Silicate mineral

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.

In mineralogy, the crystalline forms of silica () are usually considered to be tectosilicates, and they are classified as such in the Dana classification system (75.1). However, the Nickel–Strunz classification categorizes them as oxide minerals (4.DA). 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.

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.

General structure A silicate mineral is generally an inorganic compound consisting of subunits with the formula [SiO2+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, Mx+. 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 ([SiO2+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 , 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 . Replacement of one in every four silicon atoms by an aluminum atom results in the anion , whose charge is neutralized by the potassium cations .

Main groups In mineralogy, silicate minerals are classified into seven major groups according to the structure of their silicate anion:Deer, W.A.; Howie, R.A., & Zussman, J. (1992). An introduction to the rock forming minerals (2nd edition ed.). London: Longman Hurlbut, Cornelius S.; Klein, Cornelis ||1985). Manual of Mineralogy, Wiley, (20th edition ed.). Major group Structure Chemical formula Example Nesosilicates isolated silicon tetrahedra [SiO4]4− olivine, garnet, zircon... Sorosilicates double tetrahedra [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

Nesosilicates (from Greek 'island'), or orthosilicates, have the orthosilicate ion, present as isolated (insular) tetrahedra connected only by interstitial cations. The Nickel–Strunz classification is 09.A –examples include:

Phenakite – Willemite – Olivine group Forsterite – Fayalite – Tephroite – Garnet group Pyrope – Almandine – Spessartine – Grossular – Andradite – Uvarovite – Hydrogrossular – Zircon group Zircon – Thorite – Hafnon – Wollastonite group (pyroxenoid group) Wollastonite – Pectolite – Serandite – Nambulite – Fowlerite –

group Andalusite – Kyanite – Sillimanite – Dumortierite – Topaz – Staurolite – Humite group – Norbergite – Chondrodite – Humite – Clinohumite – Datolite – Titanite – Chloritoid – Mullite (aka Porcelainite) –

Sorosilicates

Sorosilicates (from Greek 'heap, mound') have isolated pyrosilicate anions , 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:

Thortveitite – Hemimorphite (calamine) – Lawsonite – Axinite – Ilvaite – Epidote group (has both and groups} Epidote – Zoisite – Tanzanite – Clinozoisite – Allanite – Dollaseite-(Ce) – Vesuvianite (idocrase) –

Cyclosilicates

Cyclosilicates (from Greek '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(s). The silicon:oxygen ratio is 1:3. Double rings have the formula (Si2xO5x)2x− or a 2:5 ratio. The Nickel–Strunz classification is 09.C. Possible ring sizes include:

Some example minerals are: 3-member single ring Benitoite – 4-member single ring Papagoite – . 6-member single ring Beryl – Bazzite – Sugilite – Tourmaline – Pezzottaite – Osumilite – Cordierite – Sekaninaite – 9-member single ring Eudialyte – 6-member double ring Milarite –

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 [genitive: ] 'fibre'), or chain silicates, have interlocking chains of silicate tetrahedra with either , 1:3 ratio, for single chains or , 4:11 ratio, for double chains. The Nickel–Strunz classification is 09.D – examples include:

Single chain inosilicates Pyroxene group Clinopyroxene subgroup Aegirine (or acmite) – Augite – Diopside – Hedenbergite – Jadeite – Pigeonite – , where 0.1 ≤ x ≤ 0.4, x + y + z = 1 and y1 + z1 = 1 Spodumene – Orthopyroxene subgroup Enstatite – Ferrosilite – Pyroxferroite - Rhodonite – Wollastonite group Pectolite – Wollastonite –

Double chain inosilicates Amphibole group Anthophyllite – Cummingtonite series Magnesiocummingtonite - Cummingtonite – Grunerite – Tremolite series Tremolite – Actinolite – Hornblende – Sodium amphibole group Glaucophane – Riebeckite (asbestos) – Arfvedsonite –

