Mineral

                                   Mineral

                                     

• A mineral is a naturally occurring substance that is solid and stable at room temperature, representable by a chemical formula, usuallyabiogenic, and has an ordered atomic structure. It is different from arock, which can be an aggregate of minerals or non-minerals, and does not have a specific chemical composition. The exact definition of a mineral is under debate, especially with respect to the requirement a valid species be abiogenic, and to a lesser extent with regards to it having an ordered atomic structure. The study of minerals is calledmineralogy.
  There are over 4,900 known mineral species; over 4,660 of these have been approved by the International Mineralogical Association (IMA). The silicate minerals compose over 90% of the Earth's crust. The diversity and abundance of mineral species is controlled by the Earth's chemistry. Silicon and oxygen constitute approximately 75% of the Earth's crust, which translates directly into the predominance of silicate minerals. Minerals are distinguished by various chemical andphysical properties. Differences in chemical composition and crystal structure distinguish various species, and these properties in turn are influenced by the mineral's geological environment of formation. Changes in the temperature, pressure, and bulk composition of a rock mass cause changes in its mineralogy; however, a rock can maintain its bulk composition, but as long as temperature and pressure change, its mineralogy can change as well.
  Minerals can be described by a variable physical properties, which relate to its chemical structure and composition. Common distinguishing characteristics include crystal structure and habit, hardness, lustre,diaphaneity, colour, streak, tenacity, cleavage, fracture, parting, and specific gravity. More specific tests for minerals include reaction to acid, magnetism, taste or smell, and radioactivity.
  Minerals are classified by key chemical constituents; the two dominant systems are the Dana classification and the Strunz classification. The silicate class of minerals is subdivided into six subclasses by the degree ofpolymerization in the chemical structure. All silicate minerals have a base unit of a [SiO₄]^4- silica tetrahedra—that is, a silicon cation coordinated by four oxygen anions, which gives the shape of a tetrahedron. These tetrahedra can be polymerized to give the subclasses: orthosilicates (no polymerization, thus single tetrahedra), disilicates (two tetrahedra bonded together), cyclosilicates (rings of tetrahedra), inosilicates (chains of tetrahedra), phyllosilicates (sheets of tetrahedra), and tectosilicates (three-dimensional network of tetrahedra). Other important mineral groups include the native elements, sulfides, oxides, halides, carbonates, sulfates, and phosphates

  Definition

  Basic definition

  The general definition of a mineral encompasses the following criteria:^[1]
  1. Naturally occurring
  2. Stable at room temperature
  3. Represented by a chemical formula
  4. Usually abiogenic
  5. Ordered atomic arrangement
  The first three general characteristics are less debated than the last two.^[1] The first criterion means that a mineral has to form by a natural process, which excludes anthropogenic compounds. Stability at room temperature, in the simplest sense, is synonymous to the mineral being solid. More specifically, a compound has to be stable or metastable at 25°C. Classical examples of exceptions to this rule include native mercury, which crystallizes at −39°C, and water ice, which is solid only below 0°C; as these two minerals were described prior to 1959, they were grandfathered by the International Mineralogical Association (IMA).^[2][3] Modern advances have included extensive study of liquid crystals, which also extensively involve mineralogy. Minerals are chemical compounds, and as such they can be described by fixed or a variable formula. Many mineral groups and species are composed of a solid solution; pure substances are not usually found because of contamination or chemical substitution. For example, the olivine group is described by the variable formula (Mg, Fe)₂SiO₄, which is a solid solution of two end-member species, magnesium-rich forsterite and iron-rich fayalite, which are described by a fixed chemical formula. Mineral species themselves could have a variable compositions, such as the sulfidemackinawite, (Fe, Ni)₉S₈, which is mostly a ferrous sulfide, but has a very significant nickel impurity that is reflected in its formula.^[1][4]
  The requirement of a valid mineral species to be abiogenic has also been described as similar to have to be inorganic; however, this criterion is imprecise and organic compounds have been assigned a separate classification branch. Finally, the requirement of an ordered atomic arrangement is usually synonymous to being crystalline; however, crystals are periodic in addition to being ordered, so the broader criterion is used instead.^[1]The presence of an ordered atomic arrangement translates to a variety of macroscopic physical properties, such as crystal form, hardness, and cleavage.^[5] There have been several recent proposals to amend the definition to consider biogenic or amorphous substances as minerals. The formal definition of a mineral approved by the IMA in 1995:

