Silicate minerals

Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust.

Lithium aluminium silicate mineral spodumene

In mineralogy, silica (silicon dioxide, SiO₂) is usually considered a silicate mineral rather than an oxide mineral. 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, a biogenic form of silica as viewed under a microscope. The imaged region measures approximately 1.13 by 0.69 mm.

Living organisms also contribute to this geologic cycle. For example, a type of plankton known as diatoms construct their exoskeletons ("frustules") from silica extracted from seawater. The frustules of dead diatoms are a major constituent of deep ocean sediment, and of diatomaceous earth.^{[citation needed]}

General structure

A silicate mineral is generally an inorganic compound consisting of subunits with the formula [SiO_2+n]²ⁿ⁻. Although depicted as such, the description of silicates as anions is a simplification. Balancing the charges of the silicate anions are metal cations, M^x+. Typical cations are Mg²⁺, Fe²⁺, and Na⁺. The Si-O-M linkage between the silicates and the metals are strong, polar-covalent bonds. Silicate anions ([SiO_2+n]²ⁿ⁻) are invariably colorless, or when crushed to a fine powder, white. The colors of silicate minerals arise from the metal component, commonly iron.

In most silicate minerals, silicon is tetrahedral, being surrounded by four oxides. The coordination number of the oxides is variable except when it bridges two silicon centers, in which case the oxide has a coordination number of two.

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
₃O
₈]
_n, the anion is a tridimensional network of tetrahedrums in which all oxygen corners are shared. If all tetrahedrums had silicon centers, the anion would be just neutral silica [SiO
₂]
_n. Replacement of one in every four silicon atoms by an aluminum atom results in the anion [AlSi
₃O⁻
₈]
_n, whose charge is neutralized by the potassium cations K⁺
.

Main groups

In mineralogy, silicate minerals are classified into seven major groups according to the structure of their silicate anion:

Major group	Structure	Chemical formula	Example
Nesosilicates	isolated silicon tetrahedrums	[SiO4]^4−	olivine, garnet, zircon...
Sorosilicates	double tetrahedrums	[Si2O7]^6−	epidote, melilite group
Cyclosilicates	rings	[SinO3n]^2n−	beryl group, 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

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

Nesosilicates or orthosilicates

Orthosilicate anion SiO⁴⁻
₄. The grey ball represents the silicon atom, and the red balls are the oxygen atoms.

Nesosilicate specimens at the Museum of Geology in South Dakota

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

• Phenakite group
  • Phenakite – Be₂SiO₄
  • Willemite – Zn₂SiO₄
• Olivine group
  • Forsterite – Mg₂SiO₄
  • Fayalite – Fe₂SiO₄
  • Tephroite – Mn₂SiO₄
• Garnet group
  • Pyrope – Mg₃Al₂(SiO₄)₃
  • Almandine – Fe₃Al₂(SiO₄)₃
  • Spessartine – Mn₃Al₂(SiO₄)₃
  • Grossular – Ca₃Al₂(SiO₄)₃
  • Andradite – Ca₃Fe₂(SiO₄)₃
  • Uvarovite – Ca₃Cr₂(SiO₄)₃
  • Hydrogrossular – Ca
    ₃Al
    ₂Si
    ₂O
    ₈(SiO
    ₄)
    _3−m(OH)
    _4m
• Zircon group
  • Zircon – ZrSiO₄
  • Thorite – (Th,U)SiO₄
  • Hafnon – (Hf,Zr)SiO₄

Kyanite crystals (unknown scale)

• Al₂SiO₅ group
  • Andalusite – Al₂SiO₅
  • Kyanite – Al₂SiO₅
  • Sillimanite – Al₂SiO₅
  • Dumortierite – Al
    _6.5–7BO
    ₃(SiO
    ₄)
    ₃(O,OH)
    ₃
  • Topaz – Al₂SiO₄(F,OH)₂
  • Staurolite – Fe₂Al₉(SiO₄)₄(O,OH)₂
• Humite group – (Mg,Fe)₇(SiO₄)₃(F,OH)₂
  • Norbergite – Mg₃(SiO₄)(F,OH)₂
  • Chondrodite – Mg₅(SiO₄)₂(F,OH)₂
  • Humite – Mg₇(SiO₄)₃(F,OH)₂
  • Clinohumite – Mg₉(SiO₄)₄(F,OH)₂
• Datolite – CaBSiO₄(OH)
• Titanite – CaTiSiO₅
• Chloritoid – (Fe,Mg,Mn)₂Al₄Si₂O₁₀(OH)₄
• Mullite (aka Porcelainite) – Al₆Si₂O₁₃

