Silicate minerals

Silicate minerals are rock-forming minerals with predominantly silicate anions. They are the largest and most important class of rock-forming minerals and make up approximately 90 percent of the Earth's crust.[1][2][3]

In mineralogy, silica, or silicon dioxide SiO2, which corresponds to x=2 in the general formula, is usually considered a silicate mineral—even though its silicate "anion" has no negative charge and it has no cations. Silica is found in nature as the mineral quartz, and its polymorphs.

On Earth, a wide variety of silicate minerals occur in an even wider range of combinations as a result of the processes that have been forming and re-working the crust for billions of years. These processes include partial melting, crystallization, fractionation, metamorphism, weathering, and diagenesis.

Diatomaceous Earth BrightField
Diatomaceous earth, a biogenic form of silica as viewed under a microscope. The imaged region measures approximately 1.13 by 0.69 mm.

Living things also contribute to this geologic cycle. For example, a type of plankton known as diatoms construct their exoskeletons ("tests") from silica extracted from seawater. The tests of dead diatoms are a major constituent of deep ocean sediment, and of diatomaceous earth.

Chrysocolla
Copper silicate mineral chrysocolla.

General structure

A silicate mineral is generally an ionic solid whose anions consist predominantly of silicon and oxygen atoms.

In most minerals in the Earth's crust, each silicon atom is the center of an ideal tetrahedron, whose corners are four oxygen atoms covalently bound to it. Two adjacent tetrahedra may share a vertex, meaning that the oxygen atom is a bridge connecting the two silicon atoms. An unpaired vertex represents an ionized oxygen atom, covalently bound to a single silicon atom, that contributes one unit of negative charge to the anion.

Some silicon centers may be replaced by atoms of other elements, still bound to the four corner oxygen corners. If the substituted atom is not normally tetravalent, it usually contributes extra charge to the anion, which then requires extra cations. For example, in the mineral orthoclase [KAlSi
3
O
8
]
n
, the anion is a tridimensional network of tetrahedra in which all oxygen corners are shared. If all tetrahedra had silicon centers, the anion would be just neutral silica [SiO
2
]
n
. Replacement of one every four silicon atoms by an aluminum atom results in the anion [AlSi
3
O
8
]
n
, whose charge is neutralized by the potassium cations K+
.

Main groups

In mineralogy, silicate minerals are classified into 7 major groups according to the structure of their silicate anion:[4][5]

Major group structure chemical formula example
Nesosilicates isolated silicon tetrahedra [SiO4]4− olivine.
Sorosilicates double tetrahedra [Si2O7]6− epidote, melilite group.
Cyclosilicates rings [SinO3n]2n tourmaline group.
Inosilicates single chain [SinO3n]2n pyroxene group.
Inosilicates double chain [Si4nO11n]6n amphibole group.
Phyllosilicates sheets [Si2nO5n]2n micas and clays.
Tectosilicates 3D framework [AlxSiyO(2x+2y)]x quartz, feldspars, zeolites.

Note that tectosilicates can only have additional cations if some of the silicon is replaced by an atom of lower valence such as aluminium. Al for Si substitution is common.

Nesosilicates or orthosilicates

Silicate-tetrahedron-3D-balls
Orthosilicate anion SiO4−
4
. The grey ball represents the silicon atom, and the red balls are the oxygen atoms.
Nesosilicates exhibit, Museum of Geology, South Dakota
Nesosilicate specimens at the Museum of Geology in South Dakota

Nesosilicates (from Greek νῆσος nēsos, island), or orthosilicates, have the orthosilicate ion, which constitute isolated (insular) [SiO4]4− tetrahedra that are connected only by interstitial cations. The Nickel–Strunz classification is 09.A –examples include:

Kyanite crystals
Kyanite crystals (unknown scale)

Sorosilicates

Silicate-double-tetrahedra-3D-balls
Pyrosilicate anion Si
2
O6−
7
.
Sorosilicates exhibit, Museum of Geology, South Dakota
Sorosilicate exhibit at Museum of Geology in South Dakota

Sorosilicates (from Greek σωρός sōros, heap, mound) have isolated pyrosilicate anions Si
2
O6−
7
, consisting of double tetrahedra with a shared oxygen vertex—a silicon:oxygen ratio of 2:7. The Nickel–Strunz classification is 09.B. Examples include:

Cyclosilicates

Cyclosilicate exhibit, Museum of Geology, South Dakota
Cyclosilicate specimens at the Museum of Geology, South Dakota

