Editing Mineral (section)

==Classification==

===Earliest classifications===

In 315 [[BCE]], [[Theophrastus]] presented his classification of minerals in his treatise ''On Stones''. His classification was influenced by the ideas of his teachers [[Plato]]  and [[Aristotle]]. Theophrastus classified minerals as stones, earths or metals.<ref name="Staples1983">{{cite encyclopedia |last=Staples |first=L.W. |chapter=Mineral classification: History |encyclopedia=Encyclopedia of Mineralogy |series=Encyclopedia of Earth Science |date=1983 |pages=247–249 |publisher=Springer |location=Boston |doi=10.1007/0-387-30720-6_76 |isbn=978-0-87933-184-9 |doi-access=free }}</ref>

[[Georgius Agricola]]'s classification of minerals in his book ''De Natura Fossilium'', published in 1546, divided minerals into three types of substance: simple (stones, earths, metals, and congealed juices), compound (intimately mixed) and composite (separable).<ref name="Staples1983"/>

===Linnaeus===
An early classification of minerals was given by [[Carl Linnaeus]] in his seminal 1735 book ''[[Systema Naturae]]''. He divided the natural world into three kingdoms&nbsp;– plants, animals, and minerals&nbsp;– and classified each with the same hierarchy.<ref name="wilk">{{cite book|last1=Wilk|first1=H|editor1-last=Wilk|editor1-first=H|title=The Magic of Minerals|date=1986|publisher=Springer|location=Berlin|isbn=978-3-642-64783-3|page=154|chapter-format=Hardcover|chapter=Systematic Classification of Minerals|chapter-url=https://link.springer.com/book/10.1007/978-3-642-61304-3|doi=10.1007/978-3-642-61304-3_7|access-date=2018-11-13|archive-date=2018-11-14|archive-url=https://web.archive.org/web/20181114010823/https://link.springer.com/book/10.1007/978-3-642-61304-3|url-status=live}}</ref> In descending order, these were Phylum, Class, Order, Family, Tribe, Genus, and Species. However, while his system was justified by [[Charles Darwin]]'s theory of species formation and has been largely adopted and expanded by [[biologist]]s in the following centuries (who still use his Greek- and Latin-based [[binomial name|binomial naming]] scheme), it had little success among mineralogists (although each distinct mineral is still formally referred to as a mineral ''species'').

===Modern classification===
{{See also|Abundance of elements in Earth's crust}}

Minerals are classified by variety, species, series and group, in order of increasing generality. The basic level of definition is that of mineral species, each of which is distinguished from the others by unique chemical and physical properties. For example, quartz is defined by its [[Chemical formula|formula]], SiO<sub>2</sub>, and a specific [[Crystal structure|crystalline structure]] that distinguishes it from other minerals with the same chemical formula (termed [[Polymorphism (materials science)|polymorphs]]). When there exists a range of composition between two minerals species, a mineral series is defined. For example, the [[biotite]] series is represented by variable amounts of the [[endmembers]] [[phlogopite]], [[siderophyllite]], [[annite]], and [[eastonite]]. In contrast, a mineral group is a grouping of mineral species with some common chemical properties that share a crystal structure. The [[pyroxene]] group has a common  formula of XY(Si,Al)<sub>2</sub>O<sub>6</sub>, where X and Y are both cations, with X typically [[ionic radius|bigger]] than Y; the pyroxenes are single-chain silicates that crystallize in either the [[orthorhombic]] or [[monoclinic]] crystal systems. Finally, a mineral variety is a specific type of mineral species that differs by some physical characteristic, such as colour or crystal habit. An example is [[amethyst]], which is a purple variety of quartz.<ref name="DG20-22">{{harvnb|Dyar|Gunter|2008}}, pp. 20–22</ref>

Two common classifications, Dana and Strunz, are used for minerals; both rely on composition, specifically with regards to important chemical groups, and structure. [[James Dwight Dana]], a leading geologist of his time, first published his ''System of Mineralogy'' in 1837; {{As of|1997|lc=y}}, it is in its eighth edition. The [[Dana classification]] assigns a four-part number to a mineral species. Its class number is based on important compositional groups; the type gives the ratio of cations to anions in the mineral, and the last two numbers group minerals by structural similarity within a given type or class. The less commonly used [[Strunz classification]], named for German mineralogist [[Karl Hugo Strunz]], is based on the Dana system, but combines both chemical and structural criteria, the latter with regards to distribution of chemical bonds.<ref>{{harvnb|Dyar|Gunter|2008}}, pp 558–59</ref>

