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Characteristics

Physical

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]:123 It is somewhat less dense than iron and has a hexagonal crystal structure. [Lehto 1968]:826

The metal is hard and brittle at most temperatures but becomes malleable between 100 and 150 °C.[CRC 2006] [Heiserman, 1992:123] 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]

Occurrence

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

A black shiny lump of solid with uneven surface.

The element is normally found in association with other base metals such as copper and lead in ores.[Lehto 1968]: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]: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]: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]: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]: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]: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 role

Zinc is an essential trace element, necessary for plants,[Broadley 2007] animals, [Prasad 2008] and microorganisms.[Sugarman 1983] Zinc is found in nearly 100 specific enzymes [NRC 2000]:443 (other sources say 300), serves as structural ions in transcription factors and is stored and transferred in metallothioneins.[Cotton 1999]: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]:131 Semen is particularly rich in zinc, which is a key factor in prostate gland function and reproductive organ growth.[Berdanier 2007]: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]

Bibliography


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