- Related Topics:
- quartz
- opal
- coesite
- tridymite
- melanophlogite
silica mineral
-
What is silica mineral?
Silica minerals constitute about 26% of the Earth's crust and are second in abundance to feldspars. They are composed of silicon and oxygen, with silicon at 46.75% and oxygen at 53.25% by weight.
Silica minerals have a crystallographic structure consisting of three-dimensional linked tetrahedrons. Each tetrahedron contains one silicon atom and four oxygen atoms. When pure, silica minerals are colorless and transparent with a vitreous luster. They don't conduct electricity, are diamagnetic, hard, and strong, and experience brittle fractures when stressed.
Quartz is the only natural silica mineral used in significant quantities. It is used to make concrete and mortar. It is also crushed for use in road construction and roofing.
2 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
Where is silica found in nature?
Silica (silicon dioxide, SiO2) is a compound of silicon and oxygen, the two most abundant elements in Earth's crust. It constitutes 59% of the Earth’s crust mass and is the main component of over 95% of known rocks.
Free silica appears as a mineral when the abundance of SiO2 exceeds that of other cations available to form silicates, with the amount of crystallized silica increasing along with the silica content of magma.
Silica is found in crystalline forms such as quartz, tridymite, and cristobalite. It is also found in glass sands, river, lake, and beach sediments. Less common forms include the hydrous silica mineral opal, coesite, and stishovite.
3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
What are the most common forms of silica mineral?
Silica minerals, with the chemical composition SiO2, appear in various forms, including quartz, tridymite, cristobalite, coesite, and stishovite.
Quartz is the only natural silica mineral used in significant quantities. Cristobalite and tridymite are high-temperature forms of SiO2 and are found in high-temperature lava flows. Coesite and stishovite are high-pressure forms of SiO2.
These minerals consist of three-dimensional arrays of linked tetrahedrons, where each silicon atom is coordinated by four oxygen atoms. The geometry of these linkages differentiates the forms.
3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How is quartz related to silica?
Quartz is a crystalline form of silica, or silicon dioxide (SiO_2). Silica minerals include quartz, tridymite, cristobalite, coesite, stishovite, lechatelierite, and chalcedony. Quartz is the most abundant of these, constituting the majority of the Earth's crust.
Tectosilicates, like quartz, have a three-dimensional lattice of silicate tetrahedrons, where each tetrahedron shares its four oxygen atoms with others, resulting in chemical formulas that are multiples of SiO_2. Quartz is found in nearly all types of igneous, metamorphic, and sedimentary rocks, and is very resistant to weathering.
5 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
What are some main uses of silica in everyday life?
Silica (silicon dioxide, or SiO2) is one of the most abundant compounds on Earth, with a wide array of uses in everyday life.
Here are some of its main applications:
- Construction materials Silica sand is a key ingredient in Portland cement, concrete, mortar, and sandstone used in buildings and roads.
- Glassware and ceramics Silica is a raw material in the production of glass for windows, containers, tableware, and lightbulbs, as well as ceramics.
- Abrasives and polishing Silica is used to grind and polish glass and stone.
- Electronics Silicon derived from silica is an extremely important semiconductor in the electronics and technology sector and is used in computer chips, integrated circuits, transistors, silicon diodes and liquid crystal displays.
- Other applications Silica is also used in foundry molds, the manufacture of silicon carbide, ferrosilicon, and silicones, as a refractory material, as gemstones, and silica gel is often used as a desiccant to remove moisture.
5 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How can silica minerals impact human health and the environment?
Silica minerals, though common in rocks and sand, can significantly impact human health and the environment.
Inhaling silica dust over long periods can cause silicosis, a chronic lung disease characterized by shortness of breath, coughing, and weakness. This occurs as the lungs react to the dust, leading to fibrosis and damage. Occupations with high silica dust exposure, such as mining, quarrying, and sandblasting, pose the greatest risk.
There is no cure for silicosis, so prevention is critical. Using protective face masks, ensuring proper workplace ventilation, and conducting regular X-ray monitoring of workers' lungs can help reduce the incidence of the disease.
3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
silica mineral, any of the forms of silicon dioxide (SiO2), including quartz, tridymite, cristobalite, coesite, stishovite, lechatelierite, and chalcedony. Various kinds of silica minerals have been produced synthetically; one is keatite.
General considerations
Silica minerals make up approximately 26 percent of Earth’s crust by weight and are second only to the feldspars in mineral abundance. Free silica occurs in many crystalline forms with a composition very close to that of silicon dioxide, 46.75 percent by weight being silicon and 53.25 percent oxygen. Quartz is by far the most commonly occurring form. Tridymite, cristobalite, and the hydrous silica mineral opal are uncommon, and vitreous (glassy) silica, coesite, and stishovite have been reported from only a few localities. Several other forms have been produced in the laboratory but have not been found in nature.
