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Mercury (element)

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


               80                 gold ← mercury → thallium
               Cd
               ↑
               Hg
               ↓
               Uub

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                      Name, Symbol, Number mercury, Hg, 80
                                         Chemical series transition metals
                                             Group, Period, Block 12, 6, d
                                                  Appearance silvery white
                                              Atomic mass 200.59 (2) g/mol
                              Electron configuration [Xe] 4f^14 5d^10 6s^2
                                   Electrons per shell 2, 8, 18, 32, 18, 2
                                                       Physical properties
                                                              Phase liquid
                            Density (near r.t.) (liquid) 13.534 g·cm^−3
                                                   Melting point 234.32  K
                                                (-38.83 ° C, -37.89 ° F)
                                                    Boiling point 629.88 K
                                                (356.73 ° C, 674.11 ° F)
                                         Critical point 1750 K, 172.00 MPa
                                          Heat of fusion 2.29 kJ·mol^−1
                                   Heat of vaporization 59.11 kJ·mol^−1
                         Heat capacity (25 °C) 27.983 J·mol^−1·K^−1

   CAPTION: Vapor pressure

                                          P/Pa   1  10  100 1 k 10 k 100 k
                                         at T/K 315 350 393 449 523   629

                                                         Atomic properties
                                            Crystal structure rhombohedral
                                                     Oxidation states 2, 1
                                                      (mildly basic oxide)
                                    Electronegativity 2.00 (Pauling scale)
                                    Ionization energies 1st: 1007.1 kJ/mol
                                                          2nd: 1810 kJ/mol
                                                          3rd: 3300 kJ/mol
                                                      Atomic radius 150 pm
                                              Atomic radius (calc.) 171 pm
                                                    Covalent radius 149 pm
                                               Van der Waals radius 155 pm
                                                             Miscellaneous
                                             Magnetic ordering diamagnetic
                                Electrical resistivity (25 °C) 961 nΩ·m
                       Thermal conductivity (300 K) 8.30 W·m^−1·K^−1
                       Thermal expansion (25 °C) 60.4 µm·m^−1·K^−1
                                Speed of sound (liquid, 20 °C) 1451.4 m/s
                                             CAS registry number 7439-97-6
                                                         Selected isotopes

                 CAPTION: Main article: Isotopes of mercury

                               iso     NA   half-life DM  DE ( MeV)   DP
                              ^194Hg syn    444 y     ε   0.040     ^194Au
                              ^195Hg syn    9.9 h     ε   1.510     ^195Au
                              ^196Hg 0.15%  Hg is stable with 116 neutrons
                              ^197Hg syn    64.14 h   ε   0.600     ^197Au
                              ^198Hg 9.97%  Hg is stable with 118 neutrons
                              ^199Hg 16.87% Hg is stable with 119 neutrons
                              ^200Hg 23.1%  Hg is stable with 120 neutrons
                              ^201Hg 13.18% Hg is stable with 121 neutrons
                              ^202Hg 29.86% Hg is stable with 122 neutrons
                              ^203Hg syn    46.612 d  β^- 0.492     ^203Tl
                              ^204Hg 6.87%  Hg is stable with 124 neutrons

                                                                References

   Mercury ( IPA: /ˈmɜːkjəˌɹi/), also called quicksilver, is a chemical
   element in the periodic table that has the symbol Hg ( Latinized Greek:
   hydrargyrum, meaning watery or liquid silver) and atomic number 80. A
   heavy, silvery transition metal, mercury is one of five elements that
   are liquid at or near standard room temperature (the others are the
   metals caesium, francium, and gallium, and the nonmetal bromine).
   Mercury is used in thermometers, barometers and other scientific
   apparatus, although the use of mercury in thermometers has been largely
   phased out in clinical and scientific environments (in favour of
   alcohol-filled, digital or thermistor-based replacements) due to
   concerns about the element's toxicity. Mercury is still used in dental
   amalgam. Mercury is mostly obtained by reduction from the mineral
   cinnabar.

