Editing Mineral (section)

===Silicates===
{{main article|Silicate minerals}}
[[File:Aegirine-233494.jpg|right|thumb|[[Aegirine]], an iron-sodium clinopyroxene, is part of the inosilicate subclass.]]
The base unit of a silicate mineral is the [SiO<sub>4</sub>]<sup>4−</sup> tetrahedron. In the vast majority of cases, silicon is in four-fold or tetrahedral coordination with oxygen. In very high-pressure situations, silicon will be in six-fold or octahedral coordination, such as in the [[perovskite structure]] or the quartz polymorph [[stishovite]] (SiO<sub>2</sub>). In the latter case, the mineral no longer has a silicate structure, but that of [[rutile]] (TiO<sub>2</sub>), and its associated group, which are simple oxides. These silica tetrahedra are then polymerized to some degree to create various structures, such as one-dimensional chains, two-dimensional sheets, and three-dimensional frameworks. The basic silicate mineral where no polymerization of the tetrahedra has occurred requires other elements to balance out the base 4- charge. In other silicate structures, different combinations of elements are required to balance out the resultant negative charge. It is common for the Si<sup>4+</sup> to be substituted by  Al<sup>3+</sup> because of similarity in ionic radius and charge; in those cases, the [AlO<sub>4</sub>]<sup>5−</sup> tetrahedra form the same structures as do the unsubstituted tetrahedra, but their charge-balancing requirements are different.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 104–20</ref>

The degree of polymerization can be described by both the structure formed and how many tetrahedral corners (or coordinating oxygens) are shared (for aluminium and silicon in tetrahedral sites):<ref>{{harvnb|Dyar|Gunter|2008}}, p. 105</ref><ref name="Dyar 2008 104–17">{{harvnb|Dyar|Gunter|2008|pp=104–17}}</ref>
;Orthosilicates (or nesosilicates): Have no linking of polyhedra, thus tetrahedra share no corners.
;Disilicates (or sorosilicates): Have two tetrahedra sharing one oxygen atom.
;Inosilicates are chain silicates: Single-chain silicates have two shared corners, whereas double-chain silicates have two or three shared corners.
;Phyllosilicates: Have a sheet structure which requires three shared oxygens; in the case of double-chain silicates, some tetrahedra must share two corners instead of three as otherwise a sheet structure would result. 
;Framework silicates (or tectosilicates): Have tetrahedra that share all four corners.
;Ring silicates (or cyclosilicates): Only need tetrahedra to share two corners to form the cyclical structure.<ref name="Dyar 2008 104–17"/>

The silicate subclasses are described below in order of decreasing polymerization.

====Tectosilicates====
[[File:Natroliteinde1.jpg|thumb|left|upright=1.15|[[Natrolite]] is a mineral series in the zeolite group; this sample has a very prominent acicular crystal habit.]]
Tectosilicates, also known as framework silicates, have the highest degree of polymerization. With all corners of a tetrahedra shared, the silicon:oxygen ratio becomes 1:2. Examples are quartz, the [[feldspar]]s, [[feldspathoid]]s, and the [[zeolite]]s. Framework silicates tend to be particularly chemically stable as a result of strong covalent bonds.{{sfn|Klein|Hurlbut|1993|p=524}}

Forming 12% of the Earth's crust, [[quartz]] (SiO<sub>2</sub>) is the most abundant mineral species. It is characterized by its high chemical and physical resistivity. Quartz has several polymorphs, including [[tridymite]] and [[cristobalite]] at high temperatures, high-pressure [[coesite]], and ultra-high pressure [[stishovite]]. The latter mineral can only be formed on Earth by meteorite impacts, and its structure has been compressed so much that it has changed from a silicate structure to that of [[rutile]] (TiO<sub>2</sub>). The silica polymorph that is most stable at the Earth's surface is α-quartz. Its counterpart, β-quartz, is present only at high temperatures and pressures (changes to α-quartz below 573&nbsp;°C at 1 bar). These two polymorphs differ by a "kinking" of bonds; this change in structure gives β-quartz greater symmetry than α-quartz, and they are thus also called high quartz (β) and low quartz (α).<ref name="{{harvnb|Dyar|Gunter|2008}}, p. 104"/><ref>{{harvnb|Dyar|Gunter|2008}}, pp. 578–83</ref>

