Mineral

For other uses, see Mineral (disambiguation).

File:Different minerals.jpg

An assortment of minerals.

A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not have a specific chemical composition. Minerals range in composition from pure elements and simple salts to very complex silicates with thousands of known forms. The study of minerals is called mineralogy.

Mineral definition and classification

To be classified as a true mineral, a substance must be a solid and have a crystalline structure. It must also be a naturally occurring, homogeneous substance with a defined chemical composition.

The International Mineralogical Association approved the following definition 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."

According to this definition and classification scheme, biogenic materials were excluded from the mineral kingdom:

"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."

However, other researchers do not adhere to this exclusion rule. Lowenstam (1981), for example, states the following:

"Organisms are capable of forming a diverse array of minerals, some of which cannot be formed inorganically in the biosphere."

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. Inclusion of these biogenic minerals requires a expanded definition of a mineral as:

"An element or compound, amorphous or crystalline, formed through biogeochemical processes."

Mineral classification schemes and their definitions are evolving to match recent advances in mineral science. More recent classifications, for example, include an organic class – in both the new Dana and the Strunz classification schemes. 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 and established seven commissions and four working groups to review and classify minerals into an official listing of their published names. 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."

Recent advances in high-resolution genetic and x-ray absorption spectroscopy is opening new revelations on the biogeochemical relations between microrganisms and minerals that may make Nickel's (1995) biogenic mineral exclusion obsolete and Skinner's (2005) biogenic mineral inclusion a necessity. For example, the IMA commissioned 'Environmental Mineralogy and Geochemistry Working Group' 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 meters below the sea floor (possibly further) and 70 kilometers into the stratosphere (possibly entering the mesosphere). 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.

Prior to the International Mineralogical Association's listing, over 60 biominerals had been discovered, named, and published. These minerals (a sub-set tabulated in Lowenstam (1981)) are considered minerals proper according to the Skinner (2005) definition. These biominerals are not listed in the International Mineral Association official list of mineral names, however, many of these biomineral representatives are distributed amongst the 78 mineral classes listed in the `Dana' classification scheme. 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 crystaline compounds have been identified.

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.

The Skinner (2005) definition 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, but these groups do help to identify at the margins of what constitutes a mineral proper.

Crystal structure

A crystal structure is the orderly geometric spatial arrangement of atoms in the internal structure of a mineral. There are 14 basic crystal lattice arrangements of atoms in three dimensions, and these are referred to as the 14 "Bravais lattices". Each of these lattices can be classified into one of the seven crystal systems, and all crystal structures currently recognized fit in one Bravais lattice and one crystal system. This crystal structure is based on regular internal atomic or ionic arrangement that is often expressed in the geometric form that the crystal takes. Even when the mineral grains are too small to see or are irregularly shaped, the underlying crystal structure is always periodic and can be determined by X-ray diffraction. Chemistry and crystal structure together define a mineral. In fact, two or more minerals may have the same chemical composition, but differ in crystal structure (these are known as polymorphs). For example, pyrite and marcasite are both iron sulfide, but their arrangement of atoms differs. Similarly, some minerals have different chemical compositions, but the same crystal structure: for example, halite (made from sodium and chlorine), galena (made from lead and sulfur) and periclase (made from magnesium and oxygen) all share the same cubic crystal structure.

Crystal structure greatly influences a mineral's physical properties. For example, though diamond and graphite have the same composition (both are pure carbon), graphite is very soft, while diamond is the hardest of all known minerals. This happens because the carbon atoms in graphite are arranged into sheets which can slide easily past each other, while the carbon atoms in diamond form a strong, interlocking three-dimensional network.

There are currently more than 4,000 known minerals, according to the International Mineralogical Association (IMA), which is responsible for the approval of and naming of new mineral species found in nature. Of these, perhaps 100 can be called "common", 50 are "occasional", and the rest are "rare" to "extremely rare".

Mineral groups and solid solution

The chemical composition may vary between end members of a mineral system. For example the plagioclase feldspars comprise a continuous series from sodium and silicon-rich albite (NaAlSi₃O₈) to calcium and aluminium-rich anorthite (CaAl₂Si₂O₈) with four recognized intermediate compositions between. Mineral-like substances that don't strictly meet the definition are sometimes classified as mineraloids.

Minerals with the same structure and forming solid solutions are named isomorphs, and form series; for example: forsterite and fayalite of the olivine series, ferberite and hubnerite of the wolframite series. Minerals with the same structure and not forming solid solutions are named isotypes, and form groups [classification of minerals (non silicates)]. Minerals with a similar structure are grouped in homeotype families: amphibole and pyroxene families [classification of minerals (silicates)].