Sorosilicates

Pyrosilicate anion Si
₂O⁶⁻
₇

Sorosilicate exhibit at Museum of Geology in South Dakota

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

• Thortveitite – (Sc,Y)₂(Si₂O₇)
• Hemimorphite (calamine) – Zn₄(Si₂O₇)(OH)₂·H₂O
• Lawsonite – CaAl₂(Si₂O₇)(OH)₂·H₂O
• Axinite – (Ca,Fe,Mn)₃Al₂(BO₃)(Si₄O₁₂)(OH)
• Ilvaite – CaFe^II₂Fe^IIIO(Si₂O₇)(OH)
• Epidote group (has both (SiO₄)⁴⁻ and (Si₂O₇)⁶⁻ groups}
  • Epidote – Ca₂(Al,Fe)₃O(SiO₄)(Si₂O₇)(OH)
  • Zoisite – Ca₂Al₃O(SiO₄)(Si₂O₇)(OH)
    • Tanzanite – Ca₂Al₃O(SiO₄)(Si₂O₇)(OH)
  • Clinozoisite – Ca₂Al₃O(SiO₄)(Si₂O₇)(OH)
  • Allanite – Ca(Ce,La,Y,Ca)Al₂(Fe^II,Fe^III)O(SiO₄)(Si₂O₇)(OH)
  • Dollaseite-(Ce) – CaCeMg₂AlSi₃O₁₁F(OH)
• Vesuvianite (idocrase) – Ca₁₀(Mg,Fe)₂Al₄(SiO₄)₅(Si₂O₇)₂(OH)₄

Cyclosilicates

Cyclosilicate specimens at the Museum of Geology, South Dakota

Pezzottaite

Bazzite

Cyclosilicates (from Greek κύκλος kýklos 'circle'), or ring silicates, have three or more tetrahedrums linked in a ring. The general formula is (Si_xO_3x)^2x−, where one or more silicon atoms can be replaced by other 4-coordinated atom(s). The silicon:oxygen ratio is 1:3. Double rings have the formula (Si_2xO_5x)^2x− or a 2:5 ratio. The Nickel–Strunz classification is 09.C. Possible ring sizes include:

• 6 units [Si₆O₁₈], beryl (red: Si, blue: O)
• 3 units [Si₃O₉], benitoite
• 4 units [Si₄O₁₂], papagoite
• 9 units [Si₉O₂₇], eudialyte
• 12 units, double ring [Si₁₂O₃₀], milarite

Some example minerals are:

• 3-member single ring
  • Benitoite – BaTi(Si₃O₉)
• 4-member single ring
  • Papagoite – CaCuAlSi
    ₂O
    ₆(OH)
    ₃.
• 6-member single ring
  • Beryl – Be₃Al₂(Si₆O₁₈)
  • Bazzite – Be₃Sc₂(Si₆O₁₈)
  • Sugilite – KNa₂(Fe,Mn,Al)₂Li₃Si₁₂O₃₀
  • Tourmaline – (Na,Ca)(Al,Li,Mg)
    _3–(Al,Fe,Mn)
    ₆(Si
    ₆O
    ₁₈)(BO
    ₃)
    ₃(OH)
    ₄
  • Pezzottaite – Cs(Be₂Li)Al₂Si₆O₁₈
  • Osumilite – (K,Na)(Fe,Mg)₂(Al,Fe)₃(Si,Al)₁₂O₃₀
  • Cordierite – (Mg,Fe)₂Al₄Si₅O₁₈
  • Sekaninaite – (Fe⁺²,Mg)₂Al₄Si₅O₁₈
• 9-member single ring
  • Eudialyte – Na
    ₁₅Ca
    ₆(Fe,Mn)
    ₃Zr
    ₃SiO(O,OH,H
    ₂O)
    ₃(Si
    ₃O
    ₉)
    ₂(Si
    ₉O
    ₂₇)
    ₂(OH,Cl)
    ₂
• 6-member double ring
  • Milarite – K₂Ca₄Al₂Be₄(Si₂₄O₆₀)H₂O