Feldspars are the most abundant group in the Earth's crust, at about 50%. In the feldspars, Al<sup>3+</sup> substitutes for Si<sup>4+</sup>, which creates a charge imbalance that must be accounted for by the addition of cations. The base structure becomes either [AlSi<sub>3</sub>O<sub>8</sub>]<sup>−</sup> or [Al<sub>2</sub>Si<sub>2</sub>O<sub>8</sub>]<sup>2−</sup>  There are 22 mineral species of feldspars, subdivided into two major subgroups – alkali and plagioclase – and two less common groups – [[celsian]] and [[banalsite]]. The alkali feldspars are most commonly in a series between potassium-rich orthoclase and sodium-rich [[albite]]; in the case of plagioclase, the most common series ranges from albite to calcium-rich [[anorthite]]. Crystal twinning is common in feldspars, especially polysynthetic twins in plagioclase and Carlsbad twins in alkali feldspars. If the latter subgroup cools slowly from a melt, it forms exsolution lamellae because the two components – orthoclase and albite – are unstable in solid solution. Exsolution can be on a scale from microscopic to readily observable in hand-sample; perthitic texture forms when Na-rich feldspar exsolve in a K-rich host. The opposite texture (antiperthitic), where K-rich feldspar exsolves in a Na-rich host, is very rare.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 583–88</ref>

Feldspathoids are structurally similar to feldspar, but differ in that they form in Si-deficient conditions, which allows for further substitution by Al<sup>3+</sup>. As a result, feldspathoids are almost never found in association with quartz. A common example of a feldspathoid is [[nepheline]] ((Na, K)AlSiO<sub>4</sub>); compared to alkali feldspar, nepheline has an Al<sub>2</sub>O<sub>3</sub>:SiO<sub>2</sub> ratio of 1:2, as opposed to 1:6 in alkali feldspar.<ref>{{harvnb|Dyar|Gunter|2008}}, p. 588</ref> Zeolites often have distinctive crystal habits, occurring in needles, plates, or blocky masses. They form in the presence of water at low temperatures and pressures, and have channels and voids in their structure. Zeolites have several industrial applications, especially in waste water treatment.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 589–93</ref>

====Phyllosilicates====
[[File:Muscovite-Albite-122886.jpg|right|upright=1.15|thumb|Muscovite, a mineral species in the mica group, within the phyllosilicate subclass]]
Phyllosilicates consist of sheets of polymerized tetrahedra. They are bound at three oxygen sites, which gives a characteristic silicon:oxygen ratio of 2:5. Important examples include the [[mica]], [[Chlorite group|chlorite]], and the [[kaolinite]]-[[Serpentine group|serpentine]] groups. In addition to the tetrahedra, phyllosilicates have a sheet of octahedra (elements in six-fold coordination by oxygen) that balance out the basic tetrahedra, which have a negative charge (e.g. [Si<sub>4</sub>O<sub>10</sub>]<sup>4−</sup>) These tetrahedra (T) and octahedra (O) sheets are stacked in a variety of combinations to create phyllosilicate layers. Within an octahedral sheet, there are three octahedral sites in a unit structure; however, not all of the sites may be occupied. In that case, the mineral is termed dioctahedral, whereas in other case it is termed trioctahedral.<ref>{{harvnb|Dyar|Gunter|2008}}, p. 110</ref> The layers are weakly bound by [[van der Waals forces]], [[hydrogen bond]]s, or sparse [[ionic bond]]s, which causes a crystallographic weakness, in turn leading to a prominent basal cleavage among the phyllosilicates.<ref>{{harvnb|Chesterman|Lowe|2008}}, p. 525</ref>