  A mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes."^[6]

  In addition, biogenic substances were explicitly excluded:

  "Biogenic substances are chemical compounds produced entirely by biological processes without a geological component (e.g., urinary calculi, oxalate crystals in plant tissues, shells of marine molluscs, etc.) and are not regarded as minerals. However, if geological processes were involved in the genesis of the compound, then the product can be accepted as a mineral."^[6]

  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 classificationschemes.^[7][8] The organic class includes a very rare group of minerals with hydrocarbons. The IMA Commission on New Minerals and Mineral Names recently adopted (in 2009) a hierarchical scheme for the naming and classification of mineral groups and group names^[9] and established seven commissions and four working groups to review and classify minerals into an official listing of their published names.^[10] 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."^[9]
  The Nickel (1995) exclusion of biogenic substances was not universally adhered to. For example, Lowenstam (1981) stated that "organisms are capable of forming a diverse array of minerals, some of which cannot be formed inorganically in the biosphere."^[11] The distinction is a matter of classification and less to do with the constituents of the minerals themselves. Skinner (2005) views all solids as potential minerals and includes biominerals in the mineral kingdom, which are those that are created by the metabolic activities of organisms. Skinner expanded the previous definition of a mineral to classify "element or compound, amorphous or crystalline, formed throughbiogeochemical processes," as a mineral.^[12]
  Recent advances in high-resolution genetic and x-ray absorption spectroscopy is opening new revelations on the biogeochemical relations between microorganisms and minerals that may make Nickel's (1995)^[6] biogenic mineral exclusion obsolete and Skinner's (2005) biogenic mineral inclusion a necessity.^[12] For example, the IMA commissioned 'Environmental Mineralogy and Geochemistry Working Group'^[13] deals with minerals in the hydrosphere, atmosphere, and biosphere. Mineral forming microorganisms inhabit the areas that this working group deals with. These organisms exist on nearly every rock, soil, and particle surface spanning the globe reaching depths at 1600 metres below the sea floor (possibly further) and 70 kilometres into the stratosphere(possibly entering the mesosphere).^[14][15][16] Biologists and geologists have recently started to research and appreciate the magnitude of mineral geoengineering that these creatures are capable of. Bacteria have contributed to the formation of minerals for billions of years and critically define the biogeochemical cycles on this planet. Microorganisms can precipitate metals from solution contributing to the formation of ore deposits in addition to their ability to catalyze mineral dissolution, to respire, precipitate, and form minerals.^[17][18][19]
  Prior to the International Mineralogical Association's listing, over 60 biominerals had been discovered, named, and published.^[20] These minerals (a sub-set tabulated in Lowenstam (1981)^[11]) are considered minerals proper according to the Skinner (2005) definition.^[12] These biominerals are not listed in the International Mineral Association official list of mineral names,^[21] however, many of these biomineral representatives are distributed amongst the 78 mineral classes listed in the Dana classification scheme.^[12] Another rare class of minerals (primarily biological in origin) include the mineral liquid crystals that are crystalline and liquid at the same time. To date over 80,000 liquid crystalline compounds have been identified.^[22][23]

  Concerning the use of the term “mineral” to name this family of liquid crystals, one can argue that the term inorganic would be more appropriate. However, inorganic liquid crystals have long been used for organometallic liquid crystals. Therefore, in order to avoid any confusion between these fairly chemically different families, and taking into account that a large number of these liquid crystals occur naturally in nature, we think that the use of the old fashioned but adequate “mineral” adjective taken sensus largo is more specific that an alternative such as “purely inorganic”, to name this subclass of the inorganic liquid crystals family.^[23]