The ring in axinite contains two B and four Si tetrahedrums 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 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

• Pyroxene group
  • Enstatite – orthoferrosilite series
    • Enstatite – MgSiO₃
    • Ferrosilite – FeSiO₃
  • Pigeonite – Ca_0.25(Mg,Fe)_1.75Si₂O₆
  • Diopside – hedenbergite series
    • Diopside – CaMgSi₂O₆
    • Hedenbergite – CaFeSi₂O₆
    • Augite – (Ca,Na)(Mg,Fe,Al)(Si,Al)₂O₆
  • Sodium pyroxene series
    • Jadeite – NaAlSi₂O₆
    • Aegirine (or acmite) – NaFe^IIISi₂O₆
  • Spodumene – LiAlSi₂O₆
  • Pyroxferroite - (Fe,Ca)SiO₃
• Pyroxenoid group
  • Wollastonite – CaSiO₃
  • Rhodonite – MnSiO₃
  • Pectolite – NaCa₂(Si₃O₈)(OH)

Double chain inosilicates

• Amphibole group
  • Anthophyllite – (Mg,Fe)₇Si₈O₂₂(OH)₂
  • Cummingtonite series
    • Cummingtonite – Fe₂Mg₅Si₈O₂₂(OH)₂
    • Grunerite – Fe₇Si₈O₂₂(OH)₂
  • Tremolite series
    • Tremolite – Ca₂Mg₅Si₈O₂₂(OH)₂
    • Actinolite – Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂
  • Hornblende – (Ca,Na)
    _2–3(Mg,Fe,Al)
    ₅Si
    ₆(Al,Si)
    ₂O
    ₂₂(OH)
    ₂
  • Sodium amphibole group
    • Glaucophane – Na₂Mg₃Al₂Si₈O₂₂(OH)₂
    • Riebeckite (asbestos) – Na₂Fe^II₃Fe^III₂Si₈O₂₂(OH)₂
    • Arfvedsonite – Na₃(Fe,Mg)₄FeSi₈O₂₂(OH)₂

• 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 phýllon 'leaf'), or sheet silicates, form parallel sheets of silicate tetrahedrums 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.

Kaolinite

Examples include:

• Serpentine subgroup
  • Antigorite – Mg₃Si₂O₅(OH)₄
  • Chrysotile – Mg₃Si₂O₅(OH)₄
  • Lizardite – Mg₃Si₂O₅(OH)₄
• Clay minerals group
  • 1:1 clay minerals (TO)
    • Halloysite – Al₂Si₂O₅(OH)₄
    • Kaolinite – Al₂Si₂O₅(OH)₄
  • 2:1 clay minerals (TOT)
    • Pyrophyllite – Al₂Si₄O₁₀(OH)₂
    • Talc – Mg₃Si₄O₁₀(OH)₂
    • Illite – (K,H₃O)(Al,Mg,Fe)₂(Si,Al)₄O₁₀[(OH)₂,(H₂O)]
    • Montmorillonite (smectite) – (Na,Ca)_0.33(Al,Mg)₂Si₄O₁₀(OH)₂·nH₂O
    • Chlorite – (Mg,Fe)₃(Si,Al)₄O₁₀(OH)₂·(Mg,Fe)₃(OH)₆
    • Vermiculite – (Mg,Fe,Al)₃(Al,Si)₄O₁₀(OH)₂·4H₂O
  • Other clay minerals
    • Sepiolite – Mg₄Si₆O₁₅(OH)₂·6H₂O
    • Palygorskite (or attapulgite) – (Mg,Al)₂Si₄O₁₀(OH)·4(H₂O)
• Mica group
  • Biotite – K(Mg,Fe)₃(AlSi₃)O₁₀(OH)₂
  • Fuchsite – K(Al,Cr)₂(AlSi₃)O₁₀(OH)₂
  • Muscovite – KAl₂(AlSi₃)O₁₀(OH)₂
  • Phlogopite – KMg₃(AlSi₃)O₁₀(OH)₂
  • Lepidolite – K(Li,Al)
    _2–3(AlSi
    ₃)O
    ₁₀(OH)
    ₂
  • Margarite – CaAl₂(Al₂Si₂)O₁₀(OH)₂
  • Glauconite – (K,Na)(Al,Mg,Fe)₂(Si,Al)₄O₁₀(OH)₂