The kaolinite-serpentine group consists of T-O stacks (the 1:1 clay minerals); their hardness ranges from 2 to 4, as the sheets are held by hydrogen bonds. The 2:1 clay minerals (pyrophyllite-talc) consist of T-O-T stacks, but they are softer (hardness from 1 to 2), as they are instead held together by van der Waals forces. These two groups of minerals are subgrouped by octahedral occupation; specifically, kaolinite and pyrophyllite are dioctahedral whereas serpentine and talc trioctahedral.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 110–13</ref>

Micas are also T-O-T-stacked phyllosilicates, but differ from the other T-O-T and T-O-stacked subclass members in that they incorporate aluminium into the tetrahedral sheets (clay minerals have Al<sup>3+</sup> in octahedral sites). Common examples of micas are [[muscovite]], and the [[biotite]] series. Mica T-O-T layers are bonded together by metal ions, giving them a greater hardness than other phyllosilicate minerals, though they retain perfect basal cleavage.{{sfn|Nesse|2000|p=238}} The chlorite group is related to mica group, but a [[brucite]]-like (Mg(OH)<sub>2</sub>) layer between the T-O-T stacks.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 602–05</ref>

Because of their chemical structure, phyllosilicates typically have flexible, elastic, transparent layers that are electrical insulators and can be split into very thin flakes. Micas can be used in electronics as insulators, in construction, as optical filler, or even cosmetics. Chrysotile, a species of serpentine, is the most common mineral species in industrial asbestos, as it is less dangerous in terms of health than the amphibole asbestos.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 593–95</ref>

====Inosilicates====
[[File:Asbestos with muscovite.jpg|left|thumb|[[Asbestiform]] [[tremolite]], part of the amphibole group in the inosilicate subclass]]
Inosilicates consist of tetrahedra repeatedly bonded in chains. These chains can be single, where a tetrahedron is bound to two others to form a continuous chain; alternatively, two chains can be merged to create double-chain silicates. Single-chain silicates have a silicon:oxygen ratio of 1:3 (e.g. [Si<sub>2</sub>O<sub>6</sub>]<sup>4−</sup>), whereas the double-chain variety has a ratio of 4:11, e.g. [Si<sub>8</sub>O<sub>22</sub>]<sup>12−</sup>. Inosilicates contain two important rock-forming mineral groups; single-chain silicates are most commonly [[pyroxene]]s, while double-chain silicates are often [[amphibole]]s.<ref>{{harvnb|Chesterman|Lowe|2008}}, p. 537</ref> Higher-order chains exist (e.g. three-member, four-member, five-member chains, etc.) but they are rare.<ref>{{cite web|url=http://webmineral.com/strunz/strunz.php?class=09&subclass=09.D|title=09.D Inosilicates|publisher=Webmineral.com|access-date=2012-08-20|archive-date=2017-07-02|archive-url=https://web.archive.org/web/20170702022444/http://webmineral.com/strunz/strunz.php?class=09&subclass=09.D|url-status=live}}</ref>

The pyroxene group consists of 21 mineral species.<ref name="DG112" /> Pyroxenes have a general structure formula of XY(Si<sub>2</sub>O<sub>6</sub>), where X is an octahedral site, while Y can vary in coordination number from six to eight. Most varieties of pyroxene consist of permutations of Ca<sup>2+</sup>, Fe<sup>2+</sup> and Mg<sup>2+</sup> to balance the negative charge on the backbone. Pyroxenes are common in the Earth's crust (about 10%) and are a key constituent of mafic igneous rocks.<ref>{{harvnb|Dyar|Gunter|2008}} pp. 612–13</ref>