Applications

   Mercury column to measure pressure
   Enlarge
   Mercury column to measure pressure

   Mercury is used primarily for the manufacture of industrial chemicals
   or for electrical and electronic applications. It is used in some
   thermometers, especially ones which are used to measure high
   temperatures (In the United States, non-prescription sale of mercury
   fever thermometers is banned by a number of different states and
   localities). Other uses:
     * Mercury sphygmomanometers.
     * Mercury barometers, diffusion pumps, coulometers, and many other
       laboratory instruments. As an opaque liquid with a very high
       density, it is ideal for this role.
     * The triple point of mercury, -38.8344 °C, is a fixed point used as
       a temperature standard for the International Temperature Scale (
       ITS-90).
     * In some gaseous electron tubes, mercury arc rectifier
     * Gaseous mercury is used in mercury-vapor lamps and some " neon
       sign" type advertising signs and fluorescent lamps.
     * Liquid mercury was sometimes used as a coolant for nuclear
       reactors. However, sodium is proposed for reactors cooled with
       liquid metal, because the high density of mercury requires much
       energy for circulating the coolant.
     * Mercury was once used in the amalgamation process of refining gold
       and silver ores. This polluting practice is still used by the
       garimpeiros (gold miners) of the Amazon basin in Brazil.
     * Mercury is still used in some cultures for folk medicine and
       ceremonial purposes which may involve ingestion, injection, or the
       sprinkling of elemental mercury around the home. It must be
       emphasized that the former two procedures, especially, are
       extremely hazardous.
     * Alexander Calder built a mercury fountain for the Spanish Pavilion
       at the 1937 World's Fair in Paris. The fountain is now on display
       at the Fundació Miró in Barcelona.
     * Used in electrochemistry as part of a secondary reference electrode
       called the calomel electrode as an alternative to the Standard
       Hydrogen Electrode. This is used to work out the electrode
       potential of half cells.

   Miscellaneous uses: mercury switches, electrodes in some types of
   electrolysis, batteries ( mercury cells, including for sodium hydroxide
   and chlorine production, and alkaline batteries), catalysts,
   insecticides, dental amalgams/preparations and liquid mirror
   telescopes.
     * Thiomersal, (called Thimerosal in the United States), an organic
       compound used as a preservative in vaccines, though this use is
       disappearing.

   Historical uses: preserving wood, developing daguerreotypes, silvering
   mirrors, anti-fouling paints (discontinued in 1990), herbicides
   (discontinued in 1995), cleaning, and in-road leveling devices in cars.
   Mercury compounds have been used in antiseptics, laxatives,
   antidepressants, and antisyphilitics. It was also allegedly used by
   allied spies to sabotage German planes. A mercury paste was applied to
   bare aluminium, causing the metal to rapidly corrode. This would cause
   mysterious structural failures.

   In Islamic Spain it was used for filling decorative pools and for
   fountains.

   In some applications, mercury can be replaced with less toxic but
   considerably more expensive galinstan alloy.

   A new type of atomic clock, using mercury instead of caesium, has been
   demonstrated. Accuracy is expected to be within one second in 100
   million years.

History

   Mercury was known to the ancient Chinese and Hindus and was found in
   Egyptian tombs that date from 1500 BC. In China, India and Tibet,
   mercury use was thought to prolong life, heal fractures, and maintain
   generally good health. China's first emperor, Qin Shi Huang Di — said
   to have been buried in a tomb that contained rivers of flowing mercury,
   representative of the rivers of China — was driven insane and killed by
   mercury pills intended to give him eternal life. The ancient Greeks
   used mercury in ointments and the Romans used it in cosmetics. By 500
   BCE mercury was used to make amalgams with other metals. The Indian
   word for alchemy is Rasavātam which means ‘the way of mercury’.
   Alchemists often thought of mercury as the first matter from which all
   metals were formed. Different metals could be produced by varying the
   quality and quantity of sulfur contained within the mercury. An ability
   to transform mercury into any metal resulted from the essentially
   mercurial quality of all metals. The purest of these was gold, and
   mercury was required for the transmutation of base (or impure) metals
   into gold. This was a primary goal of alchemy, either for material or
   spiritual gain.

