Magnesium (Mg)

There is a region of Thessaly called Magnesia, and an implausible amount of chemical vocabulary comes out of it. The lodestone found there was magnes lithos, the Magnesian stone, which gives English the magnet. Two different white and black mineral powders from the same area were called magnesia alba and magnesia nigra, and each turned out to contain a different metal. The white one produced magnesium. The black one produced manganese. Three words, one place name, and no chemical relationship between any of them.

Untangling the white powder

Magnesia alba had been sold as a laxative long before anyone asked what it was, and it was widely assumed to be a form of lime. Joseph Black settled that in 1755 with careful weighing: he showed that magnesia alba and limestone behaved differently on heating and in reaction with acids, and that the two were distinct substances. That work, and the fixed air he identified along the way, is a founding document of quantitative chemistry.

Black never had the metal. Humphry Davy obtained it in 1808 by electrolysing a mixture of moist magnesia with mercury oxide and then driving off the mercury from the resulting amalgam — the same indirect route that had finally worked for calcium. The card above pairs Black's name with Davy's date because a single credit line cannot hold both halves of the story, and both halves are genuinely required: one man proved the element was there, the other got it out.

Antoine Bussy produced a coherent sample of usable purity in 1831, which is where practical magnesium chemistry actually begins.

The same ring, a different metal, a different job

Chlorophyll is built on a porphyrin-type macrocycle with a single magnesium ion held at its centre. That structure closely resembles the haem group that carries oxygen in blood, and the difference in function comes almost entirely from which metal sits in the middle.

Magnesium is a poor redox metal — it has essentially one accessible oxidation state and will not shuttle electrons the way iron does. For a light-harvesting pigment, that is exactly right. The metal's job is to hold the ring rigid and planar, tuning the electronic structure so that it absorbs strongly in the red and blue and reflects the green in between, and then to pass the captured excitation onward without being chemically altered itself. A metal eager to change oxidation state would quench the excited state and waste the photon.

Roughly a quarter of the magnesium in a green leaf is in chlorophyll. Most of the rest is doing something equally universal: nearly every reaction involving ATP requires magnesium as a counter-ion, screening the negative charges on the phosphate chain so that an enzyme can approach it. Every kinase in every organism is really working on magnesium-ATP.

The lightest structural metal, and the reason that is difficult

Magnesium is about two-thirds the density of aluminum and a quarter that of steel, which gives it the best strength-to-weight ratio of any common structural metal. It also has two serious liabilities: it is not stiff, and it corrodes readily, particularly in salt spray and especially when bolted directly to steel or aluminum, where galvanic contact accelerates the attack.

The result is a metal used precisely where those weaknesses can be designed around — as die castings inside a structure rather than as skin. Steering wheel armatures, instrument panel beams, seat frames, gearbox and camera housings and laptop chassis are the typical products. Cast magnesium fills a thin, complicated mould better than aluminum does, which is as much a reason for its use as the weight saving.

But the largest single destination is not structural at all. Most magnesium is alloyed into aluminum: the 5000-series alloys used in ship hulls, beverage cans and automotive body panels depend on it for strength and corrosion resistance. The can in your hand is aluminum with a few per cent of magnesium, and the lid has a different composition from the body.

Its corrosion weakness is also sold deliberately. A magnesium rod inside a domestic water heater, and magnesium anodes buried alongside steel pipelines, exist to corrode preferentially so that the steel does not. The metal's job there is to be destroyed.

One country, one process, and the autumn of 2021

Around eighty-five to ninety per cent of world magnesium comes from China, and most of that from a cluster of plants in Shaanxi province running the Pidgeon process — a silicothermic reduction in which calcined dolomite is heated with ferrosilicon in rows of retorts under vacuum. Lloyd Montgomery Pidgeon developed it in Canada during the Second World War. It is simple, batch-based, labour-intensive and extremely energy-hungry, and it survives because those inputs were cheap in one place.

In September and October 2021, Chinese electricity rationing shut a large share of that capacity almost overnight. European magnesium prices rose several-fold within weeks, and European industry bodies publicly warned that aluminum alloy production across the continent could halt within months, because there is no aluminum sheet without magnesium and no meaningful non-Chinese supply. Output resumed before the worst case arrived, though not before European manufacturers had discovered how much of their own supply chain ran through one Chinese province.

The irony is that the raw material is not scarce in the slightest. Seawater holds well over a kilogram of magnesium per tonne, and the Dow plant at Freeport, Texas, extracted wartime magnesium directly from the Gulf of Mexico from 1941, using lime burnt from oyster shells to precipitate it. The resource is effectively unlimited; what is concentrated is the cheap way of reducing it.

The reagent that built organic synthesis

In 1900 Victor Grignard found that magnesium turnings react with an organic halide in dry ether to give a compound with a carbon-magnesium bond, and that this compound will attack a carbonyl group to form a new carbon-carbon bond.

That last capability was the bottleneck of organic chemistry. Building a bigger molecule from smaller ones requires joining carbon to carbon, and before Grignard reagents the options were few and bad. He shared the 1912 Nobel prize with Paul Sabatier, and named reactions rarely earn one that quickly. Organomagnesium chemistry remains a first-choice tool more than a century later.

Burning bright in the ultraviolet

Magnesium burns with an intense white light that is unusually rich in ultraviolet, and that spectral quality is why it dominated early flash photography — magnesium ribbon and then magnesium flash powder gave photographers usable exposures indoors decades before electric flashbulbs. The same property made it the core of incendiary munitions in both world wars, designed small and dropped in enormous numbers rather than as large individual bombs.

The fire is difficult to put out because burning magnesium is hot enough to strip oxygen from water and from carbon dioxide, which means the substances used to smother most fires become fuel instead. That is a fact about the metal's thermodynamics, and it is why magnesium fires are given their own hazard classification.

Weathering, recorded in a ratio

Magnesium has three stable isotopes, and the ratio between magnesium-26 and magnesium-24 varies measurably between reservoirs because chemical and biological processes discriminate slightly by mass.

Silicate weathering strongly fractionates magnesium — clay minerals preferentially retain the heavier isotope — while carbonate weathering barely does. The magnesium isotope signature of ancient seawater, read from marine carbonates, therefore carries information about the balance between those two kinds of rock breakdown through geological time. Since silicate weathering is the long-term sink that draws carbon dioxide out of the atmosphere, that ratio is one of the threads used to reconstruct the climate of the deep past.