Editing Mineral (section)

==Chemistry==
[[File:Hubnerite-Quartz-216455.jpg|left|thumb|upright|[[Hübnerite]], the manganese-rich end-member of the [[wolframite]] series, with minor quartz in the background]]
The abundance and diversity of minerals is controlled directly by their chemistry, in turn dependent on elemental abundances in the Earth. The majority of minerals observed are derived from the [[Earth's crust]]. Eight elements account for most of the key components of minerals, due to their abundance in the crust. These eight elements, summing to over 98% of the crust by weight, are, in order of decreasing abundance: [[oxygen]], [[silicon]], [[aluminium]], [[iron]], [[magnesium]], [[calcium]], [[sodium]] and [[potassium]]. Oxygen and silicon are by far the two most important&nbsp;– oxygen composes 47% of the crust by weight, and silicon accounts for 28%.<ref name="DG4-7">{{harvnb|Dyar|Gunter|2008}}, pp. 4–7</ref>

The minerals that form are those that are most stable at the temperature and pressure of formation, within the limits imposed by the bulk chemistry of the parent body.<ref>{{cite book |last1=Sinkankas |first1=John |title=Mineralogy for amateurs. |date=1964 |publisher=Van Nostrand |location=Princeton, N.J. |isbn=0-442-27624-9 |page=237}}</ref> For example, in most igneous rocks, the aluminium and alkali metals (sodium and potassium) that are present are  primarily found in combination with oxygen, silicon, and calcium as feldspar minerals. However, if the rock is unusually rich in alkali metals, there will not be enough aluminium to combine with all the sodium as feldspar, and the excess sodium will form sodic amphiboles such as [[riebeckite]]. If the aluminium abundance is unusually high, the excess aluminium will form [[muscovite]] or other aluminium-rich minerals.<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Tracy |first2=Robert J. |title=Petrology: igneous, sedimentary, and metamorphic. |date=1996 |publisher=W.H. Freeman |location=New York |isbn=0-7167-2438-3 |edition=2nd |page=185}}</ref> If silicon is deficient, part of the feldspar will be replaced by feldspathoid minerals.{{sfn|Nesse|2000|p=226}} Precise predictions of which minerals will be present in a rock of a particular composition formed at a particular temperature and pressure requires complex thermodynamic calculations. However, approximate estimates may be made using relatively simple [[rules of thumb]], such as the [[Normative mineralogy|CIPW norm]], which gives reasonable estimates for volcanic rock formed from dry magma.<ref>{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=978-0-521-88006-0 |edition=2nd |pages=133–137}}</ref>

The chemical composition may vary between [[Endmember (mineralogy)|end member]] species of a [[solid solution]] series. For example, the [[plagioclase]] [[feldspar]]s comprise a continuous series from [[sodium]]-rich end member [[albite]] (NaAlSi<sub>3</sub>O<sub>8</sub>) to [[calcium]]-rich [[anorthite]] (CaAl<sub>2</sub>Si<sub>2</sub>O<sub>8</sub>) with four recognized intermediate varieties between them (given in order from sodium- to calcium-rich): [[oligoclase]], [[andesine]], [[labradorite]], and [[bytownite]].<ref>{{harvnb|Dyar|Gunter|2008}}, p. 586</ref> Other examples of series include the olivine series of magnesium-rich forsterite and iron-rich fayalite, and the [[wolframite]] series of [[manganese]]-rich [[hübnerite]] and iron-rich [[ferberite]].{{sfn|Nesse|2000|pp=308, 352}}

Chemical substitution and coordination polyhedra explain this common feature of minerals. In nature, minerals are not pure substances, and are contaminated by whatever other elements are present in the given chemical system. As a result, it is possible for one element to be substituted for another.<ref>{{harvnb|Dyar|Gunter|2008}}, p. 141</ref> Chemical substitution will occur between ions of a similar size and charge; for example, K<sup>+</sup> will not substitute for Si<sup>4+</sup> because of chemical and structural incompatibilities caused by a big difference in size and charge. A common example of chemical substitution is that of Si<sup>4+</sup> by Al<sup>3+</sup>, which are close in charge, size, and abundance in the crust. In the example of plagioclase, there are three cases of substitution. Feldspars are all framework silicates, which have a silicon-oxygen ratio of 2:1, and the space for other elements is given by the substitution of Si<sup>4+</sup> by Al<sup>3+</sup> to give a base unit of [AlSi<sub>3</sub>O<sub>8</sub>]<sup>−</sup>; without the substitution, the formula would be charge-balanced as SiO<sub>2</sub>, giving quartz.<ref>{{harvnb|Dyar|Gunter|2008}}, p. 14</ref> The significance of this structural property will be explained further by coordination polyhedra. The second substitution occurs between Na<sup>+</sup> and Ca<sup>2+</sup>; however, the difference in charge has to accounted for by making a second substitution of Si<sup>4+</sup> by Al<sup>3+</sup>.<ref>{{harvnb|Dyar|Gunter|2008}}, p. 585</ref>

