Difference between revisions of "Mineral" - New World Encyclopedia

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A mineral is defined as a naturally occurring, [[inorganic]] [[solid]] with a definite [[chemical]] composition and [[crystal]]line structure. Mineral-like substances that do not strictly meet this definition are sometimes classified as [[mineraloid]]*s. Other naturally occurring substances are called ''nonminerals''. "Industrial minerals" is a market term and refers to commercially valuable, mined materials.
 
A mineral is defined as a naturally occurring, [[inorganic]] [[solid]] with a definite [[chemical]] composition and [[crystal]]line structure. Mineral-like substances that do not strictly meet this definition are sometimes classified as [[mineraloid]]*s. Other naturally occurring substances are called ''nonminerals''. "Industrial minerals" is a market term and refers to commercially valuable, mined materials.
  
A collection of minerals with slightly varying chemical composition is known as a mineral system<?>.
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A number of minerals may be grouped together when they are closely related in chemical composition. For example, the plagioclase [[feldspar]]*s consist of a continuous series of minerals, from [[sodium]]-rich albite (NaAlSi<sub>3</sub>O<sub>8</sub>) to [[calcium]]-rich anorthite (CaAl<sub>2</sub>Si<sub>2</sub>O<sub>8</sub>), with four recognized intermediate compositions.
  
The [[chemical]] composition may vary between end members of a mineral system<<?>>.  
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The '''crystal structure''' of a mineral is the orderly, geometric arrangement of [[atom]]s or [[ion]]s in the mineral's internal structure. 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 six "crystal systems." All currently recognized crystal structures fit into one Bravais lattice and one crystal system. 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 a technique known as [[X ray|X-ray]] diffraction.
  
For example, the [[plagioclase]] [[feldspar]]s are composed of a continuous series from [[sodium]]-rich albite (NaAlSi<sub>3</sub>O<sub>8</sub>) to [[calcium]]-rich anorthite (CaAl<sub>2</sub>Si<sub>2</sub>O<sub>8</sub>), with four recognized intermediate compositions.
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The typical, outward appearance of a mineral is called the "crystal habit." Some crystal habits are distinctive of certain minerals, but in most cases, a mineral exhibits a variety of habits that are influenced by the growth conditions of the crystals. An inexperienced observer may be misled by a mineral's crystal habit, as the crystal system can be hidden or disguised.
 
 
A '''crystal structure''' is an orderly, geometric arrangement of [[atom]]s or [[ion]]s 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 six "crystal systems." All currently recognized crystal structures fit into one Bravais lattice and one crystal system.
 
 
 
A mineral's crystal structure 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|X-ray]] diffraction.
 
  
 
Chemistry and crystal structure together define a mineral. When two or more minerals have the same chemical composition but differ in crystal structure, they 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.
 
Chemistry and crystal structure together define a mineral. When two or more minerals have the same chemical composition but differ in crystal structure, they 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.
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Crystal structure greatly influences a mineral's physical properties. For example, both [[diamond]] and [[graphite]] are pure [[carbon]]), but graphite is very soft, while diamond is the hardest of all known minerals. The reason for this difference is that the carbon atoms in graphite are arranged into sheets that can easily slide past one another, while the carbon atoms in diamond form a strong, interlocking three-dimensional network.
 
Crystal structure greatly influences a mineral's physical properties. For example, both [[diamond]] and [[graphite]] are pure [[carbon]]), but graphite is very soft, while diamond is the hardest of all known minerals. The reason for this difference is that the carbon atoms in graphite are arranged into sheets that can easily slide past one another, while the carbon atoms in diamond form a strong, interlocking three-dimensional network.
  
There are currently just over 4,000 known minerals, according to the [[International Mineralogical Association]], which is responsible for the approval and naming of mineral species newly discovered in nature.
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There are currently just over 4,000 known minerals, according to the [[International Mineralogical Association]], which is responsible for the approval and naming of newly discovered mineral species.
  
 
=== Rocks ===
 
=== Rocks ===
  
A [[Rock (geology)|rock]] is an aggregate of two or more minerals, and it may also include organic remains. In some rocks, one mineral may be predominant. 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 one or two locations worldwide. Over half the known mineral species are so rare that they have been found in only a handful of samples, and many are known from only one or two small grains.
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A [[Rock (geology)|rock]] is an aggregate of two or more minerals, and it may also include organic remains. In some rocks, one mineral may be predominant. 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.
  
 
=== Physical properties of minerals ===
 
=== Physical properties of minerals ===

Revision as of 22:39, 4 June 2006

For other uses, see Mineral (disambiguation).

