Question: What does Mercury have that's large and made of iron?
About the article "Solar System", section "Mercury".

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Mercury relays consist of a vertical (usually glass) tube containing liquid mercury.
They have isolated contacts at the bottom of the tube and partway up, usually in a side arm of the glass.
The relay works by displacement.
A pool of mercury fills the lower portion of the tube, but is insufficient to bridge the contacts.
A magnetic slug of iron or steel is placed in the tube, where it sinks by gravity so as to displace the mercury.
The displaced mercury rises in the tube, sufficiently to bridge the contacts and complete the circuit between them.
Around the top part of the tube is placed the coil.
When energised, this coil attracts the slug, lifting it upwards and out of the mercury pool.
The mercury is no longer displaced, so flows downwards, away from the upper contact and the circuit opens.
This allows for normally closed operation.
For the more traditional normally open relay operation the side contact is arranged somewhat higher up (or the volume of mercury is reduced) so contact is not made when the iron slug is free floating on the pool of mercury.
The control coil is mounted below the rest level of the slug and when energised draws down the slug deeper into the pool displacing additional mercury thus raising the level to the previously uncovered side contact and closing the circuit.
The mercury relay thus allows for switching of higher currents with a small control current for a large number of cycles, they are often installed into automatic controllers that required extended periods of unattended continuous switching operation.
The glass tube of a mercury relay must be mounted near-vertically.
Their sensitivity can be adjusted by adjusting their angle relative to vertical.
As sensitivity depends upon angle, they are unsuitable for use on mobile equipment or with conditions of high vibration. 
Iron forms compounds mainly in the +2 and +3 oxidation states.
Traditionally, iron(II) compounds are called ferrous, and iron(III) compounds ferric.
Iron also occurs in higher oxidation states, an example being the purple potassium ferrate (K2FeO4) which contains iron in its +6 oxidation state.
Iron(IV) is a common intermediate in many biochemical oxidation reactions.
Numerous organometallic compounds contain formal oxidation states of +1, 0, −1, or even −2.
The oxidation states and other bonding properties are often assessed using the technique of Mössbauer spectroscopy.
There are also many mixed valence compounds that contain both iron(II) and iron(III) centers, such as magnetite and Prussian blue (Fe4(Fe6)3).
The latter is used as the traditional "blue" in blueprints.
The iron compounds produced on the largest scale in industry are iron(II) sulfate (FeSO4·7H2O) and iron(III) chloride (FeCl3).
The former is one of the most readily available sources of iron(II), but is less stable to aerial oxidation than Mohr's salt ((NH4)2Fe(SO4)2·6H2O).
Iron(II) compounds tend to be oxidized to iron(III) compounds in the air.
Unlike many other metals, iron does not form amalgams with mercury.
As a result, mercury is traded in standardized 76 pound flasks (34 kg) made of iron. 
Several possible classifications for terrestrial planets have been proposed:  Silicate planet  The standard type of terrestrial planet seen in the Solar System, made primarily of silicon-based rocky mantle with a metallic (iron) core.
Iron planet  A theoretical type of terrestrial planet that consists almost entirely of iron and therefore has a greater density and a smaller radius than other terrestrial planets of comparable mass.
Mercury in the Solar System has a metallic core equal to 60–70% of its planetary mass.
Iron planets are believed to form in the high-temperature regions close to a star, like Mercury, and if the protoplanetary disk is rich in iron.
Coreless planet  A theoretical type of terrestrial planet that consists of silicate rock but has no metallic core, i.e.
the opposite of an iron planet.
Although the Solar System contains no coreless planets, chondrite asteroids and meteorites are common in the Solar System.
Coreless planets are believed to form farther from the star where volatile oxidizing material is more common.
Carbon planet (also called "diamond planet")  A theoretical class of planets, composed of a metal core surrounded by primarily carbon-based minerals.
They may be considered a type of terrestrial planet if the metal content dominates.
The Solar System contains no carbon planets, but does have carbonaceous asteroids.  
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 (ppm).
However, because it does not blend geochemically with those elements that constitute 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).
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.
Beginning in 1558, with the invention of the patio process to extract silver from ore using mercury, mercury became an essential resource in the economy of Spain and its American colonies.
Mercury was used to extract silver from the lucrative mines in New Spain and Peru.
Initially, the Spanish Crown's mines in Almaden in Southern Spain supplied all the mercury for the colonies.