Cyclosilicates (from Greek κύκλος kuklos, circle), or ring silicates, have three or more tetrahedra linked in a ring. The general formula is (SixO3x)2x, where one or more silicon atoms can be replaced by other 4-coordinated atom. The silicon:oxygen ratio is 1:3. Double rings have the formula (Si2xO6x)2x. The Nickel–Strunz classification is 09.C. Possible ring sizes include:

Beryll.ring.combined

6 units [Si6O18], beryl (red: Si, blue: O)

Benitoid.2200

3 units [Si3O9], benitoite

Papagoite.2200

4 units [Si4O12], papagoite

Eudialyte.2200

9 units [Si9O27], eudialyte

Milarite

6 units, double ring [Si6O18], milarite

Some example minerals are:

  • 3-member single ring
  • 4-member single ring
  • 6-member single ring
  • 9-member single ring
    • EudialyteNa
      15
      Ca
      6
      (Fe,Mn)
      3
      Zr
      3
      SiO(O,OH,H
      2
      O)
      3
      (Si
      3
      O
      9
      )
      2
      (Si
      9
      O
      27
      )
      2
      (OH,Cl)
      2
  • 6-member double ring
    • Milarite – K2Ca4Al2Be4(Si24O60)H2O

Note that the ring in axinite contains two B and four Si tetrahedra and is highly distorted compared to the other 6-member ring cyclosilicates.

Inosilicates

Inosilicates (from Greek ἴς is [genitive: ἰνός inos], fibre), or chain silicates, have interlocking chains of silicate tetrahedra with either SiO3, 1:3 ratio, for single chains or Si4O11, 4:11 ratio, for double chains. The Nickel–Strunz classification is 09.D – examples include:

Single chain inosilicates

Double chain inosilicates

Inosilicate, pyroxene family, with 2-periodic single chain (Si2O6), diopside

Inosilicate, clinoamphibole, with 2-periodic double chains (Si4O11), tremolite

Inosilicate, unbranched 3-periodic single chain of wollastonite

Inosilicate with 5-periodic single chain, rhodonite

Inosilicate with cyclic branched 8-periodic chain, pellyite

Phyllosilicates

Phyllosilicates (from Greek φύλλον phyllon, leaf), or sheet silicates, form parallel sheets of silicate tetrahedra with Si2O5 or a 2:5 ratio. The Nickel–Strunz classification is 09.E. All phyllosilicate minerals are hydrated, with either water or hydroxyl groups attached.

KaolinUSGOV
Kaolinite

Examples include:

Muskovite.sheet

Phyllosilicate, mica group, muscovite (red: Si, blue: O)

Apophyllite.sheet

Phyllosilicate, single net of tetrahedra with 4-membered rings, apophyllite-(KF)-apophyllite-(KOH) series

Pyrosmalite.sheet

Phyllosilicate, single tetrahedral nets of 6-membered rings, pyrosmalite-(Fe)-pyrosmalite-(Mn) series

Zeophyllite.sheet

Phyllosilicate, single tetrahedral nets of 6-membered rings, zeophyllite

Carletonite.sheet

Phyllosilicate, double nets with 4- and 6-membered rings, carletonite

Tectosilicates

Beta-quartz-CM-2D-balls
Silica family (SiO2 3D network), β-quartz.
Zeolite-ZSM-5-vdW
The 3D aluminosilicate anion of synthetic zeolite ZSM-5.
Lunar Ferroan Anorthosite (60025)
Lunar ferroan anorthosite (plagioclase feldspar) collected by Apollo 16 astronauts from the Lunar Highlands near Descartes Crater

Tectosilicates, or "framework silicates," have a three-dimensional framework of silicate tetrahedra with SiO2 or a 1:2 ratio. This group comprises nearly 75% of the crust of the Earth.[6] Tectosilicates, with the exception of the quartz group, are aluminosilicates. The Nickel–Strunz classifications are 09.F and 09.G, 04.DA (Quartz/ silica family). Examples include:

See also

References

  1. ^ "Mineral - Silicates". britannica.com. Archived from the original on 25 October 2017. Retrieved 8 May 2018.
  2. ^ Deer, W.A.; Howie, R.A.; Zussman, J. (1992). An introduction to the rock-forming minerals (2nd ed.). London: Longman. ISBN 0-582-30094-0.
  3. ^ Hurlbut, Cornelius S.; Klein, Cornelis (1985). Manual of Mineralogy (20th ed.). Wiley. ISBN 0-47180580-7.
  4. ^ Deer, W.A.; Howie, R.A., & Zussman, J. (1992). An introduction to the rock forming minerals (2nd edition ed.). London: Longman ISBN 0-582-30094-0
  5. ^ Hurlbut, Cornelius S.; Klein, Cornelis ||1985). Manual of Mineralogy, Wiley, (20th edition ed.). ISBN 0-471-80580-7
  6. ^ Deer, W.A.; Howie, R.A.; Wise, W.S.; Zussman, J. (2004). Rock-forming minerals. Volume 4B. Framework silicates: silica minerals. Feldspathoids and the zeolites (2nd ed.). London: Geological Society of London. p. 982 pp.