Coordination polyhedra are geometric representations of how a cation is surrounded by an anion. In mineralogy, coordination polyhedra are usually considered in terms of oxygen, due its abundance in the crust. The base unit of silicate minerals is the silica tetrahedron&nbsp;– one Si<sup>4+</sup> surrounded by four O<sup>2−</sup>. An alternate way of describing the coordination of the silicate is by a number: in the case of the silica tetrahedron, the silicon is said to have a coordination number of 4. Various cations have a specific range of possible coordination numbers; for silicon, it is almost always 4, except for very high-pressure minerals where the compound is compressed such that silicon is in six-fold (octahedral) coordination with oxygen. Bigger cations have a bigger coordination numbers because of the increase in relative size as compared to oxygen (the last [[Atomic orbital|orbital subshell]] of heavier atoms is different too). Changes in coordination numbers leads to physical and mineralogical differences; for example, at high pressure, such as in the [[Mantle (geology)|mantle]], many minerals, especially silicates such as [[olivine]] and [[garnet]], will change to a [[perovskite structure]], where silicon is in octahedral coordination. Other examples are the aluminosilicates [[kyanite]], [[andalusite]], and [[sillimanite]] (polymorphs, since they share the formula Al<sub>2</sub>SiO<sub>5</sub>), which differ by the coordination number of the Al<sup>3+</sup>; these minerals transition from one another as a response to changes in pressure and temperature.<ref name="DG4-7"/> In the case of silicate materials, the substitution of Si<sup>4+</sup> by Al<sup>3+</sup> allows for a variety of minerals because of the need to balance charges.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 12–17</ref>

Because the eight most common elements make up over 98% of the Earth's crust, the small quantities of the other elements that are typically present are substituted into the common rock-forming minerals. The distinctive minerals of most elements are quite rare, being found only where these elements have been concentrated by geological processes, such as [[hydrothermal circulation]], to the point where they can no longer be accommodated in common minerals.{{sfn|Sinkankas|1964|pp=238-239}}

[[File:Kaolinite-Orthoclase-lw18c.jpg|thumb|When minerals react, the products will sometimes assume the shape of the reagent; the product mineral is termed a pseudomorph of (or after) the reagent. Illustrated here is a pseudomorph of [[kaolinite]] after [[orthoclase]]. Here, the pseudomorph preserved the Carlsbad [[Crystal twinning|twinning]] common in orthoclase.]]

Changes in temperature and pressure and composition alter the mineralogy of a rock sample. Changes in composition can be caused by processes such as [[weathering]] or [[metasomatism]] ([[hydrothermal alteration]]). Changes in temperature and pressure occur when the host rock undergoes [[tectonic]] or [[magmatic]] movement into differing physical regimes. Changes in [[thermodynamic]] conditions make it favourable for mineral assemblages to react with each other to produce new minerals; as such, it is possible for two rocks to have an identical or a very similar bulk rock chemistry without having a similar mineralogy. This process of mineralogical alteration is related to the [[rock cycle]]. An example of a series of mineral reactions is illustrated as follows.<ref name="DG549">{{harvnb|Dyar|Gunter|2008}}, p. 549</ref>

[[Orthoclase]] feldspar (KAlSi<sub>3</sub>O<sub>8</sub>) is a mineral commonly found in [[granite]], a [[plutonic]] [[igneous rock]]. When exposed to weathering, it reacts to form [[kaolinite]] (Al<sub>2</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub>, a sedimentary mineral, and [[silicic acid]]):

:2 KAlSi<sub>3</sub>O<sub>8</sub> + 5 H<sub>2</sub>O + 2 H<sup>+</sup> → Al<sub>2</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub> + 4 H<sub>2</sub>SiO<sub>3</sub> + 2 K<sup>+</sup>

Under low-grade metamorphic conditions, kaolinite reacts with [[quartz]] to form [[pyrophyllite]] (Al<sub>2</sub>Si<sub>4</sub>O<sub>10</sub>(OH)<sub>2</sub>):

:Al<sub>2</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub> + SiO<sub>2</sub> → Al<sub>2</sub>Si<sub>4</sub>O<sub>10</sub>(OH)<sub>2</sub> + H<sub>2</sub>O

As metamorphic grade increases, the pyrophyllite reacts to form [[kyanite]] and quartz:

:Al<sub>2</sub>Si<sub>4</sub>O<sub>10</sub>(OH)<sub>2</sub> → Al<sub>2</sub>SiO<sub>5</sub> + 3 SiO<sub>2</sub> + H<sub>2</sub>O

Alternatively, a mineral may change its crystal structure as a consequence of changes in temperature and pressure without reacting. For example, quartz will change into a variety of its SiO<sub>2</sub> [[Polymorphism (materials science)|polymorphs]], such as [[tridymite]] and [[cristobalite]] at high temperatures, and [[coesite]] at high pressures.<ref>{{harvnb|Dyar|Gunter|2008}}, p. 579</ref>