   Hg is the modern chemical symbol for mercury. It comes from
   hydrargyrum, a Latinized form of the Greek word `Υδραργυρος
   (hydrargyros), which is a compound word meaning 'water' and 'silver' —
   since it is liquid, like water, and yet has a silvery metallic sheen.
   The element was named after the Roman god Mercury, known for speed and
   mobility. It is associated with the planet Mercury. The astrological
   symbol for the planet is also one of the alchemical symbols for the
   metal (above left). Mercury is the only metal for which the alchemical
   planetary name became the common name.

Hatting

   From the mid-18th to the mid-19th centuries, a process called
   "carroting" was used in the making of felt hats. Animal skins were
   rinsed in an orange solution of the mercury compound mercuric nitrate,
   Hg(NO[3])[2]·2H[2]O. This process separated the fur from the pelt and
   matted it together. This solution and the vapors it produced were
   highly toxic. Its use resulted in widespread cases of mercury poisoning
   among hatters. Symptoms included tremors, emotional lability, insomnia,
   dementia and hallucinations. The United States Public Health Service
   banned the use of mercury in the felt industry in December 1941. The
   psychological symptoms associated with mercury poisoning may have
   inspired the phrase "mad as a hatter"; see the hatter article on the
   origin of the phrase.

Dentistry

   Elemental mercury is the main ingredient in dental amalgams.
   Controversy over the health effects from the use of mercury amalgams
   began shortly after its introduction into the western world, nearly 200
   years ago. In 1843, The American Society of Dental Surgeons, concerned
   about mercury poisoning, required its members to sign a pledge that
   they would not use amalgam. In 1859, The American Dental Association
   was formed by dentists who believed amalgam was "safe and effective."
   The ADA "continues to believe that amalgam is a valuable, viable and
   safe choice for dental patients," as written in their statement on
   dental amalgam. In 1993, the United States Public Health Service
   reported that "amalgam fillings release small amounts of mercury
   vapor," but in such a small amount that it "has not been shown to cause
   any … adverse health effects." This position is not shared by all
   governments and there is an ongoing dental amalgam controversy. A
   recent review by an FDA appointed advisory panel, the panelists, in a
   13-7 vote, rejected the current FDA report on amalgam safety stating
   the report's conclusions weren't reasonable, given the quantity and
   quality of information currently available. Panelists said remaining
   uncertainties about the risk of so-called silver fillings demanded
   further research, in particular, on the effects of mercury-laden
   fillings on children and the fetuses of pregnant women with fillings
   and the release of mercury vapor on insertion and removal of mercury
   fillings.

Medicine

   Mercury and its compounds have been used in medicine for centuries,
   although they are much less common today than they once were, now that
   the toxic effects of mercury and its compounds are more widely known
   and understood.

   Mercury(I) chloride (also known as calomel or mercurous chloride) has
   traditionally been used as a diuretic, topical disinfectant, and
   laxative. Mercury(II) chloride (also known as mercuric chloride or
   corrosive sublimate) was once used to treat syphilis (along with other
   mercury compounds), although it is so toxic that sometimes the symptoms
   of its toxicity were confused with those of the syphilis it was
   believed to treat (Pimple 2004); it was also used as a disinfectant.
   Blue mass, a pill or syrup in which mercury is the main ingredient, was
   prescribed throughout the 1800s for numerous conditions including
   constipation, depression, child-bearing and toothaches. In the early
   20th century, mercury was administered to children yearly as a laxative
   and dewormer, and it was used in teething powders for infants. The
   mercury containing organohalide Mercurochrome is still widely used but
   has been banned in some countries such as the U.S.