Physical and chemical properties
The crystallographic structures of the silica minerals, except stishovite, are three-dimensional arrays of linked tetrahedrons, each consisting of a silicon atom coordinated by four oxygen atoms. The tetrahedrons are usually quite regular, and the silicon-oxygen bond distances are 1.61 ± 0.02 Å. Principal differences are related to the geometry of the tetrahedral linkages, which may cause small distortions within the silica tetrahedrons. High pressure forces silicon atoms to coordinate with six oxygen atoms, producing nearly regular octahedrons in the stishovite structure.
" height="20" alt="HEW Logo">
The silica minerals when pure are colourless and transparent and have a vitreous lustre. They are nonconductors of electricity and are diamagnetic. All are hard and strong and fail by brittle fracture under an imposed stress.
Some important physical properties of the silica minerals are compared in the Table. All except low tridymite and coesite (among the crystalline varieties) have relatively high symmetry. There is a linear relationship between the specific gravity values listed in the Table and the arithmetic mean of the indices of refraction (measures of the velocity of light that is transmitted in different crystallographic directions) for silica minerals composed of linked tetrahedrons. This relationship does not extend to stishovite because it is not made up of silica tetrahedrons. Melanophlogite is notable because it plots below vitreous silica on the graph. The specific gravities of silica minerals are less than those of most of the dark-coloured silicate minerals associated with them in nature; in general, the lighter-coloured rocks have lower specific gravity for this reason. Silica minerals are insoluble to sparingly soluble in strong acids except hydrofluoric acid, in which there is a correlation between specific gravity and solubility.
| phase | symmetry | specific gravity | hardness |
|---|---|---|---|
| quartz (alpha-quartz) | hexagonal; trigonal trapezohedral | 2.651 | 7 |
| high quartz (beta-quartz) | hexagonal; hexagonal trapezohedral | 2.53 at 600 degrees Celsius | 7 |
| low tridymite | monoclinic? | 2.26 | 7 |
| high tridymite | orthorhombic | 2.20 at 200 degrees Celsius | 7? |
| low cristobalite | tetragonal | 2.32 | 6–7 |
| high cristobalite | isometric | 2.20 at 500 degrees Celsius | 6–7 |
| keatite | tetragonal | 2.50 | ? |
| coesite | monoclinic | 2.93 | 7.5 |
| stishovite | tetragonal | 4.28 | ? |
| vitreous silica | amorphous | 2.203 | 6 |
| opal | poorly crystalline or amorphous | 1.99–2.05 | 5½–6½ |
Individual silica minerals
Quartz
Quartz occurs in many varieties in almost all types of igneous, sedimentary, and metamorphic rocks. It has also been found in meteorites and in some lunar rocks.
Quartz crystals lack a centre of symmetry or planes of symmetry and have one crystallographic axis (c) perpendicular to three polar axes (a) that are 120° apart. One end of a polar axis is different from its other end; when mechanical stress is applied on such an axis, opposite electrical charges develop on each end. This leads to important applications in electronics as a frequency control and in pressure gauges and other devices. The lack of symmetry planes parallel to the vertical axis allows quartz crystals to occur as two types: left-handed or right-handed (enantiomorphism). Left-handed quartz is less than 1 percent more abundant than right-handed quartz. The structural tetrahedrons spiral upward through the crystal in the sense of the handedness parallel to the c axis. Similarly, if polarized light is transmitted by a quartz crystal along the c-axis direction, the plane is rotated in the direction of the handedness by tens of degrees per millimetre, the amount depending on the wavelength of the light. This property is used in optical instruments such as monochromators.
The trusted destination for professionals, college students, and lifelong learners.
SUBSCRIBE
The requirements for high-purity crystals for electronic frequency applications have led to the development of industrial synthesis methods based on the differences in silica solubility with temperature. Nutrient crystals are placed in the hot portion of a solution-filled autoclave, and an oriented seed crystal free from twinning is placed in the cooler portion. Crystals weighing thousands of grams can be grown in a few weeks.
Quartz shows less range in chemical composition than do most other minerals, but it commonly contains tens to hundreds of parts per million of aluminum atoms substituting for silicon atoms, with charge balance maintained by the incorporation of small atoms, such as hydrogen, lithium, or sodium. Titanium, magnesium, or iron atoms substituting for silicon atoms also have been reported, but anionic substitution (i.e., substitution for the negative ion, oxygen) is limited because the linkage of the tetrahedrons is disrupted.
Coloured varieties of quartz are numerous and have many causes. Most colours result from mechanically incorporated admixtures within fine-crystallized or granular quartz, but some coarse-crystallized varieties, such as amethyst (violet), citrine (yellow), milky quartz, smoky quartz or morion (black), or rose quartz, may be coloured by ions other than silicon and oxygen that occur within the crystal structure. Small fractions of 1 percent by weight of iron, aluminum, manganese, titanium, hydrogen, and small alkali atoms, such as lithium and sodium, have been shown to be the cause of different colours. Heat treatment or various irradiation treatments under oxidizing or reducing atmospheres are used to change one coloured variety to another. Citrine is commonly produced by heat-treating amethyst at 250–400 °C (482–752 °F), for example.
Quartz may contain inclusions of other minerals, such as rutile (rutilated quartz), tourmaline, asbestiform amphiboles, or platy minerals, such as mica, iron oxides, or chlorite (aventurine).