External links

Media related to Silicates at Wikimedia Commons

Babingtonite

Babingtonite is a calcium iron manganese inosilicate mineral with the formula Ca2(Fe,Mn)FeSi5O14(OH). It is unusual in that iron(III) completely replaces the aluminium so typical of silicate minerals. It is a very dark green to black translucent (in thin crystals or splinters) mineral crystallizing in the triclinic system with typically radial short prismatic clusters and druzy coatings. It occurs with zeolite minerals in cavities in volcanic rocks. Babingtonite contains both iron(II) and iron(III) and shows weak magnetism. It has a Mohs hardness of 5.5 to 6 and a specific gravity of 3.3.

It was first described in 1824 from samples from Arendal, Aust-Agder, Norway (which is its type locality) and was named after the Irish physician and mineralogist William Babington (1757–1833).It is the official mineral (mineral emblem) of the Commonwealth of Massachusetts.

Calc–silicate rock

A calc–silicate rock is a rock produced by metasomatic alteration of existing rocks in which calcium silicate minerals such as diopside and wollastonite are produced. Calc–silicate skarn or hornfels occur within impure limestone or dolomite strata adjacent to an intruding igneous rock.

Cancrinite

Cancrinite is a complex carbonate and silicate of sodium, calcium and aluminium with the formula Na6Ca2[(CO3)2|Al6Si6O24]·2H2O. It is classed as a member of the feldspathoid group of minerals; the alkali feldspars that are poor in silica. Yellow, orange, pink, white or even blue, it has a vitreous or pearly luster; a hardness of 5–6 and an uneven conchoidal fracture. It is unusual among the silicate minerals in that it will effervesce with hydrochloric acid due to the associated carbonate ions.

Found originally in 1839 in the Ural Mountains, it is named after Georg von Cancrin, a Russian minister of finance.

Chlorastrolite

Chlorastrolite also known as Isle Royale Greenstone, is a green or bluish green stone. Chlorastrolite has finely radiating or stellate (for examples, see crystal habits) masses that have a "turtleback" pattern. The stellate masses tend to be chatoyant, meaning they have a varying luster. This chatoyancy can be subtranslucent to opaque. Cholorastrolite is a variety of pumpellyite: Ca2(Mg,Fe)Al2(SiO4)(Si2O7)(OH)2·H2O. Chlorastrolite was once thought to be an impure variety of prehnite or thomsonite.

Classification of silicate minerals

This list gives an overview of the classification of minerals (silicates) and includes mostly IMA recognized minerals and its groupings. This list complements the alphabetical list on List of minerals (complete) and List of minerals. Rocks, ores, mineral mixtures, non-IMA approved minerals and non-named

Felsic

In geology, felsic refers to igneous rocks that are relatively rich in elements that form feldspar and quartz. It is contrasted with mafic rocks, which are relatively richer in magnesium and iron. Felsic refers to those rocks rich in silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium.

They are usually light in color and have specific gravities less than 3. The most common felsic rock is granite. Common felsic minerals include quartz, muscovite, orthoclase, and the sodium-rich plagioclase feldspars (albite-rich).

List of minerals

This is a list of minerals for which there are articles on Wikipedia.

Minerals are distinguished by various chemical and physical properties. Differences in chemical composition and crystal structure distinguish the various species. Within a mineral species there may be variation in physical properties or minor amounts of impurities that are recognized by mineralogists or wider society as a mineral variety.

Mineral variety names and mineraloids are to be listed after the valid minerals for each letter.

For a complete listing (about 5,000) of all mineral names, see List of minerals (complete).

Lodranite

Lodranites are a small group of primitive achondrite meteorites that consists of meteoric iron and silicate minerals. Olivine and pyroxene make up most of the silicate minerals. Like all primitive achondrites lodranites share similarities with chondrites and achondrites.

Metakaolin

Metakaolin is the anhydrous calcined form of the clay mineral kaolinite. Minerals that are rich in kaolinite are known as china clay or kaolin, traditionally used in the manufacture of porcelain. The particle size of metakaolin is smaller than cement particles, but not as fine as silica fume.