   Some vaccines have contained the preservative Thimerosal (partly ethyl
   mercury) since the 1930s FDA report. It has been widely speculated that
   this mercury-based preservative can trigger autism in children who are
   already genetically predisposed to it; however, medical evidence in
   recent studies has shown no evidence supporting any such link.

   Mercury in the form of one of its common ores, cinnabar, remains an
   important component of Chinese, Tibetan, and Ayurvedic medicine. As
   problems may arise when these medicines are exported to countries that
   prohibit the use of mercury in medicines, in recent times, less toxic
   substitutes have been devised.

   Today, the use of mercury in medicine has greatly declined in all
   respects, especially in developed countries. Thermometers and
   sphygmomanometers containing mercury were invented in the early 18th
   and late 19th centuries, respectively. In the early 21st century, their
   use is declining and has been banned in some countries, states and
   medical institutions. In 2002, the U.S. Senate passed legislation to
   phase out the sale of non-prescription mercury thermometers. In 2003,
   Washington and Maine became the first states to ban mercury blood
   pressure devices. Mercury compounds are found in some over-the-counter
   drugs, including topical antiseptics, stimulant laxatives, diaper-rash
   ointment, eye drops, and nasal sprays. The FDA (FDA) has “inadequate
   data to establish general recognition of the safety and effectiveness,”
   of the mercury ingredients in these products. Mercury is still used in
   some diuretics, although substitutes now exist for most therapeutic
   uses.

   In the European Union, RoHS legislation being introduced will ban
   mercury from certain products, and limit the amount of mercury in other
   products to less than 1000 ppm (except for certain exemptions).

Mineral occurrence

   Mercury ore
   Enlarge
   Mercury ore

   Mercury is an extremely rare element in the earth's crust, having an
   average crustal abundance by mass of only 0.08 parts per million.
   However, because it does not blend geochemically with those elements
   that comprise the majority of the crustal mass, mercury ores can be
   extraordinarily concentrated considering the element's abundance in
   ordinary rock. The richest mercury ores contain up to 2.5% mercury by
   mass, and even the leanest concentrated deposits are at least 0.1%
   mercury (12,000 times average crustal abundance). This makes mercury
   ore the most easily depleted of all metal ores. Depletion of mercury
   ores has been a major concern since the 1960s and it is now almost
   certain that the last mineable deposits were discovered in Algeria in
   the mid-1970s. Since the early 1970s, total world production of mercury
   has fallen from 9,000 tons to 1,600 tons due to depletion of reserve.

   It is found either as a native metal (rare) or in cinnabar, corderoite,
   livingstonite, and other minerals with cinnabar (HgS) being the most
   common ore. Mercury ores usually occur in very young orogenic belts
   where rock of high density are forced to the crust of the Earth, often
   in hot springs or other volcanic regions. Most present-day production
   occurs in Spain, Kyrgyzstan, China and Tajikistan. Over 100,000 tons of
   mercury were mined from the region of Huancavelica, Peru, over the
   course of three centuries following the discovery of deposits there in
   1563; mercury from Huancavelica was crucial in the production of silver
   in colonial Spanish America. Many former ores in Italy, Slovenia, the
   United States and Mexico which once produced a large proportion of the
   world's supply have now been completely mined out. The metal is
   extracted by heating cinnabar in a current of air and condensing the
   vapor. The equation for this extraction is

          HgS + O[2] → Hg + SO[2]

Compounds

   The most important salts are:
     * Mercury(I) chloride (AKA calomel) is sometimes still used in
       medicine and acousto-optical filters
     * Mercury(II) chloride (which is very corrosive, sublimates and is a
       violent poison)
     * Mercury fulminate, (a detonator widely used in explosives),
     * Mercury(II) oxide, the main oxide of mercury
     * Mercury(II) sulfide (AKA cinnabar mercuric ore still used in
       oriental medicine, or vermilion which is a high-grade paint
       pigment),
     * Mercury(II) selenide a semiconductor,
     * Mercury(II) telluride a semiconductor, and
     * Mercury cadmium telluride and mercury zinc telluride, infrared
       detector materials.