Some ion groups with a similar radius can occupy the same structural site in the crystal cell:

• O^2- and OH^- with 1.32 and 1.33 Å respectively.
• Si⁴⁺ and Al³⁺ with 0.42 and 0.51 Å respectively, the charge is neutralized through an exchange of the other cations:
  • Si⁴⁺ -- (Al³⁺ and Na⁺) or (Si⁴⁺ and Na⁺) -- (Al³⁺ and Ca²⁺).
• Larger molecules may have an unoccupied structural site by occupying another unoccupied structural site or by using a divalent cation instead of two monovalent cations, for instance (amphibole family).
• By the end of the 18th century, the minerals were getting chemical formulas. There were some difficulties, as elements were being discovered and isolated for the first time. The minerals: gadolinite-(Y) (first publication: 1802, 09.AJ.20), aeschynite-(Ce) (first publication: 1830, 04.DF.05), vanadinite (first publication: 1838, 08.BN.05), aenigmatite (first publication: 1865, 09.DH.40), labyrinthite (IMA 2002-065, 09.CO.10), illustrate these difficulties.

More recent definitions:

• "A mineral group consists of two or more minerals with the same (isotypic) or essentially the same (homeotypic) structure, and composed of chemically similar elements" (IMA-CNMNC).
• "two structures are considered homeotypic if all essential features of topology are preserved between them" (IUCr).

Differences between minerals and rocks

A mineral is a naturally occurring solid with a definite chemical composition and a specific crystalline structure. A rock is an aggregate of one or more minerals. (A rock may also include organic remains and mineraloids.) Some rocks are predominantly composed of just one mineral. For example, limestone is a sedimentary rock composed almost entirely of the mineral calcite. Other rocks contain many minerals, and the specific minerals in a rock can vary widely. Some minerals, like quartz, mica or feldspar are common, while others have been found in only four or five locations worldwide. The vast majority of the rocks of the Earth's crust consist of quartz, feldspar, mica, chlorite, kaolin, calcite, epidote, olivine, augite, hornblende, magnetite, hematite, limonite and a few other minerals. Over half of the mineral species known are so rare that they have only been found in a handful of samples, and many are known from only one or two small grains.

Commercially valuable minerals and rocks are referred to as industrial minerals. Rocks from which minerals are mined for economic purposes are referred to as ores (the rocks and minerals that remain, after the desired mineral has been separated from the ore, are referred to as tailings).

Mineral composition of rocks

A main determining factor in the formation of minerals in a rock mass is the chemical composition of the mass, for a certain mineral can be formed only when the necessary elements are present in the rock. Calcite is most common in limestones, as these consist essentially of calcium carbonate; quartz is common in sandstones and in certain igneous rocks which contain a high percentage of silica.

Other factors are of equal importance in determining the natural association or paragenesis of rock-forming minerals, principally the mode of origin of the rock and the stages through which it has passed in attaining its present condition. Two rock masses may have very much the same bulk composition and yet consist of entirely different assemblages of minerals. The tendency is always for those compounds to be formed which are stable under the conditions under which the rock mass originated. A granite arises by the consolidation of a molten magma at high temperatures and great pressures and its component minerals are those stable under such conditions. Exposed to moisture, carbonic acid and other subaerial agents at the ordinary temperatures of the Earth's surface, some of these original minerals, such as quartz and white mica are relatively stable and remain unaffected; others weather or decay and are replaced by new combinations. The feldspar passes into kaolinite, muscovite and quartz, and any mafic minerals such as pyroxenes, amphiboles or biotite have been present they are often altered to chlorite, epidote, rutile and other substances. These changes are accompanied by disintegration, and the rock falls into a loose, incoherent, earthy mass which may be regarded as a sand or soil. The materials thus formed may be washed away and deposited as sandstone or siltstone. The structure of the original rock is now replaced by a new one; the mineralogical constitution is profoundly altered; but the bulk chemical composition may not be very different. The sedimentary rock may again undergo metamorphism. If penetrated by igneous rocks it may be recrystallized or, if subjected to enormous pressures with heat and movement during mountain building, it may be converted into a gneiss not very different in mineralogical composition though radically different in structure to the granite which was its original state.

Physical properties of minerals

Classifying minerals can range from simple to very 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 commonly used are:

• Crystal structure and habit: See the above discussion of crystal structure. A mineral may show good crystal habit or form, or it may be massive, granular or compact with only microscopically visible crystals.