Minerals are natural compounds formed through geological processes. The term "mineral" encompasses not only the material's chemical composition, but also the mineral's structure. Minerals range in composition from pure elements and simple salts to very complex silicates with thousands of known forms (organic compounds are usually excluded). The study of minerals is called mineralogy.

An assortment of minerals. Photo from US Geological Survey.

Mineral definition and classification

A mineral is defined as a naturally occurring, inorganic solid with a definite chemical composition and crystalline structure. Mineral-like substances that do not strictly meet this definition are sometimes classified as mineraloids. Other naturally occurring substances are called nonminerals. "Industrial minerals" is a market term and refers to commercially valuable, mined materials.

A number of minerals may be grouped together when they are closely related in chemical composition. For example, the plagioclase feldspars consist of a continuous series of minerals, from sodium-rich albite (NaAlSi3O8) to calcium-rich anorthite (CaAl2Si2O8), with four recognized intermediate compositions.

The crystal structure of a mineral is the orderly, geometric arrangement of atoms or ions in the mineral's internal structure. 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 six "crystal systems." All currently recognized crystal structures fit into one Bravais lattice and one crystal system. 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 a technique known as X-ray diffraction.

The typical, outward appearance of a mineral is called the "crystal habit." Some crystal habits are distinctive of certain minerals, but in most cases, a mineral exhibits a variety of habits that are influenced by the growth conditions of the crystals. An inexperienced observer may be misled by a mineral's crystal habit, as the crystal system can be hidden or disguised.

Chemistry and crystal structure together define a mineral. When two or more minerals have the same chemical composition but differ in crystal structure, they 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, both diamond and graphite are pure carbon), but graphite is very soft, while diamond is the hardest of all known minerals. The reason for this difference is that the carbon atoms in graphite are arranged into sheets that can easily slide past one another, while the carbon atoms in diamond form a strong, interlocking three-dimensional network.

There are currently just over 4,000 known minerals, according to the International Mineralogical Association, which is responsible for the approval and naming of newly discovered mineral species.

Rocks

A rock is an aggregate of two or more minerals, and it may also include organic remains. In some rocks, one mineral may be predominant. 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.

Physical properties of minerals

Classifying minerals can range from simple to very difficult.

A mineral is identified and classified based on several physical and chemical properties. Some of the properties may be sufficient for full identification without equivocation. In other cases, classification of a mineral may require more complex chemical or X-ray diffraction analyses. These methods can be costly and time-consuming, and may even risk damaging the sample.

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.
  • 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. The minerals that define the scale are given below:

1- talc 2- gypsum 3- calcite 4- fluorite 5- apatite 6- orthoclase feldspar 7- quartz 8- topaz 9- corundum 10- diamond

  • Luster indicates the way a mineral's surface interacts with light and can range from dull to glassy (vitreous).

Metallic -high reflectivity like metal, e.g. galena

Sub-metallic -slightly less than metallic reflectivity, e.g. magnetite

Vitreous -the lustre of a broken glass, e.g. quartz

Pearly -a very soft light shown by some layer silicates, e.g. talc

Silky -a soft light shown by fibrous materials, e.g. gypsum

Dull/earthy -shown by finely crystallized minerals, e.g. the kidney ore variety of hematite

  • 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).
  • Streak refers to the color of the powder a mineral leaves after rubbing it on an unglazed porcelain streak plate.
  • Cleavage describes the way a mineral may split apart along various planes. In thin section, cleavage is visible as thin parallel lines across a mineral.
  • Fracture describes how a mineral breaks when broken contrary to its natural cleavage planes, e.g. a chonchoidal fracture is a smooth fracture with concentric ridges of the type shown by glass.
  • 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 rock-forming minerals, have a specific gravity of 2.5 - 3.5, a few are noticably more or less dense, e.g. several sulphide 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), 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.

Silicate class

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

Sulfates 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), celestite (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

The halides 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 evaporitic settings such as playa lakes and landlocked seas such as the Dead Sea and Great Salt Lake. The halide class includes the fluoride, chloride, and iodide minerals.

Oxide class

Oxides are extremely important in mining as they form many of the ores from which valuable metals can be extracted. 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 (chromium oxide), spinel (magnesium aluminium oxide - a common component of the mantle), rutile (titanium dioxide), and ice (hydrogen oxide). The oxide class includes the oxide and the hydroxide minerals.

Sulfide class

Many sulfides 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 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).

See also

  • A list of minerals with associated Wikipedia articles
  • A comprehensive list of minerals
  • Industrial minerals
  • Mineral water, water containing minerals or other dissolved substances that alter its taste or give it therapeutic value
  • Mineral wool
  • Mining
  • Norman L. Bowen
  • Quarry
  • Dietary mineral

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