   Laboratory tests have found that an electrical discharge causes the
   noble gases to combine with mercury vapor. These compounds are held
   together with van der Waals forces and result in HgNe, HgAr, HgKr, and
   HgXe. Organic mercury compounds are also important. Methylmercury is a
   dangerous compound that is widely found as a pollutant in water bodies
   and streams.

Isotopes

   There are seven stable isotopes of mercury with Hg-202 being the most
   abundant (29.86%). The longest-lived radioisotopes are ^194Hg with a
   half-life of 444 years, and ^203Hg with a half-life of 46.612 days.
   Most of the remaining radioisotopes have half-lifes that are less than
   a day.

Reactivity

   Dissolves to form amalgams with gold, zinc and many metals, but with
   some exceptions such as iron, therefore iron flasks have been
   traditionally used to trade mercury. Reacts with oxygen in air when
   heated to form mercury oxide, which then can be decomposed by further
   heating to higher temperatures. Being below hydrogen in the reactivity
   series of metals, it does not react with most acids, such as dilute
   sulfuric acid, though oxidizing acids, such as concentrated sulfuric
   acid and nitric acid or aqua regia dissolve it to give sulfate and
   nitrate and chloride. Similar to silver, mercury reacts with
   atmospheric hydrogen sulfide. Mercury even reacts with solid sulfur
   flakes, which is used in mercury spill kits to absorb mercury vapors
   (spill kits also use activated charcoal and powdered zinc).

Occurrence in the environment

   Amount of atmospheric mercury deposited at Wyoming's Fremont Glacier
   over the last 270 years
   Enlarge
   Amount of atmospheric mercury deposited at Wyoming's Fremont Glacier
   over the last 270 years

   Abundance
     * Crustal ~7×10^-2 mg/kg
     * Oceans ~3×10^-5 mg/l

   Preindustrial deposition rates of mercury from the atmosphere may be in
   the range of 4 ng/L in the western USA. Although that can be considered
   a natural level of exposure, regional or global sources have
   significant effects. Volcanic eruptions can increase the atmospheric
   source by 4–6 times.

   Mercury enters the environment as a pollutant from various industries:
     * coal-fired power plants are the largest source (40% of USA
       emissions in 1999).
     * industrial processes
          + chlorine, steel, phosphate & gold production
          + metal smelting
          + manufacture & repair of weather and electronic devices
          + incineration of municipal waste streams
     * medical applications, including vaccinations
          + dentistry
          + cosmetic industries
     * laboratory work involving mercury or sulfur compounds

   Mercury also enters into the environment through the disposal (e.g.,
   landfilling, incineration) of certain products. Products containing
   mercury include: auto parts, batteries, fluorescent bulbs, medical
   products, thermometers, and thermostats. Due to health concerns (see
   below), toxics use reduction efforts are cutting back or eliminating
   mercury in such products. For example, most thermometers now use
   pigmented alcohol instead of mercury. Mercury thermometers are still
   occasionally used in the medical field because they are more accurate
   than alcohol thermometers, though both are being replaced by electronic
   thermometers. Mercury thermometers are still widely used for certain
   scientific applications because of their greater accuracy and working
   range.

   One of the worst industrial disasters in history was caused by the
   dumping of mercury compounds into Minamata Bay, Japan. The Chisso
   Corporation, a fertilizer and later petrochemical company, was found
   responsible for polluting the bay from 1932–1968. It is estimated that
   over 3,000 people suffered various deformities, severe mercury
   poisoning symptoms or death from what became known as Minamata disease.