File:Talc block.jpg

Talc

File:Rough diamond.jpg

Rough diamond.

• Hardness: the physical hardness of a mineral is usually measured according to the Mohs scale. This scale is relative and goes from 1 to 10. Minerals with a given Mohs hardness can scratch the surface of any mineral that has a lower hardness than itself.
  • Mohs hardness scale:

1. Talc Mg₃Si₄O₁₀(OH)₂
2. Gypsum CaSO₄·2H₂O
3. Calcite CaCO₃
4. Fluorite CaF₂
5. Apatite Ca₅(PO₄)₃(OH,Cl,F)
6. Orthoclase KAlSi₃O₈
7. Quartz SiO₂
8. Topaz Al₂SiO₄(OH,F)₂
9. Corundum Al₂O₃
10. Diamond C (pure carbon)

• Luster indicates the way a mineral's surface interacts with light and can range from dull to glassy (vitreous).
  • Metallic – high reflectivity like metal: galena and pyrite
  • Sub-metallic – slightly less than metallic reflectivity: magnetite
  • Non-metallic lusters:
    • Adamantine – brilliant, the luster of diamond also cerussite and anglesite
    • Vitreous – the luster of a broken glass: quartz
    • Pearly – iridescent and pearl-like: talc and apophyllite
    • Resinous – the luster of resin: sphalerite and sulfur
    • Silky – a soft light shown by fibrous materials: gypsum and chrysotile
    • Dull/earthy – shown by finely crystallized minerals: the kidney ore variety of hematite
• Diaphaneity describes how well light passes through a mineral; there are three basic degrees of transparency:
  • Transparent objects can be seen through a transparent mineral, such as a clear quartz crystal
  • Translucent light passes through the mineral but no objects can be seen
  • Opaque no light passes through the mineral

Many minerals range from transparent to translucent or translucent to opaque. Calcite, for instance, can be translucent or opaque. Some minerals that are naturally translucent become opaque with weathering.

• Color indicates the appearance of the mineral in reflected light or transmitted light for translucent minerals (i.e. what it looks like to the naked eye).
  • Iridescence – the play of colors due to surface or internal interference. Labradorite exhibits internal iridescence whereas hematite and sphalerite often show the surface effect.
• Streak refers to the color of the powder a mineral leaves after rubbing it on an unglazed porcelain streak plate. Note that this is not always the same color as the original mineral.
• Cleavage describes the way a mineral may split apart along various planes. In thin sections, cleavage is visible as thin parallel lines across a mineral.
• Fracture describes how a mineral breaks when broken contrary to its natural cleavage planes.
  • Chonchoidal fracture is a smooth curved fracture with concentric ridges of the type shown by glass.
  • Hackley is jagged fracture with sharp edges.
  • Fibrous
  • Irregular
• Specific gravity relates the mineral mass to the mass of an equal volume of water, namely the density of the material. While most minerals, including all the common rock-forming minerals, have a specific gravity of 2.5–3.5, a few are noticeably more or less dense, e.g. several sulfide minerals have high specific gravity compared to the common rock-forming minerals.
• Other properties: fluorescence (response to ultraviolet light), magnetism, radioactivity, tenacity (response to mechanical induced changes of shape or form), piezoelectricity and reactivity to dilute acids.

Chemical properties of minerals

Minerals may be classified according to chemical composition. They are here categorized by anion group. The list below is in approximate order of their abundance in the Earth's crust. The list follows the Dana classification system which closely parallels the Strunz classification.

Silicate class

File:Quartz, Tibet.jpg

Quartz

The largest group of minerals by far are the silicates (most rocks are ≥95% silicates), which are composed largely of silicon and oxygen, with the addition of ions such as aluminium, magnesium, iron, and calcium. Some important rock-forming silicates include the feldspars, quartz, olivines, pyroxenes, amphiboles, garnets, and micas.

Carbonate class

The carbonate minerals consist of those minerals containing the anion (CO₃)²⁻ and include calcite and aragonite (both calcium carbonate), dolomite (magnesium/calcium carbonate) and siderite (iron carbonate). Carbonates are commonly deposited in marine settings when the shells of dead planktonic life settle and accumulate on the sea floor. Carbonates are also found in evaporitic settings (e.g. the Great Salt Lake, Utah) and also in karst regions, where the dissolution and reprecipitation of carbonates leads to the formation of caves, stalactites and stalagmites. The carbonate class also includes the nitrate and borate minerals.