As the composition of the Earth's crust is dominated by silicon and oxygen, silicates are by far the most important class of minerals in terms of rock formation and diversity. However, non-silicate minerals are of great economic importance, especially as ores.<ref>{{harvnb|Dyar|Gunter|2008}}, p. 641</ref><ref name="{{harvnb|Dyar|Gunter|2008}}, p. 681">{{harvnb|Dyar|Gunter|2008}}, p. 681</ref> Non-silicate minerals are subdivided into several other classes by their dominant chemistry, which includes native elements, sulfides, halides, oxides and hydroxides, carbonates and nitrates, borates, sulfates, phosphates, and organic compounds. Most non-silicate mineral species are rare (constituting in total 8% of the Earth's crust), although some are relatively common, such as calcite, [[pyrite]], [[magnetite]], and [[hematite]]. There are two major structural styles observed in non-silicates: close-packing and silicate-like linked tetrahedra. [[Close-packing of equal spheres|Close-packed structures]] are a way to densely pack atoms while minimizing interstitial space. Hexagonal close-packing involves stacking layers where every other layer is the same ("ababab"), whereas cubic close-packing involves stacking groups of three layers ("abcabcabc"). Analogues to linked silica tetrahedra include {{Chem|S|O|4|4-}} ([[sulfate]]), {{Chem|P|O|4|4-}} ([[phosphate]]), {{Chem|As|O|4|4-}} ([[arsenate]]), and {{Chem|V|O|4|4-}} ([[vanadate]]) structures. The non-silicates have great economic importance, as they concentrate elements more than the silicate minerals do.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 641–43</ref>

The largest grouping of minerals by far are the [[Silicate minerals|silicates]]; most rocks are composed of greater than 95% silicate minerals, and over 90% of the Earth's crust is composed of these minerals.<ref name="{{harvnb|Dyar|Gunter|2008}}, p. 104">{{harvnb|Dyar|Gunter|2008}}, p. 104</ref> The two main constituents of silicates are silicon and oxygen, which are the two most abundant elements in the Earth's crust. Other common elements in silicate minerals correspond to other common elements in the Earth's crust, such as aluminium, magnesium, iron, calcium, sodium, and potassium.<ref>{{harvnb|Dyar|Gunter|2008}}, p. 5</ref> Some important rock-forming silicates include the [[feldspar]]s, quartz, [[olivine]]s, [[pyroxene]]s, [[amphibole]]s, [[garnet]]s, and [[mica]]s.

===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>

===Non-silicates===

====Native elements====
{{main article|Native element minerals}}
[[File:Gold-mz4b.jpg|left|thumb|Native gold. Rare specimen of stout crystals growing off of a central stalk, size 3.7 x 1.1 x 0.4 cm, from Venezuela.]]
[[native element minerals|Native elements]] are those that are not chemically bonded to other elements. This mineral group includes [[native metal]]s, semi-metals, and non-metals, and various alloys and solid solutions. The metals are held together by metallic bonding, which confers distinctive physical properties such as their shiny metallic lustre, ductility and malleability, and electrical conductivity. Native elements are subdivided into groups by their structure or chemical attributes.

The gold group, with a cubic close-packed structure, includes metals such as gold, silver, and copper. The platinum group is similar in structure to the gold group. The iron-nickel group is characterized by several iron-nickel alloy species. Two examples are [[kamacite]] and [[taenite]], which are found in iron meteorites; these species differ by the amount of Ni in the alloy; kamacite has less than 5–7% nickel and is a variety of [[telluric iron|native iron]], whereas the nickel content of taenite ranges from 7–37%. Arsenic group minerals consist of semi-metals, which have only some metallic traits; for example, they lack the malleability of metals. Native carbon occurs in two allotropes, graphite and diamond; the latter forms at very high pressure in the mantle, which gives it a much stronger structure than graphite.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 644–48</ref>

