Manganese

• 1. Introduction

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  Manganese, the chemical element with the symbol Mn and atomic number 25, sits quietly inside almost every can of soda and every batch of stainless steel, yet most people have never heard its name. It is a hard, brittle, silvery metal that rusts like iron in oxygenated water and tarnishes slowly in air. The human body holds about 12 milligrams of it, mostly in the bones. First isolated in the 1770s, it has since become a metal that no metallurgist can replace. How did a mineral once used to wash the green tint out of glass end up sustaining steel mills, enzymes, and the oxygen that plants release? Why does too much of it mimic Parkinson's disease, while too little stalls wound healing? And what made a CIA spy ship pretend to mine it from the deep ocean floor? The answers run from the caves of the Stone Age to the active sites of living enzymes.

• 2. A Name Born From Confusion

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  Two black minerals from the lands of the Magnetes, either Magnesia in modern Greece or Magnesia ad Sipylum in modern Turkey, started the long tangle behind the word manganese. Both were called magnes after their origin, but ancient observers thought they differed in sex. The male magnes attracted iron and was the ore now known as lodestone or magnetite, the source of the word magnet. The female magnes did not attract iron at all. Instead it was used to decolorize glass, and that ore is what we now call pyrolusite, or manganese dioxide. Neither it nor elemental manganese is actually magnetic. In the 16th century glassmakers called manganese dioxide manganesum, with two N's, possibly a garbled blend of words used to separate the black ore, magnesia nigra, from a useful white ore called magnesia alba. The Italian physician Michele Mercati called the black ore manganesa, and the metal drawn from it eventually became manganese. The name magnesia drifted to the white ore, magnesium oxide, which is why a different element entirely ended up called magnesium when it was isolated much later.

• 3. From Cave Walls To Glassmakers Soap

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  The cave paintings at Gargas, between 30,000 and 24,000 years old, were drawn with the mineral form of manganese dioxide. That dark brown pigment gave Stone Age artists a lasting color. Egyptian and Roman glassmakers later turned the same compound into a tool, using it both to add color and to remove it. Known as glassmakers soap, this use carried through the Middle Ages and survives in 14th-century glass from Venice. The chemistry behind it is precise. Manganese in the +3 state reacts with iron in the +2 state, producing less-colored iron and a slightly pink manganese that cancels the residual green tint left by iron contamination. Larger doses of manganese push glass the other way, into a pink color. The metal also reaches into ceramics, where manganese compounds account for the brown shade of some pottery. In 2009, Mas Subramanian and colleagues at Oregon State University combined manganese with yttrium and indium to make YInMn Blue, a non-toxic, fade-resistant pigment described as the first new blue discovered in 200 years.

• 4. The Chemist's Purple Reagent

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  Common oxidation states of manganese are +2, +4, and +7, though every state from minus 3 to +7 has been observed. The +7 state gives the intensely purple permanganate anion, and potassium permanganate is a workhorse oxidizer in laboratories. It serves as a topical medicine, including treatment for fish diseases, and as a biocide in water treatment. Solutions of it were among the first stains and fixatives used to prepare biological cells and tissues for electron microscopy. The +2 state tells a quieter story, glowing a pale pink in water. Manganese in this state usually carries five unpaired electrons in a high-spin arrangement because of its high pairing energy, and it has no spin-allowed d-d transitions, which is why its color stays faint. This is the state of the mineral rhodochrosite, manganese carbonate. The +4 state is the puzzle of the family, common in nature yet rare in synthetic chemistry. Its most familiar face is pyrolusite, the dark brown manganese dioxide that colored cave drawings and still fills dry cell batteries.