• Phyllosilicate, mica group, muscovite (red: Si, blue: O)
• Phyllosilicate, single net of tetrahedrums with 4-membered rings, apophyllite-(KF)-apophyllite-(KOH) series
• Phyllosilicate, single tetrahedral nets of 6-membered rings, -(Fe)-pyrosmalite-(Mn) series
• Phyllosilicate, single tetrahedral nets of 6-membered rings,
• Phyllosilicate, double nets with 4- and 6-membered rings, carletonite

Tectosilicates

Silica family (SiO₂ 3D network), β-quartz

Aluminosilicate family, the 3D model of synthetic zeolite ZSM-5

Quartz

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 with SiO₂ in a 1:2 ratio. This group comprises nearly 75% of the crust of the Earth. 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:

• 3D-Silicates, quartz family
  • Quartz – SiO₂
  • Tridymite – SiO₂
  • Cristobalite – SiO₂
  • Coesite – SiO₂
  • Stishovite – SiO₂
  • Moganite – SiO₂
  • Chalcedony – SiO₂
• Tectosilicates, feldspar group
  • Alkali feldspars (potassium feldspars)
    • Microcline – KAlSi₃O₈
    • Orthoclase – KAlSi₃O₈
    • Anorthoclase – (Na,K)AlSi₃O₈
    • Sanidine – KAlSi₃O₈
  • Plagioclase feldspars
    • Albite – NaAlSi₃O₈
    • Oligoclase – (Na,Ca)(Si,Al)₄O₈    (Na:Ca 4:1)
    • Andesine – (Na,Ca)(Si,Al)₄O₈    (Na:Ca 3:2)
    • Labradorite – (Ca,Na)(Si,Al)₄O₈    (Na:Ca 2:3)
    • Bytownite – (Ca,Na)(Si,Al)₄O₈    (Na:Ca 1:4)
    • Anorthite – CaAl₂Si₂O₈
• Tectosilicates, feldspathoid family
  • Nosean – Na₈Al₆Si₆O₂₄(SO₄)
  • Cancrinite –Na₆Ca₂(CO₃,Al₆Si₆O₂₄)·₂H₂O
  • Leucite – KAlSi₂O₆
  • Nepheline – (Na,K)AlSiO₄
  • Sodalite – Na₈(AlSiO₄)₆Cl₂
  • Hauyne – (Na,Ca)
    _4–8Al
    ₆Si
    ₆(O,S)24(SO
    ₄,Cl)
    _1–2
    • Lazurite – (Na,Ca)₈(AlSiO₄)₆(SO₄,S,Cl)₂
• Tectosilicates, scapolite group
  • Marialite – Na₄(AlSi₃O₈)₃(Cl₂,CO₃,SO₄)
  • Meionite – Ca₄(Al₂Si₂O₈)₃(Cl₂CO₃,SO₄)
• Tectosilicates, zeolite family
  • Natrolite – Na₂Al₂Si₃O₁₀·2H₂O
  • Erionite – (Na₂,K₂,Ca)₂Al₄Si₁₄O₃₆·15H₂O
  • Chabazite – CaAl₂Si₄O₁₂·6H₂O
  • Heulandite – CaAl₂Si₇O₁₈·6H₂O
  • Stilbite – NaCa₂Al₅Si₁₃O₃₆·17H₂O
  • Scolecite – CaAl₂Si₃O₁₀·3H₂O
  • Mordenite – (Ca,Na₂,K₂)Al₂Si₁₀O₂₄·7H₂O
  • Analcime – NaAlSi₂O₆·H₂O

See also

• Earth sciences portal

• Classification of non-silicate minerals – List of IMA recognized minerals and groupings
• Classification of silicate minerals – List of IMA recognized minerals and groupings
• Silicate mineral paint – Paint coats with mineral binding agents

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

The Wikibook Historical Geology has a page on the topic of: Silicate minerals

Wikimedia Commons has media related to Silicate minerals.

• Mindat.org, Dana classification
• Webmineral : Dana's New Silicate Classification