====Sulfides====
{{main article|Sulfide minerals}}
[[File:Cinnabar on Dolomite.jpg|right|thumb|Red cinnabar (HgS), a mercury ore, on dolomite.]]
[[File:Geodized brachiopod.jpg|right|thumb|Sphalerite crystal partially encased in [[calcite]] from the [[Devonian]] [[Milwaukee Formation]] of [[Wisconsin]]]]
The [[sulfide minerals]] are chemical compounds of one or more metals or semimetals with a [[chalcogen]] or [[pnictogen]], of which sulfur is most common. Tellurium, arsenic, or selenium can substitute for the sulfur. Sulfides tend to be soft, brittle minerals with a high specific gravity. Many powdered sulfides, such as pyrite, have a sulfurous smell when powdered. Sulfides are susceptible to weathering, and many readily dissolve in water; these dissolved minerals can be later redeposited, which creates enriched secondary ore deposits.<ref>{{harvnb|Chesterman|Lowe|2008}}, p. 357</ref> Sulfides are classified by the ratio of the metal or semimetal to the sulfur, such as M:S equal to 2:1, or 1:1.<ref>{{harvnb|Dyar|Gunter|2008}}, p. 649</ref> Many [[sulfide mineral]]s are economically important as metal [[ore]]s; examples include [[sphalerite]] (ZnS), an ore of zinc, [[galena]] (PbS), an ore of lead, [[cinnabar]] (HgS), an ore of mercury, and [[molybdenite]] (MoS<sub>2</sub>, an ore of molybdenum.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 651–54</ref> Pyrite (FeS<sub>2</sub>), is the most commonly occurring sulfide, and can be found in most geological environments. It is not, however, an ore of iron, but can be instead oxidized to produce [[sulfuric acid]].<ref>{{harvnb|Dyar|Gunter|2008}}, p. 654</ref> Related to the sulfides are the rare [[Sulfosalt mineral|sulfosalts]], in which a metallic element is bonded to sulfur and a semimetal such as [[antimony]], [[arsenic]], or [[bismuth]]. Like the sulfides, sulfosalts are typically soft, heavy, and brittle minerals.<ref>{{harvnb|Chesterman|Lowe|2008}}, p. 383</ref>

====Oxides====
{{main article|Oxide minerals}}
[[Oxide minerals]] are divided into three categories: simple oxides, hydroxides, and multiple oxides. Simple oxides are characterized by O<sup>2−</sup> as the main anion and primarily ionic bonding. They can be further subdivided by the ratio of oxygen to the cations. The [[periclase]] group consists of minerals with a 1:1 ratio. Oxides with a 2:1 ratio include [[cuprite]] (Cu<sub>2</sub>O) and water ice. Corundum group minerals have a 2:3 ratio, and includes minerals such as [[corundum]] (Al<sub>2</sub>O<sub>3</sub>), and [[hematite]] (Fe<sub>2</sub>O<sub>3</sub>). Rutile group minerals have a ratio of 1:2; the eponymous species, rutile (TiO<sub>2</sub>) is the chief ore of [[titanium]]; other examples include [[cassiterite]] (SnO<sub>2</sub>; ore of [[tin]]), and [[pyrolusite]] (MnO<sub>2</sub>; ore of [[manganese]]).<ref>{{harvnb|Chesterman|Lowe|2008}}, pp. 400–03</ref><ref>{{harvnb|Dyar|Gunter|2008}}, pp. 657–60</ref>  In hydroxides, the dominant anion is the hydroxyl ion, OH<sup>−</sup>. [[Bauxite]]s are the chief aluminium ore, and are a heterogeneous mixture of the hydroxide minerals [[diaspore]], [[gibbsite]], and [[bohmite]]; they form in areas with a very high rate of chemical weathering (mainly tropical conditions).<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 663–64</ref>  Finally, multiple oxides are compounds of two metals with oxygen. A major group within this class are the [[spinel group|spinels]], with a general formula of X<sup>2+</sup>Y<sup>3+</sup><sub>2</sub>O<sub>4</sub>. Examples of species include [[spinel]] (MgAl<sub>2</sub>O<sub>4</sub>), [[chromite]] (FeCr<sub>2</sub>O<sub>4</sub>), and [[magnetite]] (Fe<sub>3</sub>O<sub>4</sub>). The latter is readily distinguishable by its strong magnetism, which occurs as it has iron in two [[oxidation state]]s (Fe<sup>2+</sup>Fe<sup>3+</sup><sub>2</sub>O<sub>4</sub>), which makes it a multiple oxide instead of a single oxide.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 660–63</ref>

====Halides====
{{main article|Halide minerals}}
[[File:Halite-Nahcolite-51411.jpg|left|thumb|Pink cubic [[halite]] (NaCl; halide class) crystals on a [[nahcolite]] matrix (NaHCO<sub>3</sub>; a carbonate, and mineral form of sodium bicarbonate, used as [[baking soda]]).]]