Precautions and regulation

   Mercury should be handled with care. Containers of mercury should be
   securely sealed to avoid spills and evaporation. Heating of mercury, or
   compounds of mercury that may decompose when heated, should always be
   carried out with adequate ventilation in order to avoid human exposure
   to mercury vapor. Mercury should never be used as decoration and or
   displayed in open containers. Most of its compounds are highly toxic,
   especially its inorganic compounds.

Occupational exposure

   Due to the health effects of mercury exposure, industrial and
   commercial uses are regulated in many countries. The World Health
   Organization, OSHA, and NIOSH all treat mercury as an occupational
   hazard, and have established specific occupational exposure limits.
   Environmental releases and disposal of mercury are regulated in the
   U.S. primarily by the Environmental Protection Agency.

Mercury in fish

   Fish and shellfish have a natural tendency to concentrate mercury in
   their bodies, often in the form of methylmercury, a highly toxic
   organic compound of mercury. Species of fish that are high on the food
   chain, such as shark, swordfish, king mackerel, albacore tuna, and
   tilefish contain higher concentrations of mercury than others. This is
   because mercury is stored in the muscle tissues of fish, and when a
   predatory fish eats another fish, it assumes the entire body burden of
   mercury in the consumed fish. Since fish are less efficient at
   depurating than accumulating methylmercury, fish-tissue concentrations
   increase over time. Thus species that are high on the food chain amass
   body burdens of mercury that can be ten times higher, or more, than the
   species they consume. This process is called biomagnification. The
   complexities associated with mercury fate and transport are relatively
   succinctly described by USEPA in their 1997 Mercury Study Report to
   Congress. Because methylmercury and high levels of elemental mercury
   can be particularly toxic to unborn or young children, organizations
   such as the U.S. EPA and FDA recommend that women who are pregnant or
   plan to become pregnant within the next one or two years, as well as
   young children avoid eating more than 6 ounces (one average meal) of
   fish per week. In the United States the FDA has an action level for
   methyl mercury in commercial marine and freshwater fish that is 1.0
   parts per million (ppm), and in Canada the limit for the total of
   mercury content is 0.5 (ppm).

A common warning

   This is a Common risk with eating seafood.
   Enlarge
   This is a Common risk with eating seafood.

   Species with characteristically low levels of mercury include shrimp,
   tilapia, salmon, pollock, and catfish (FDA March 2004). The FDA
   characterizes shrimp, catfish, pollock, salmon, and canned light tuna
   as low-mercury seafood, although recent tests have indicated that up to
   6 percent of canned light tuna may contain high levels.

The danger of avoiding fish

   The effects of consuming fish high in mercury is in dispute with the
   University of Rochester's study of people in the Republic of the
   Seychelles. While there is no doubt high level exposure to methyl
   mercury is definitely toxic, low level exposure isn't. A recent Harvard
   Medical School study of mothers and their infants suggests that the
   nutritional benefits of fish outweigh the effects of mercury. In the
   HMS study, each additional weekly serving of fish consumed by the
   mother during pregnancy was associated with an increase in infant
   cognition.

   What one Professor Emeritus of Engineering (University of Denver) says
   about mercury in fish:

     Recently (December 2003), news reports stated that tuna fish
     contained "large amounts of mercury" in an alarming tone. What they
     really should have said is that tuna fish contains only extremely
     small, scarcely detectable, amounts of methyl mercury. The
     extrapolated threat appears to be to the nerves of developing
     fetuses. I wonder how many cases have been observed of fetal mercury
     damage by normal tuna fish. I suspect that the number is zero. The
     authorities probably have no idea of the magnitude of the hazard,
     only that a hazard could be possible. Perhaps we have homeopaths at
     work here! Generally, they deal in extrapolations of inexact data to
     minute concentrations, and probably do not know the rate at which
     organisms take up and excrete such tiny quantities. The problem is
     that methyl mercury could be a threat, but I have no confidence that
     the authorities know enough about it to protect the public, only to
     enjoy the creation of panic. Tuna fish, indeed, also contains many
     nutrients valuable to fetuses.