Amphiboles have great variability in chemistry, described variously as a "mineralogical garbage can" or a "mineralogical shark swimming a sea of elements". The backbone of the amphiboles is the [Si<sub>8</sub>O<sub>22</sub>]<sup>12−</sup>; it is balanced by cations in three possible positions, although the third position is not always used, and one element can occupy both remaining ones. Finally, the amphiboles are usually hydrated, that is, they have a hydroxyl group ([OH]<sup>−</sup>), although it can be replaced by a fluoride, a chloride, or an oxide ion.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 606–12</ref> Because of the variable chemistry, there are over 80 species of amphibole, although variations, as in the pyroxenes, most commonly involve mixtures of Ca<sup>2+</sup>, Fe<sup>2+</sup> and Mg<sup>2+</sup>.<ref name="DG112">{{harvnb|Dyar|Gunter|2008}}, p. 112</ref> Several amphibole mineral species can have an [[asbestiform]] crystal habit. These asbestos minerals form long, thin, flexible, and strong fibres, which are electrical insulators, chemically inert and heat-resistant; as such, they have several applications, especially in construction materials. However, asbestos are known carcinogens, and cause various other illnesses, such as [[asbestosis]]; amphibole asbestos ([[anthophyllite]], [[tremolite]], [[actinolite]], [[grunerite]], and [[riebeckite]]) are considered more dangerous than [[chrysotile]] serpentine asbestos.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 611–12</ref>

====Cyclosilicates====
[[File:Elbaite-121353.jpg|right|thumb|An example of elbaite, a species of tourmaline, with distinctive colour banding.]]
Cyclosilicates, or ring silicates, have a ratio of silicon to oxygen of 1:3. Six-member rings are most common, with a base structure of [Si<sub>6</sub>O<sub>18</sub>]<sup>12−</sup>; examples include the [[tourmaline]] group and [[beryl]]. Other ring structures exist, with 3, 4, 8, 9, 12 having been described.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 113–15</ref>  Cyclosilicates tend to be strong, with elongated, striated crystals.<ref>{{harvnb|Chesterman|Lowe|2008}}, p. 558</ref>

Tourmalines have a very complex chemistry that can be described by a general formula XY<sub>3</sub>Z<sub>6</sub>(BO<sub>3</sub>)<sub>3</sub>T<sub>6</sub>O<sub>18</sub>V<sub>3</sub>W. The T<sub>6</sub>O<sub>18</sub> is the basic ring structure, where T is usually Si<sup>4+</sup>, but substitutable by Al<sub>3+</sub> or B<sup>3+</sup>. Tourmalines can be subgrouped by the occupancy of the X site, and from there further subdivided by the chemistry of the W site. The Y and Z sites can accommodate a variety of cations, especially various transition metals; this variability in structural transition metal content gives the tourmaline group greater variability in colour. Other cyclosilicates include beryl, Al<sub>2</sub>Be<sub>3</sub>Si<sub>6</sub>O<sub>18</sub>, whose varieties include the gemstones emerald (green) and aquamarine (bluish). [[Cordierite]] is structurally similar to beryl, and is a common metamorphic mineral.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 617–21</ref>

====Sorosilicates====
[[File:Epidote Oisans.jpg|thumb|left|upright=1.25|Epidote often has a distinctive pistachio-green colour.]]
Sorosilicates, also termed disilicates, have tetrahedron-tetrahedron bonding at one oxygen, which results in a 2:7 ratio of silicon to oxygen. The resultant common structural element is the [Si<sub>2</sub>O<sub>7</sub>]<sup>6−</sup> group. The most common disilicates by far are members of the [[epidote]] group. Epidotes are found in variety of geologic settings, ranging from mid-ocean ridge to granites to [[pelite|metapelites]]. Epidotes are built around the structure [(SiO<sub>4</sub>)(Si<sub>2</sub>O<sub>7</sub>)]<sup>10−</sup> structure; for example, the mineral ''species'' epidote has calcium, aluminium, and ferric iron to charge balance: Ca<sub>2</sub>Al<sub>2</sub>(Fe<sup>3+</sup>, Al)(SiO<sub>4</sub>)(Si<sub>2</sub>O<sub>7</sub>)O(OH). The presence of iron as Fe<sup>3+</sup> and Fe<sup>2+</sup> helps buffer oxygen [[fugacity]], which in turn is a significant factor in petrogenesis.<ref name="DG612-627">{{harvnb|Dyar|Gunter|2008}}, pp. 612–27</ref>