Mineral

A mineral is a naturally occurring chemical compound, usually of crystalline form and not produced by life processes. A mineral has one specific chemical composition, whereas a rock can be an aggregate of different minerals or mineraloids. The study of minerals is called mineralogy.

Minerals are classified by variety, species, series and group, in order of increasing generality. As of November 2018, there are more than 5,500 known mineral species; 5,389 of these have been approved by the International Mineralogical Association (IMA).Minerals are distinguished by various chemical and physical properties. Differences in chemical composition and crystal structure distinguish the various species, which were determined by the mineral's geological environment when formed. Changes in the temperature, pressure, or bulk composition of a rock mass cause changes in its minerals. Within a mineral species there may be variation in physical properties or minor amounts of impurities that are recognized by mineralogists or wider society as a mineral variety, for example amethyst, a purple variety of the mineral species quartz.

Minerals can be described by their various physical properties, which are related to their chemical structure and composition. Common distinguishing characteristics include crystal structure and habit, hardness, lustre, diaphaneity, colour, streak, tenacity, cleavage, fracture, parting, specific gravity, magnetism, taste or smell, radioactivity, and reaction to acid.

Minerals are classified by key chemical constituents; the two dominant systems are the Dana classification and the Strunz classification. Silicon and oxygen constitute approximately 75% of the Earth's crust, which translates directly into the predominance of silicate minerals. The silicate minerals compose over 90% of the Earth's crust. The silicate class of minerals is subdivided into six subclasses by the degree of polymerization in the chemical structure. All silicate minerals have a base unit of a [SiO4]4− silica tetrahedron – that is, a silicon cation coordinated by four oxygen anions, which gives the shape of a tetrahedron. These tetrahedra can be polymerized to give the subclasses: orthosilicates (no polymerization, thus single tetrahedra), disilicates (two tetrahedra bonded together), cyclosilicates (rings of tetrahedra), inosilicates (chains of tetrahedra), phyllosilicates (sheets of tetrahedra), and tectosilicates (three-dimensional network of tetrahedra). Other important mineral groups include the native elements, sulfides, oxides, halides, carbonates, sulfates, and phosphates.

Peridot

Peridot ( or ) (sometimes called Chrysolite) is gem-quality olivine, which is a silicate mineral with the formula of (Mg, Fe)2SiO4. As peridot is the magnesium-rich variety of olivine (forsterite), the formula approaches Mg2SiO4.

Plagioclase

Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more properly known as the plagioclase feldspar series (from the Ancient Greek for "oblique fracture", in reference to its two cleavage angles). This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or 'record-groove' effect.

Plagioclase is a major constituent mineral in the Earth's crust, and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks. Plagioclase is also a major constituent of rock in the highlands of the Earth's moon. Analysis of thermal emission spectra from the surface of Mars suggests that plagioclase is the most abundant mineral in the crust of Mars.

Planar deformation features

Planar deformation features, or PDFs, are optically recognizable microscopic features in grains of silicate minerals (usually quartz or feldspar), consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain's crystal structure.

PDFs are only produced by extreme shock compressions on the scale of meteor impacts. They are not found in volcanic environments. Their presence therefore is a primary criterion for recognizing that an impact event has occurred.

Riebeckite

Riebeckite is a sodium-rich member of the amphibole group of silicate minerals, chemical formula Na2(Fe2+3Fe3+2)Si8O22(OH)2. It forms a solid solution series with magnesioriebeckite. It crystallizes in the monoclinic system, usually as long prismatic crystals showing a diamond-shaped cross section, but also in fibrous, bladed, acicular, columnar, and radiating forms. Its Mohs hardness is 5.0–6.0, and its specific gravity is 3.0–3.4. Cleavage is perfect, two directions in the shape of a diamond; fracture is uneven, splintery. It is often translucent to nearly opaque.