The [[halide minerals]] are compounds in which a [[halogen]] (fluorine, chlorine, iodine, or bromine) is the main anion. These minerals tend to be soft, weak, brittle, and water-soluble. Common examples of halides include halite (NaCl, table salt), [[sylvite]] (KCl), and [[fluorite]] (CaF<sub>2</sub>). Halite and sylvite commonly form as [[evaporite]]s, and can be dominant minerals in chemical sedimentary rocks. [[Cryolite]], Na<sub>3</sub>AlF<sub>6</sub>, is a key mineral in the extraction of aluminium from [[bauxite]]s; however, as the only significant occurrence at [[Ivittuut]], [[Greenland]], in a granitic pegmatite, was depleted, synthetic cryolite can be made from fluorite.<ref>{{harvnb|Chesterman|Lowe|2008}}, pp. 425–30</ref>

====Carbonates====
{{main article|Carbonate minerals}}
The [[carbonate minerals]] are those in which the main anionic group is carbonate, [CO<sub>3</sub>]<sup>2−</sup>. Carbonates tend to be brittle, many have rhombohedral cleavage, and all react with acid.<ref>{{harvnb|Chesterman|Lowe|2008}}, p. 431</ref> Due to the last characteristic, field geologists often carry dilute hydrochloric acid to distinguish carbonates from non-carbonates. The reaction of acid with carbonates, most commonly found as the polymorph calcite and [[aragonite]] (CaCO<sub>3</sub>), relates to the dissolution and precipitation of the mineral, which is a key in the formation of limestone caves, features within them such as stalactite and stalagmites, and [[karst]] landforms. Carbonates are most often formed as biogenic or chemical sediments in marine environments. The carbonate group is structurally a triangle, where a central C<sup>4+</sup> cation is surrounded by three O<sup>2−</sup> anions; different groups of minerals form from different arrangements of these triangles.<ref>{{harvnb|Dyar|Gunter|2008}}, p. 667</ref> The most common carbonate mineral is calcite, which is the primary constituent of sedimentary limestone and metamorphic marble. Calcite, CaCO<sub>3</sub>, can have a significant percentage of magnesium substituting for calcium. Under high-Mg conditions, its polymorph aragonite will form instead; the marine geochemistry in this regard can be described as an [[aragonite sea|aragonite]] or [[calcite sea]], depending on which mineral preferentially forms. [[Dolomite (mineral)|Dolomite]] is a double carbonate, with the formula CaMg(CO<sub>3</sub>)<sub>2</sub>. Secondary dolomitization of limestone is common, in which calcite or aragonite are converted to dolomite; this reaction increases pore space (the unit cell volume of dolomite is 88% that of calcite), which can create a reservoir for oil and gas. These two mineral species are members of eponymous mineral groups: the calcite group includes carbonates with the general formula XCO<sub>3</sub>, and the dolomite group constitutes minerals with the general formula XY(CO<sub>3</sub>)<sub>2</sub>.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 668–69</ref>

====Sulfates====
{{main article|Sulfate minerals}}
[[File:Roses des Sables Tunisie.jpg|right|thumb|upright=1.15|Gypsum desert rose]]
The [[sulfate mineral]]s all contain the sulfate anion, [SO<sub>4</sub>]<sup>2−</sup>. They tend to be transparent to translucent, soft, and many are fragile.<ref>{{harvnb|Chesterman|Lowe|2008}}, p. 453</ref> Sulfate minerals commonly form as [[evaporite]]s, where they precipitate out of evaporating saline waters. Sulfates can also be found in hydrothermal vein systems associated with sulfides,<ref>{{harvnb|Chesterman|Lowe|2008}}, pp. 456–57</ref> or as oxidation products of sulfides.<ref>{{harvnb|Dyar|Gunter|2008}}, p. 674</ref> Sulfates can be subdivided into anhydrous and hydrous minerals. The most common hydrous sulfate by far is [[gypsum]], CaSO<sub>4</sub>⋅2H<sub>2</sub>O. It forms as an evaporite, and is associated with other evaporites such as calcite and halite; if it incorporates sand grains as it crystallizes, gypsum can form [[Desert rose (crystal)|desert roses]]. Gypsum has very low thermal conductivity and maintains a low temperature when heated as it loses that heat by dehydrating; as such, gypsum is used as an insulator in materials such as plaster and drywall. The anhydrous equivalent of gypsum is [[anhydrite]]; it can form directly from seawater in highly arid conditions. The barite group has the general formula XSO<sub>4</sub>, where the X is a large 12-coordinated cation. Examples include [[barite]] (BaSO<sub>4</sub>), [[Celestine (mineral)|celestine]] (SrSO<sub>4</sub>), and [[anglesite]] (PbSO<sub>4</sub>); anhydrite is not part of the barite group, as the smaller Ca<sup>2+</sup> is only in eight-fold coordination.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 672–73</ref>