Other examples of sorosilicates include [[lawsonite]], a  metamorphic mineral forming in the [[blueschist]] facies (subduction zone setting with low temperature and high pressure), [[vesuvianite]], which takes up a significant amount of calcium in its chemical structure.<ref name="DG612-627" /><ref>{{harvnb|Chesterman|Lowe|2008}}, pp. 565–73</ref>

====Orthosilicates====
[[File:Andradite-172390.jpg|right|thumb|Black andradite, an end-member of the orthosilicate garnet group.]]
Orthosilicates consist of isolated tetrahedra that are charge-balanced by other cations.<ref name="DG116-117">{{harvnb|Dyar|Gunter|2008}}, pp. 116–17</ref> Also termed nesosilicates, this type of silicate has a silicon:oxygen ratio of 1:4 (e.g. SiO<sub>4</sub>). Typical orthosilicates tend to form blocky equant crystals, and are fairly hard.<ref>{{harvnb|Chesterman|Lowe|2008}}, p. 573</ref> Several rock-forming minerals are part of this subclass, such as the aluminosilicates, the olivine group, and the garnet group.

The aluminosilicates –bkyanite, andalusite, and sillimanite, all Al<sub>2</sub>SiO<sub>5</sub> – are structurally composed of one [SiO<sub>4</sub>]<sup>4−</sup> tetrahedron, and one Al<sup>3+</sup> in octahedral coordination. The remaining Al<sup>3+</sup> can be in six-fold coordination (kyanite), five-fold (andalusite) or four-fold (sillimanite); which mineral forms in a given environment is depend on pressure and temperature conditions. In the olivine structure, the main olivine series of (Mg, Fe)<sub>2</sub>SiO<sub>4</sub> consist of magnesium-rich forsterite and iron-rich fayalite. Both iron and magnesium are in octahedral by oxygen. Other mineral species having this structure exist, such as [[tephroite]], Mn<sub>2</sub>SiO<sub>4</sub>.<ref>{{harvnb|Chesterman|Lowe|2008}}, pp. 574–75</ref> The garnet group has a general formula of X<sub>3</sub>Y<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub>, where X is a large eight-fold coordinated cation, and Y is a smaller six-fold coordinated cation. There are six ideal endmembers of garnet, split into two group. The pyralspite garnets have Al<sup>3+</sup> in the Y position: [[pyrope]] (Mg<sub>3</sub>Al<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub>), [[almandine]] (Fe<sub>3</sub>Al<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub>), and [[spessartine]] (Mn<sub>3</sub>Al<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub>). The ugrandite garnets have Ca<sup>2+</sup> in the X position: [[uvarovite]] (Ca<sub>3</sub>Cr<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub>), [[grossular]] (Ca<sub>3</sub>Al<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub>) and [[andradite]] (Ca<sub>3</sub>Fe<sub>2</sub>(SiO<sub>4</sub>)<sub>3</sub>). While there are two subgroups of garnet, solid solutions exist between all six end-members.<ref name="DG116-117"/>

Other orthosilicates include [[zircon]], [[staurolite]], and [[topaz]]. Zircon (ZrSiO<sub>4</sub>) is useful in geochronology as U<sup>6+</sup> can substitute for Zr<sup>4+</sup>; furthermore, because of its very resistant structure, it is difficult to reset it as a chronometer. Staurolite is a common metamorphic intermediate-grade index mineral. It has a particularly complicated crystal structure that was only fully described in 1986. Topaz (Al<sub>2</sub>SiO<sub>4</sub>(F, OH)<sub>2</sub>, often found in granitic pegmatites associated with [[tourmaline]], is a common gemstone mineral.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 627–34</ref>