Editing Mineral (section)

====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>