Sulfate class

File:Hanksite.JPG

Hanksite, Na₂₂K(SO₄)₉(CO₃)₂Cl, one of the few minerals that is considered a carbonate and a sulfate

Sulfate minerals all contain the sulfate anion, SO₄²⁻. Sulfates commonly form in evaporitic settings where highly saline waters slowly evaporate, allowing the formation of both sulfates and halides at the water-sediment interface. Sulfates also occur in hydrothermal vein systems as gangue minerals along with sulfide ore minerals. Another occurrence is as secondary oxidation products of original sulfide minerals. Common sulfates include anhydrite (calcium sulfate), celestine (strontium sulfate), barite (barium sulfate), and gypsum (hydrated calcium sulfate). The sulfate class also includes the chromate, molybdate, selenate, sulfite, tellurate, and tungstate minerals.

Halide class

File:Halite(Salt)USGOV.jpg

Halite

The halide minerals are the group of minerals forming the natural salts and include fluorite (calcium fluoride), halite (sodium chloride), sylvite (potassium chloride), and sal ammoniac (ammonium chloride). Halides, like sulfates, are commonly found in evaporite settings such as salt lakes and landlocked seas such as the Dead Sea and Great Salt Lake. The halide class includes the fluoride, chloride, bromide and iodide minerals.

Oxide class

Oxide minerals are extremely important in mining as they form many of the ores from which valuable metals can be extracted. They also carry the best record of changes in the Earth's magnetic field. They commonly occur as precipitates close to the Earth's surface, oxidation products of other minerals in the near surface weathering zone, and as accessory minerals in igneous rocks of the crust and mantle. Common oxides include hematite (iron oxide), magnetite (iron oxide), chromite (iron chromium oxide), spinel (magnesium aluminium oxide – a common component of the mantle), ilmenite (iron titanium oxide), rutile (titanium dioxide), and ice (hydrogen oxide). The oxide class includes the oxide and the hydroxide minerals.

Sulfide class

Many sulfide minerals are economically important as metal ores. Common sulfides include pyrite (iron sulfide – commonly known as fools' gold), chalcopyrite (copper iron sulfide), pentlandite (nickel iron sulfide), and galena (lead sulfide). The sulfide class also includes the selenides, the tellurides, the arsenides, the antimonides, the bismuthinides, and the sulfosalts (sulfur and a second anion such as arsenic).

Phosphate class

The phosphate mineral group actually includes any mineral with a tetrahedral unit AO₄ where A can be phosphorus, antimony, arsenic or vanadium. By far the most common phosphate is apatite which is an important biological mineral found in teeth and bones of many animals. The phosphate class includes the phosphate, arsenate, vanadate, and antimonate minerals.

Element class

The elemental group includes native metals and intermetallic elements (gold, silver, copper), semi-metals and non-metals (antimony, bismuth, graphite, sulfur). This group also includes natural alloys, such as electrum (a natural alloy of gold and silver), phosphides, silicides, nitrides and carbides (which are usually only found naturally in a few rare meteorites).

Organic class

The organic mineral class includes biogenic substances in which geological processes have been a part of the genesis or origin of the existing compound. Minerals of the organic class include various oxalates, mellitates, citrates, cyanates, acetates, formates, hydrocarbons and other miscellaneous species. Examples include whewellite, moolooite, mellite, fichtelite, carpathite, evenkite and abelsonite.

See also

A list of minerals with associated Oilfield Wiki articles A comprehensive list of minerals Bowen's reaction series Dietary mineral Goldich dissolution series Industrial minerals Mineral industry Mineral processing Mineral water Mineral wool Mining

Nonmineral Norman L. Bowen Ores Quarry Rocks Strunz classification Mineral collecting Tucson Gem & Mineral Show, the world's largest

Mineralientage, the Munich Mineral Show, Europe's largest

External links

• Mindat mineralogical database, largest mineral database on the Internet
• "Mineralogy Database" by David Barthelmy (2009)
• "Vibrational Spectroscopy and Photo Atlas of Minerals", Mineral atlas with properties, photos, etc.
• Ontogeny of minerals in drawings. Drawings of crystals, druses, and mineral aggregates, showing genetic features indicative of their history, ontogenesis, and formative processes
• "Mineral Identification Key II" Mineralogical Society of America
• "The Mineral and Gemstone Kingdom" interactive reference guide to minerals and gemstones
• "American Mineralogist Crystal Structure Database"