• 5. Indispensable To Steel

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  Robert Forester Mushet, who lived from 1811 to 1891, introduced manganese to steelmaking in 1856 in the form of spiegeleisen, an iron alloy holding roughly 15 percent manganese. The element fixes sulfur, removes oxygen, and alloys cleanly, and metallurgy has found no satisfactory substitute for it. Steelmaking now drives 85 to 90 percent of all manganese demand. Small amounts make steel easier to work at high temperatures by forming a high-melting sulfide that blocks brittle iron sulfide at the grain boundaries. The relationship is not linear. At 4 percent manganese the steel turns brittle, but the embrittlement eases at higher concentrations and reaches an acceptable level near 8 percent. Steel with 8 to 15 percent manganese can reach a tensile strength of 863 megapascals. Robert Hadfield discovered a 12 percent manganese steel in 1882, and Hadfield steel, also called mangalloy, went on to make British military helmets and later equipped the U.S. military.

• 6. Cans, Coins, And Batteries

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  Aluminium with roughly 1.5 percent manganese resists corrosion because its grains absorb impurities that would otherwise drive galvanic corrosion. The alloys 3004 and 3104, holding 0.8 to 1.5 percent manganese, make most beverage cans. Before 2000 more than 1.6 million tonnes of those alloys were used, which at 1 percent manganese consumed 16,000 tonnes of the metal. Manganese dioxide quietly powers everyday electronics as the blackish electron acceptor in carbon-zinc flashlight cells, where it reduces to manganese oxide-hydroxide during discharge and stops hydrogen from forming at the anode. The same material runs newer alkaline batteries, and in 2002 more than 230,000 tons of manganese dioxide went toward this use. Coins carry the metal too. The only United States coin to use it before 2000 was the wartime nickel of 1942 to 1945, when a nickel shortage forced an alloy of 56 percent copper, 35 percent silver, and 9 percent manganese. Since 2000, dollar coins such as the Sacagawea dollar and the Presidential dollars use brass with 7 percent manganese over a pure copper core.

• 7. The Ocean Floor And A Spy Ship

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  Manganese nodules, composed of 29 percent manganese, blanket the ocean floor in vast numbers. A 1978 estimate put the seafloor's supply at 500 billion tons, and the search for an economically viable way to harvest them continues without a single commercial success. The pursuit once served as the perfect lie. In 1972 the CIA's Project Azorian, channeled through the billionaire Howard Hughes, commissioned the ship Hughes Glomar Explorer under the cover story of harvesting manganese nodules. The story worked too well, setting off a rush to collect nodules across the sector. The vessel's true mission was to raise the sunken Soviet submarine K-129 and recover its code books. Manganese also lives in seawater as dissolved manganese, 90 percent of which comes from hydrothermal vents. Enriched fluid from these vents can travel up to 4,000 kilometers, kept aloft by microbial capsules that slow its sinking. On land, about 80 percent of known world resources sit in South Africa, where the Kalahari manganese fields near Hotazel carried a 2011 estimate of 15 billion tons. That year South Africa produced 3.4 million tons, more than any other nation.

• 8. The Two Faces Of A Trace Metal

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  Manganese is essential to human health, but only in milligram amounts, and the line between need and harm is narrow. It works as a coenzyme in macronutrient metabolism, bone formation, and free radical defense, and it is a critical component in dozens of proteins and enzymes. In the brain it binds to manganese metalloproteins, most notably glutamine synthetase in astrocytes. Far from the body, four atoms of manganese form the oxygen-evolving complex inside photosystem II, the catalyst that produces the oxygen plants release during photosynthesis. The danger lies in excess. Overexposure causes manganism, a rare neurological disorder first described in 1837 by the British academic John Couper, who studied two manganese grinders. Manganism is biphasic. Early stages bring depression, mood swings, compulsive behaviors, and psychosis, and the later stage resembles Parkinson's disease with tremor, an expressionless face, and a forward-leaning gait. Unlike Parkinson's, it does not cost patients their sense of smell, and they typically do not respond to L-DOPA. Worse still, the symptoms can keep worsening even after the exposure ends and brain manganese levels return to normal.