Release of mercury into the environment

   The environmental consideration of mercury use in a particular product
   can sometimes be complicated. For instance compact fluorescent light
   bulbs, which use a very small amount of mercury (in 2004 two-thirds of
   CFL lamps sold contained 5 mg Hg or less per bulb, while 96 percent
   contained 10 mg or less), due to their far higher efficiency over
   incandescent bulbs actually emit less mercury to the environment when
   they are powered using energy from a coal power plant.
   Enlarge
   The environmental consideration of mercury use in a particular product
   can sometimes be complicated. For instance compact fluorescent light
   bulbs, which use a very small amount of mercury (in 2004 two-thirds of
   CFL lamps sold contained 5 mg Hg or less per bulb, while 96 percent
   contained 10 mg or less), due to their far higher efficiency over
   incandescent bulbs actually emit less mercury to the environment when
   they are powered using energy from a coal power plant.
   Mercury use of compact fluorescent bulb vs. incandescent bulb when
   powered by electricity generated from coal.
   Enlarge
   Mercury use of compact fluorescent bulb vs. incandescent bulb when
   powered by electricity generated from coal.

   Historically, one of the largest releases was from the Colex plant, a
   Lithium isotope separation plant at Oak Ridge. The plant operated in
   the 1950's and 1960's. Records are incomplete and unclear, but govt
   commissions have estimated 2 million pounds of mercury are unaccounted
   for.

   The primary sources of mercury to the environment are fossil fuel
   burning (primarily coal) and solid waste incineration (Nriagu & Pacyna,
   1988). Power plants in the U.S., according to U.S. Environmental
   Protection Agency, are one of the main emitters of mercury — 48 tonnes
   a year

   The United States Clean Air Act, passed in 1990, put mercury on a list
   of toxic pollutants which need to be controlled to the greatest
   possible extent. Thus, certain industries that emit mercury into the
   environment must install maximum achievable control technologies
   (MACT). However, a March 2005 EPA rule took power plants off the list
   of sources which must reduce mercury to the maximum extent. Instead, a
   cap and trade rule was issued, with most of the reductions in mercury
   pollution from power plants beginning in the year 2018. States were
   also given until November, 2006 to impose stricter controls, and
   several states are doing so. The rule was being subjected to legal
   challenges from several states in 2005.

Mercury and aluminium

   Mercury readily combines with aluminium to form an amalgam when the two
   pure metals come into contact. However, when the amalgam is exposed to
   air, the aluminium oxidizes, leaving behind mercury. The oxide flakes
   away, exposing more mercury amalgam, which repeats the process. This
   process continues until the supply of amalgam is exhausted, and since
   it releases mercury, a small amount of mercury can “eat through” a
   large amount of aluminium over time, by progressively forming amalgam
   and relinquishing the aluminium as oxide.

   Aluminium in air is ordinarily protected by a molecule-thin layer of
   its own oxide (which is not porous to oxygen). Mercury coming into
   contact with this oxide does no harm. However, if any elemental
   aluminium is exposed (even by a recent scratch), the mercury may
   combine with it, starting the process described above, and potentially
   damaging a large part of the aluminium before it finally ends (Ornitz
   1998).

   For this reason, restrictions are placed on the use and handling of
   mercury in proximity with aluminium. In particular, mercury is not
   allowed aboard aircraft under most circumstances because of the risk of
   it forming amalgam with exposed aluminium parts in the aircraft.

Reclamation of mercury mines

   Due to minimal surface disruption, mercury mines lend themselves to
   constructive re-use. For example, in 1976 the Santa Clara County,
   California purchased the historic Almaden Quicksilver Mine and
   proceeded to create a county park on the site, after conducting
   extensive safety and environmental analysis of the property.
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