  The Skinner (2005) definition^[12] of a mineral takes this matter into account by stating that a mineral can be crystalline or amorphous. Liquid mineral crystals are amorphous. Biominerals and liquid mineral crystals, however, are not the primary form of minerals, most are geological in origin,^[24] but these groups do help to identify at the margins of what constitutes a mineral proper. The formal Nickel (1995) definition explicitly mentioned crystalline nature as a key to defining a substance as a mineral. A 2011 article defined icosahedrite, an aluminium-iron-copper alloy as mineral; named for its unique natural icosahedral symmetry, it is also a quasicrystal. Unlike a true crystal, quasicrystals are ordered but not periodic.^[25][26]

  Rocks, ores, and gems

  

  

  Schist is a metamorphic rockcharacterized by an abundance of platy minerals. In this example, the rock has prominent sillimaniteporphyroblasts as large as 3 cm (1.2 in).
  Minerals are not equivalent to rocks. Whereas a mineral is a naturally occurring usually solid substance, stable at room temperature, representable by a chemical formula, usually abiogenic, and has an ordered atomic structure, arock is either an aggregate of one or more minerals, or not composed of minerals at all.^[27] Rocks like limestone or quartzite are composed primarily of one mineral—calcite or aragonite in the case of limestone, and quartz in the latter case.^[28][29] Other rocks can be defined by relative abundances of key (essential) minerals; a granite is defined by proportions of quartz, alkali feldspar, and plagioclase feldspar.^[30] The other minerals in the rock are termed accessory, and do not greatly affect the bulk composition of the rock. Rocks can also be composed entirely of non-mineral material; coal is a sedimentary rock composed primarily of organically derived carbon.^[27][31]
  In rocks, some mineral species and groups are much more abundant than others; these are termed the rock-forming minerals. The major examples of these are quartz, the feldspars, the micas, the amphiboles, the pyroxenes, theolivines, and calcite; except the last one, all of the minerals are silicates.^[32]Overall, around 150 minerals are considered particularly important, whether in terms of their abundance or aesthetic value in terms of collecting.^[33]
  Commercially valuable minerals and rocks are referred to as industrial minerals. For example, muscovite, a white mica, can be used for windows (sometimes referred to as isinglass), as a filler, or as an insulator.^[34] Ores are minerals that have a high concentration of a certain element, typically a metal. Examples are cinnabar (HgS), an ore of mercury, sphalerite (ZnS), an ore of zinc, or cassiterite (SnO₂), an ore of tin. Gems are minerals with an ornamental value, and are distinguished from non-gems by their beauty, durability, and usually, rarity. There are about 20 mineral species that qualify as gem minerals, which constitute about 35 of the most common gemstones. Gem minerals are often present in several varieties, and so one mineral can account for several different gemstones; for example, ruby and sapphire are bothcorundum, Al₂O₃.^[35]

  Nomenclature and classification

  In general, a mineral is defined as naturally occurring solid, that is stable at room temperature, representable by a chemical formula, usually abiogenic, and has an ordered atomic structure. However, a mineral can be also narrowed down in terms of a mineral group, series, species, or variety, in order from most broad to least broad. The basic level of definition is that of mineral species, which is distinguished from other species by specific and unique chemical and physical properties. For example, quartz is defined by its formula, SiO₂, and a specificcrystalline structure that distinguishes it from other minerals with the same chemical formula (termedpolymorphs). 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)₂O₆, where X and Y are both cations, with X typically 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.^[36]
  Two common classifications are used for minerals; both the Dana and Strunz classifications rely on the composition of the mineral, specifically with regards to important chemical groups, and its structure. The DanaSystem of Mineralogy was first published in 1837 by James Dwight Dana, a leading geologist of his time; it is presently in its eighth edition (1997 ed.). The Dana classification, assigns a four-part number to a mineral species. First is its class, based on important compositional groups; next, the type gives the ratio of cations to anions in the mineral; finally, the last two numbers group minerals by structural similarity with 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.^[37]
  There are presently over 4,660 approved mineral species.^[38] They are most commonly named after a person (45%), followed by discovery location (23%); names based on chemical composition (14%) and physical properties (8%) are the two other major groups of mineral name etymologies.^[36][39]

