page_title: Mineral
section_title: Chemistry
hierarchical_section_title: Mineral / Chemistry
caption_attribution_description: English: Kaolinite, Orthoclase Locality: St Austell District, Cornwall, England, UK (Locality at mindat.org) Size: miniature, 4.5 x 2.6 x 1.1 cm Kaolinite ps. Orthoclase twin Very good and relatively large pseudomorph of Kaolinite after an Orthoclase Karlsbad twin. The off-white color is attractive, but what really makes this is the well-preserved habit of the twinned crystal. Overall, a very fine specimen from one of England�s most famous localities. Deutsch: Kaolinit, Orthoklas Fundort: Distrikt St Austell, Cornwall, England, Vereinigtes Königreich (Fundort bei mindat.org) Größe: 4.5 x 2.6 x 1.1 cm
context_page_description: In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure, that occurs naturally in pure form.
  The geological definition of mineral normally excludes compounds that occur only in living beings. However some minerals are often biogenic or are organic compounds in the sense of chemistry. Moreover, living beings often synthesize inorganic minerals that also occur in rocks.
  The concept of mineral is distinct from rock, any bulk solid geologic material that is relatively homogeneous at a large enough scale. A rock may consist of one type of mineral, or may be an aggregate of two or more different types of minerals, spacially segregated into distinct phases.
  Some natural solid substances without a definite crystalline structure, such as opal or obsidian, are more properly called mineraloids. If a chemical compound may occur naturally with different crystal structures, each structure is considered a different mineral species. Thus, for example, quartz and stishovite are two different minerals consisting of the same compound, silicon dioxide.
context_section_description: 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 – oxygen composes 47% of the crust by weight, and silicon accounts for 28%.
  The minerals that form are directly controlled by the bulk chemistry of the parent body. For example, a magma rich in iron and magnesium will form mafic minerals, such as olivine and the pyroxenes; in contrast, a more silica-rich magma will crystallize to form minerals that incorporate more SiO₂, such as the feldspars and quartz. In a limestone, calcite or aragonite (both CaCO₃) form because the rock is rich in calcium and carbonate.  A corollary is that a mineral will not be found in a rock whose bulk chemistry does not resemble the bulk chemistry of a given mineral with the exception of trace minerals. For example, kyanite, Al₂SiO₅ forms from the metamorphism of aluminium-rich shales; it would not likely occur in aluminium-poor rock, such as quartzite.
  The chemical composition may vary between end member species of a solid solution series. For example, the plagioclase feldspars comprise a continuous series from sodium-rich end member albite (NaAlSi₃O₈) to calcium-rich anorthite (CaAl₂Si₂O₈) with four recognized intermediate varieties between them (given in order from sodium- to calcium-rich): oligoclase, andesine, labradorite, and bytownite. 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.
  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. Chemical substitution will occur between ions of a similar size and charge; for example, K⁺ will not substitute for Si⁴⁺ because of chemical and structural incompatibilities caused by a big difference in size and charge. A common example of chemical substitution is that of Si⁴⁺ by Al³⁺, 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 Si⁴⁺ by Al³⁺ to give a base unit of [AlSi₃O₈]⁻; without the substitution, the formula would be charge-balanced as SiO₂, giving quartz. The significance of this structural property will be explained further by coordination polyhedra. The second substitution occurs between Na⁺ and Ca²⁺; however, the difference in charge has to accounted for by making a second substitution of Si⁴⁺ by Al³⁺.
  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 – one Si⁴⁺ surrounded by four O²⁻. 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 orbital subshell 
caption: 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 twinning common in orthoclase.
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