Phyllosilicates

Phyllosilicates (from Greek 'leaf'), or sheet silicates, form parallel sheets of silicate tetrahedra with in a 2:5 ratio. The Nickel–Strunz classification is 09.E. All phyllosilicate minerals are hydrated, with either water or hydroxyl groups attached. Many phyllosilicates are clay-forming and may be further classified as 1:1 clay minerals (one tetrahedral sheet and one octahedral sheet) and 2:1 clay minerals (one octahedral sheet between two tetrahedral sheets). Below are some major phyllosilicate mineral species and their chemical formulas, with group and series names in italics:

Ajoite – Apophyllite group Fluorapophyllite-(K) – Bannisterite – Carletonite – Cavansite – (dimorph of pentagonite) Chlorite group – (2:1:1 clays) Chamosite – (Fe endmember) Clinochlore – (Mg endmember) Cookeite – Chrysocolla – Ekanite – Gyrolite – Hisingerite – Imogolite – Kaolinite-Serpentine group Greenalite – Kaolinite subgroup(1:1 clays) Dickite – Kaolinite – Halloysite – Serpentine subgroup Amesite – Antigorite – Chrysotile – Lizardite – Mica group Brittle mica group Clintonite – Margarite – Dioctahedral mica group Celadonite subgroup Celadonite – Glauconite – Muscovite – Paragonite – Roscoelite – Trioctahedral mica group Aspidolite – Biotite subgroup – Annite – (Fe endmember) Phlogopite – (Mg endmember) Lepidolite (polylithionite-trilithionite series) – Zinnwaldite series – Neptunite – Okenite – Palygorskite group(2:1 clays) Palygorskite – Tuperssuatsiaite – Pentagonite – (dimorph of cavansite) Pyrophyllite-Talc group Pyrophyllite – Talc – (2:1 clay) Sepiolite group Sepiolite – (2:1 clay) Falcondoite – (Ni analogue of sepiolite) Smectite group(2:1 clays) Hectorite – Montmorillonite – Nontronite – Saponite – Stevensite – Stilpnomelane group Stilpnomelane – Vermiculite – (2:1 clay)

Tectosilicates

Tectosilicates, or "framework silicates," have a three-dimensional framework of silicate tetrahedra with 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 9.F (tectosilicates without zeolitic ), 9.G (tectosilicates with zeolitic ), and 4.DA (quartz/silica group). Below are some major tectosilicate mineral species and their chemical formulas, with group and series names in italics: Quartz group (silica) – Chalcedony – cryptocrystalline variety of silica composed mostly of quartz with some moganite Polymorphs of silica α-quartz – trigonal, "normal" quartz under β-quartz – hexagonal, high-temperature quartz Coesite – monoclinic Cristobalite – tetragonal Melanophlogite – cubic or tetragonal, rare Moganite – monoclinic Stishovite – tetragonal, extremely hard and dense Tridymite – orthorhombic Feldspar group Alkali feldspar series (potassium feldspars or K-spar) Microcline – Orthoclase – Anorthoclase – Sanidine – Plagioclase feldspar series Albite – (Na endmember) Oligoclase – (Na:Ca 90:10 to 70:30) Andesine – (Na:Ca 50:50 to 70:30) Labradorite – (Na:Ca 30:70 to 50:50) Bytownite – (Na:Ca 10:90 to 30:70) Anorthite – (Ca endmember) Other feldspars Buddingtonite — Celsian – Hyalophane – Feldspathoid group Cancrinite subgroup Afghanite – Cancrinite – Sacrofanite – Leucite – Nepheline subgroup Nepheline – Sodalite subgroup Hauyne – Lazurite – Nosean – Sodalite – Tugtupite – Scapolite group Marialite – Meionite – Zeolite group Amicite – Analcime – Brewsterite subgroup – Chabazite-Lévyne subgroup Chabazite – Lévyne – Clinoptilolite subgroup – Erionite subgroup – Faujasite subgroup – Ferrierite subgroup – (Ferrierite-Mg) Heulandite subgroup – Laumontite – Mordenite – Natrolite subgroup Mesolite – Natrolite – Scolecite – Paulingite subgroup – (Paulingite-K) Phillipsite subgroup Phillipsite – (Phillipsite-Ca) Pollucite – Stilbite subgroup Stellerite – Stilbite – Thomsonite subgroup – (Thomsonite-Ca) Yugawaralite –

Silicate mineral

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