====Phosphates====
{{main article|Phosphate minerals}}
The [[phosphate minerals]] are characterized by the tetrahedral [PO<sub>4</sub>]<sup>3−</sup> unit, although the structure can be generalized, and phosphorus is replaced by antimony, arsenic, or vanadium. The most common phosphate is the [[apatite]] group; common species within this group are fluorapatite (Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>F), chlorapatite (Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>Cl) and hydroxylapatite (Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>(OH)). Minerals in this group are the main crystalline constituents of teeth and bones in vertebrates. The relatively abundant [[monazite]] group has a general structure of ATO<sub>4</sub>, where T is phosphorus or arsenic, and A is often a [[rare-earth element]] (REE). Monazite is important in two ways: first, as a REE "sink", it can sufficiently concentrate these elements to become an ore; secondly, monazite group elements can incorporate relatively large amounts of uranium and thorium, which can be used in [[monazite geochronology]] to date the rock based on the decay of the U and Th to lead.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 675–80</ref>

====Organic minerals====
{{main article|Organic mineral}}
The Strunz classification includes a class for [[Strunz classification#Class: organic compounds|organic minerals]]. These rare compounds contain [[Organic compound|organic carbon]], but can be formed by a geologic process. For example, [[whewellite]], CaC<sub>2</sub>O<sub>4</sub>⋅H<sub>2</sub>O is an [[oxalate]] that can be deposited in hydrothermal ore veins. While hydrated calcium oxalate can be found in coal seams and other sedimentary deposits involving organic matter, the hydrothermal occurrence is not considered to be related to biological activity.<ref name="{{harvnb|Dyar|Gunter|2008}}, p. 681"/>

===Recent advances===
Mineral classification schemes and their definitions are evolving to match recent advances in mineral science. Recent changes have included the addition of an organic class, in both the new Dana and the [[Strunz classification]] schemes.<ref name=Dana>{{cite web|title=Dana Classification 8th edition – Organic Compounds|url=http://www.mindat.org/dana.php?a=50|website=mindat.org|access-date=3 April 2018|archive-date=12 November 2016|archive-url=https://web.archive.org/web/20161112130718/http://www.mindat.org/dana.php?a=50|url-status=live}}</ref><ref name=Strunz>{{cite web|title=Nickel-Strunz Classification – silicates (Germanates) 10th edition|url=https://www.mindat.org/strunz.php?a=9|website=mindat.org|access-date=3 April 2018|archive-date=5 November 2018|archive-url=https://web.archive.org/web/20181105215152/https://www.mindat.org/strunz.php?a=9|url-status=live}}</ref> The organic class includes a very rare group of minerals with [[hydrocarbons]]. The IMA Commission on New Minerals and Mineral Names adopted in 2009 a hierarchical scheme for the naming and classification of mineral groups and group names and established seven commissions and four working groups to review and classify minerals into an official listing of their published names.<ref name="Mills09" /><ref>[http://www.ima-mineralogy.org/sciactivities.html IMA divisions] {{webarchive|url=https://web.archive.org/web/20110810045840/http://www.ima-mineralogy.org/sciactivities.html |date=2011-08-10 }}. Ima-mineralogy.org (2011-01-12). Retrieved on 2011-10-20.</ref>  According to these new rules, "mineral species can be grouped in a number of different ways, on the basis of chemistry, crystal structure, occurrence, association, genetic history, or resource, for example, depending on the purpose to be served by the classification."<ref name="Mills09">{{cite journal | last1=Mills | first1=J.S. | last2=Hatert | first2=F. | last3=Nickel | first3=E.H. | last4=Ferraris | first4=G. | title=The standardisation of mineral group hierarchies: application to recent nomenclature proposals | journal=European Journal of Mineralogy | volume=21 | pages=1073–80 | doi=10.1127/0935-1221/2009/0021-1994 | date=2009 | issue=5 | bibcode=2009EJMin..21.1073M | hdl=2268/29163 | hdl-access=free }}</ref>