calcium Aluminosilicates

Calcium aluminosilicate, an aluminosilicate compound with calcium cations, most typically has the chemical formula, CaAl2Si2O8. As a food additive, it is sometimes designated “E556”. In minerals, as in feldspar, it can be found as anorthite, an end-member of the plagioclase series.

Sodium Aluminosilicates

Sodium aluminosilicates are acidic salts that is composed of sodium, aluminum, silicon and oxygen. These can be found as synthetic, amorphous, sodium aluminosilicates, a few naturally-occurring minerals, and synthetic zeolites. Synthetic, amorphous, sodium aluminosilicate is widely used as a food additive, E-554.