  Mineral chemistry

  

  

  Hübnerite, the manganese-rich end-member of the wolframiteseries, 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 — oxygen composes 46.6% of the crust by weight, and silicon accounts for 27.7%.^[40]
  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 maficminerals, such as olivine and the pyroxenes; in contrast, a more silica-rich magma will crystallize to form minerals than 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 themetamorphism of aluminium-rich shales; it would not likely occur in aluminium-poor rock, such quartzite.
  The chemical composition may vary between end member species of a mineral series. For example, theplagioclase 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.^[41] Other examples of series include the olivine series of magnesium-rich forsterite and iron-rich fayalite, and the wolframite series of manganese-rich hübneriteand 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.^[42] 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.^[43] 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⁺.^[44]
  Coordination polyhedra are geometric representation of how a cation is surrounded by an anion. In mineralogy, due its abundance in the crust, coordination polyhedra are usually considered in terms of oxygen. The base unit of silicate minerals is the silica tetrahedron — one Si⁴⁺ surrounded by four O^2-. 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 compound is compressed such that silicon is in six-fold (octahedral) coordination by oxygen. Bigger cations have a bigger coordination number because of the increase in relative size as compared to oxygen (the last orbital subshell of heavier atoms is different too). Changes in coordination numbers between leads to physical and mineralogical differences; for example, at high pressure such as in the mantle, many minerals, especially silicates such as olivine and garnet will change to aperovskite structure, where silicon is in octahedral coordination. Another example are the aluminosilicateskyanite, andalusite, and sillimanite (polymorphs, as they share the formula Al₂SiO₅), which differ by the coordination number of the Al³⁺; these minerals transition from one another as a response to changes in pressure and temperature.^[40] In the case of silicate materials, the substitution of Si⁴⁺ by Al³⁺ allows for a variety of minerals because of the need to balance charges.^[45]
  

  

  When minerals react, the products will sometimes assume the shape of the reagent; the product mineral is termed to be a pseudomorph of (or after) the reagent. Illustrated here is a pseudomorph of kaoliniteafter orthoclase. Here, the pseudomorph preserved the Carlsbad 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.^[46]
  Orthoclase feldspar (KAlSi₃O₈) is a mineral commonly found in granite, a plutonic igneous rock. When exposed to weathering, it reacts to formkaolinite (Al₂Si₂O₅(OH)₄, a sedimentary mineral, and silicic acid):

  2 KAlSi₃O₈ + 5 H₂O + 2 H⁺ → Al₂Si₂O₅(OH)₄ + 4 H₂SiO₃ + 2 K⁺

  Under low-grade metamorphic conditions, kaolinite reacts with quartz to form pyrophyllite (Al₂Si₄O₁₀(OH)₂):

  Al₂Si₂O₅(OH)₄ + SiO₂ → Al₂Si₄O₁₀(OH)₂ + H₂O

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

  Al₂Si₄O₁₀(OH)₂ → Al₂SiO₅ + 3 SiO₂ + H₂O

  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 SiO₂ polymorphs, such as tridymiteand cristobalite at high temperatures, and coesite at high pressures.^[47]

  Physical properties of minerals

  Classifying minerals ranges from simple to difficult. A mineral can be identified by several physical properties, some of them being sufficient for full identification without equivocation. In other cases, minerals can only be classified by more complex optical, chemical or X-ray diffraction analysis; these methods, however, can be costly and time-consuming. Physical properties applied for classification include crystal structure and habit, hardness, lustre, diaphaneity, colour, streak, cleavage and fracture, and specific gravity. Other less general tests includefluorescence, phosphorescence, magnetism, radioactivity, tenacity (response to mechanical induced changes of shape or form), piezoelectricity and reactivity to dilute acids.