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Zinc

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CharacteristicsПравить

PhysicalПравить

Zinc, also referred to in nonscientific contexts as spelter, [encyclo.co.uk] is a bluish-white, lustrous, diamagnetic metal, [CRC, 2006] though most common commercial grades of the metal have a dull finish. [Heiserman, 1992| p.123] It is somewhat less dense than iron and has a hexagonal crystal structure. [Lehto, 1968| p.826]

The metal is hard and brittle at most temperatures but becomes malleable between 100 and 150 °C.[CRC, 2006] [Heiserman, 1992| p.123] Above 210 °C, the metal becomes brittle again and can be pulverized by beating. [Scoffern, 1861] Zinc is a fair conductor of electricity. [CRC, 2006] For a metal, zinc has relatively low melting (420 °C) and boiling points (900 °C). [galvanizeit.org] Its melting point is the lowest of all the transition metals aside from mercury and cadmium. [galvanizeit.org]

Many alloys contain zinc, including brass, an alloy of zinc and copper. Other metals long known to form binary alloys with zinc are aluminium, antimony, bismuth, gold, iron, lead, mercury, silver, tin, magnesium, cobalt, nickel, tellurium and sodium. [Ingalls, 1902] While neither zinc nor zirconium are ferromagnetic, their alloy ZrZn2 exhibits ferromagnetism below 35 K. [CRC, 2006]

OccurrenceПравить

Zinc makes up about 75 ppm (0.007%) of the Earth's crust, making it the 24th most abundant element there. [Emsley, 2001|p.503] Soil contains 5–770 ppm of zinc with an average of 64 ppm.[Emsley, 2001|p.503] Seawater has only 30 ppb zinc and the atmosphere contains 0.1–4 µg/m3. [Emsley, 2001|p.503]

A black shiny lump of solid with uneven surface.

Sphalerite (ZnS)

The element is normally found in association with other base metals such as copper and lead in ores.[Lehto, 1968| p.822] Zinc is a chalcophile ("sulfur loving"), meaning the element has a low affinity for oxygen and prefers to bond with sulfur in highly insoluble sulfides. Chalcophiles formed as the crust solidified under the reducing conditions of the early Earth's atmosphere. [Greenwood, 1997| p.1202] Sphalerite, which is a form of zinc sulfide, is the most heavily mined zinc-containing ore because its concentrate contains 60–62% zinc. [Lehto, 1968| p.822]

Other minerals, from which zinc is extracted, include smithsonite (zinc carbonate), hemimorphite (zinc silicate), wurtzite (another zinc sulfide), and sometimes hydrozincite (basic zinc carbonate).[Emsley, 2001|p.502] With the exception of wurtzite, all these other minerals were formed as a result of weathering processes on the primordial zinc sulfides. [Greenwood, 1997| p.1202]

World zinc resources total about 1.8 gigatonnes.[usgs.gov, 2009] Nearly 200 megatonnes were economically viable in 2008; adding marginally economic and subeconomic reserves to that number, a total reserve base of 500 megatonnes has been identified. [usgs.gov, 2009] Large deposits are in Australia, Canada and the United States.[Greenwood, 1997| p.1202] At the current rate of consumption, these reserves are estimated to be depleted sometime between 2027 and 2055.[Cohen, 2007] [idtechex.com] About 346 megatonnes have been extracted throughout history to 2002, and one estimate found that about 109 megatonnes of that remains in use. [Gordon, 2006]

Biological roleПравить

Zinc is an essential trace element, necessary for plants,[Broadley, 2007] animals, [Prasad A. S., 2008] and microorganisms.[Sugarman, 1983] Zinc is found in nearly 100 specific enzymes [NRC, 2000| p.443] (other sources say 300), serves as structural ions in transcription factors and is stored and transferred in metallothioneins.[Cotton, 1999| pp.625–629] It is "typically the second most abundant transition metal in organisms" after iron and it is the only metal which appears in all enzyme classes.[Broadley, 2007]

There are 2–4 grams of zinc[Rink, 2000] distributed throughout the human body. Most zinc is in the brain, muscle, bones, kidney, and liver, with the highest concentrations in the prostate and parts of the eye.[Wapnir, 1990| p.131] Semen is particularly rich in zinc, which is a key factor in prostate gland function and reproductive organ growth.[Berdanier, 2007| p.210]

In humans, zinc plays "ubiquitous biological roles".[Hambridge, 2007] It interacts with "a wide range of organic ligands",[Hambridge, 2007] and has roles in the metabolism of RNA and DNA, signal transduction, and gene expression. It also regulates apoptosis. A 2006 study estimated that about 10% of human proteins (2800) potentially bind zinc, in addition to hundreds which transport and traffic zinc; a similar in silico study in the plant Arabidopsis thaliana found 2367 zinc-related proteins.[Broadley, 2007]

In the brain, zinc is stored in specific synaptic vesicles by glutamatergic neurons [Bitanihirwe, 2009] and can "modulate brain excitability".[Hambridge, 2007] It plays a key role in synaptic plasticity and so in learning.[Nakashima, 2009] However it has been called "the brain's dark horse"[Bitanihirwe, 2009] since it also can be a neurotoxin, suggesting zinc homeostasis plays a critical role in normal functioning of the brain and central nervous system.[Bitanihirwe, 2009]

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