Seifertite

Seifertite is a silicate mineral with the formula SiO2 and is one of the densest polymorphs of silica. It has only been found in Martian and lunar meteorites, where it is presumably formed from either tridymite or cristobalite – other polymorphs of quartz – as a result of heating during the atmospheric re-entry and impact to the Earth, at an estimated minimal pressure of 35 GPa. It can also be produced in the laboratory by compressing cristobalite in a diamond anvil cell to pressures above 40 GPa. The mineral is named after Friedrich Seifert (born 1941), the founder of the Bayerisches Geoinstitut at University of Bayreuth, Germany, and is officially recognized by the International Mineralogical Association.Seifertite forms micrometre-sized crystalline lamellae embedded into a glassy SiO2 matrix. The lamellae are rather difficult to analyze, as they vitrify within seconds under laser or electron beams used for standard Raman spectroscopy or electron-beam microanalysis, even at much reduced beam intensities. Nevertheless, it was possible to verify that it is mainly composed of SiO2 with minor inclusions of Na2O (0.40 wt.%) and Al2O3 (1.14 wt.%). X-ray diffraction reveals that the mineral has scrutinyite (α-PbO2) type structure with an orthorhombic symmetry and Pbcn or Pb2n space group. Its lattice constants a = 4.097, b = 5.0462, c = 4.4946, Z = 4 correspond to the density of 4.294 g/cm3, which is among the highest for any forms of silica (for example, density of quartz is 2.65 g/cm3). Only stishovite has a comparable density of about 4.3 g/cm3.

Silicate

In chemistry, a silicate is any member of a family of anions consisting of silicon and oxygen, usually with the general formula [SiO(4−2x)−4−x]n, where 0 ≤ x < 2. The family includes orthosilicate SiO4−4 (x = 0), metasilicate SiO2−3 (x = 1), and pyrosilicate Si2O6−7 (x = 0.5, n = 2). The name is also used for any salt of such anions, such as sodium metasilicate; or any ester containing the corresponding chemical group, such as tetramethyl orthosilicate.Silicate anions are often large polymeric molecules with an extense variety of structures, including chains and rings (as in polymeric metasilicate [SiO2−3]n), double chains (as in [Si2O2−5]n, and sheets (as in [Si2O2−5]n.In geology and astronomy, the term silicate is used to mean silicate minerals, ionic solids with silicate anions; as well as rock types that consist predominantly of such minerals. In that context, the term also includes the non-ionic compound silicon dioxide SiO2 (silica, quartz), which would correspond to x = 2 in the general formula. The term also includes minerals where aluminum or other tetravalent atoms replace some of the silicon atoms, as in the aluminosilicates. Such silicates comprise most of Earth's crust and mantle, as well as the other terrestrial planets, rocky moons, and asteroids.Silicates are extremely important materials, both natural (such as granite, gravel, and garnet) and artificial (such as Portland cement, ceramics, glass, and waterglass), for all sorts of technological and artistic activities.

The name "silicate" is sometimes extended to any anions containing silicon, even if they do not fit the general formula or contain other atoms besides oxygen; such as the hexahydroxysilicate [Si(OH)6]2− or hexafluorosilicate [SiF6]2−.

Silicate perovskite

Silicate perovskite is either (Mg,Fe)SiO3 (the magnesian end-member is called bridgmanite) or CaSiO3 (calcium silicate) when arranged in a perovskite structure. Silicate perovskites are not stable at Earth's surface, and are mainly found in the lower part of Earth's mantle, between about 670 and 2,700 km (420 and 1,680 mi) depth. They are thought to form the main mineral phases, together with ferropericlase.

Siliciclastic

Siliciclastic rocks (commonly misspelled siliclastic) are clastic noncarbonate sedimentary rocks that are almost exclusively silica-bearing, either as forms of quartz or other silicate minerals. All siliciclastic rocks are formed by inorganic processes, or deposited through some mechanical process, such as stream deposits (delta deposits) that are subsequently lithified. They are sandstone based rocks accounting for about 50 - 60% of the world oil and gas exploration.The other silicate minerals that are generally present in siliciclastic sedimentary rocks are feldspar, biotite

etc....siliciclastic sediments are silica-based sediments, lacking carbon compounds, which are formed from pre-existing rocks, by breakage, transportation and redeposition to form sedimentary rock.

Skarn

Skarns or tactites are hard, coarse-grained metamorphic rocks that form by a process called metasomatism. Skarns tend to be rich in calcium-magnesium-iron-manganese-aluminium silicate minerals, which are also referred to as calc-silicate minerals. These minerals form as a result of alteration which occurs when hydrothermal fluids interact with a protolith of either igneous or sedimentary origin. In many cases, skarns are associated with the intrusion of a granitic pluton found in and around faults or Shear zones that intrude into a carbonate layer such as a dolomite or limestone. Skarns can form by regional, or contact metamorphism and therefore form in relatively high temperature environments. The hydrothermal fluids associated with the metasomatic processes can originate from either magmatic, metamorphic, meteoric, marine, or even a mix of these. The resulting skarn may consist of a variety of different minerals which are highly dependent on the original composition of both the hydrothermal fluid and the original composition of the protolith.If a skarn has a respectable amount of ore mineralization that can be mined for a profit, it can therefore be classified as a skarn deposit.

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