Mercury deposits were discovered in the New World, and more than 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.
The patio process and later pan amalgamation process continued to create great demand for mercury to treat silver ores until the late 19th century.
Former mines in Italy, the United States and Mexico which once produced a large proportion of the world supply have now been completely mined out or, in the case of Slovenia (Idrija) and Spain (Almadén), shut down due to the fall of the price of mercury.
Nevada's McDermitt Mine, the last mercury mine in the United States, closed in 1992.
The price of mercury has been highly volatile over the years and in 2006 was $650 per 76-pound (34.46 kg) flask.
Mercury is extracted by heating cinnabar in a current of air and condensing the vapor.
The equation for this extraction is HgS + O2 → Hg + SO2 In 2005, China was the top producer of mercury with almost two-thirds global share followed by Kyrgyzstan.
Several other countries are believed to have unrecorded production of mercury from copper electrowinning processes and by recovery from effluents.
Because of the high toxicity of mercury, both the mining of cinnabar and refining for mercury are hazardous and historic causes of mercury poisoning.
In China, prison labor was used by a private mining company as recently as the 1950s to create new cinnabar mercury mines.
Thousands of prisoners were used by the Luo Xi mining company to establish new tunnels.
In addition, worker health in functioning mines is at high risk.
The European Union directive calling for compact fluorescent bulbs to be made mandatory by 2012 has encouraged China to re-open deadly cinnabar mines to obtain the mercury required for CFL bulb manufacture.
As a result, environmental dangers have been a concern, particularly in the southern cities of Foshan and Guangzhou, and in the Guizhou province in the south west.
Abandoned mercury mine processing sites often contain very hazardous waste piles of roasted cinnabar calcines.
Water run-off from such sites is a recognized source of ecological damage.
Former mercury mines may be suited for constructive re-use.
For example, in 1976 Santa Clara County, California purchased the historic Almaden Quicksilver Mine and created a county park on the site, after conducting extensive safety and environmental analysis of the property. 
Mercury content in tuna can vary widely.
For instance, testing by Rutgers University reportedly found that a can of StarKist had 10 times more mercury than another can of similarly identified tuna.
This has prompted a Rutgers University scientist whose staff conducted the mercury analysis to say, "That's one of the reasons pregnant women have to be really careful ...
If you happen to get a couple or three cans in the high range at a critical period when you are pregnant, it would not be good."
Among those calling for improved warnings about mercury in tuna is the American Medical Association, which adopted a policy that physicians should help make their patients more aware of the potential risks.
A study published in 2008 found that mercury distribution in the meat of farmed tuna is inversely related to the lipid content, suggesting that higher lipid concentration within edible tissues of tuna raised in captivity might, other factors remaining equal, have a diluting effect on mercury content.
These findings suggest that choosing to consume a type of tuna that has a relatively higher natural fat content might help reduce the amount of mercury intake, compared to consuming tuna with a low fat content.
Due to their high position in the food chain and the subsequent accumulation of heavy metals from their diet, mercury levels can be high in larger species such as bluefin and albacore.
In 2009 a California appeals court upheld a ruling that canned tuna does not need warning labels as the methylmercury is naturally occurring.
In March 2004, the United States FDA issued guidelines recommending that pregnant women, nursing mothers, and children limit their intake of tuna and other predatory fish.
The Environmental Protection Agency provides guidelines on how much canned tuna it is safe to eat.
Roughly speaking, the guidelines recommend one 6 oz.
can of light tuna a week for those weighing less than 110 pounds and two cans a week for those who weigh more.
In 2007 it was reported that some canned light tuna such as yellowfin tuna is significantly higher in mercury than skipjack, and caused Consumers Union and other activist groups to advise pregnant women to refrain from consuming canned tuna.
The Eastern little tuna ("Euthynnus affinis") has been available for decades as a low-mercury, less expensive canned tuna.
However, of the five major species of canned tuna imported by the United States it is the least commercially attractive, primarily due to its dark color and more pronounced 'fishy' flavor.
Its use has traditionally been restricted to institutional (non-retail) commerce.
A January 2008 investigation conducted by the "New York Times" found potentially dangerous levels of mercury in certain varieties of sushi tuna, reporting levels "so high that the Food and Drug Administration could take legal action to remove the fish from the market.
"A book by Jane Hightower, "", published in 2008, discusses human exposure to mercury through eating large predatory fish such as large tuna.