Magnesium Metal Mg · 12

The lightest structural metal, about a third lighter than aluminium, and the ingredient that makes most aluminium alloys work.

Por que razão é importante?

Aluminium alloying is the largest single use, so a magnesium shortage becomes an aluminium problem within weeks.

Where it is in the Earth

Magnesium is the eighth most abundant element in the Earth's crust, and it occurs in a wider variety of rock types than almost any other metal of commercial importance. The most significant sources are carbonate rocks — dolomite, which is a double carbonate of calcium and magnesium, and magnesite, which is magnesium carbonate on its own. These minerals formed over geological time as seawater and freshwater rich in dissolved magnesium ions reacted with existing carbonate sediments, or as magnesium-bearing hydrothermal fluids moved through older limestones and altered them. The result is thick, laterally extensive beds of dolomite and lenses of crystalline magnesite found on most continents. Because the source rock (limestone) is common and the chemical substitution that creates dolomite requires only time and the right fluid chemistry, large deposits are not rare.

A second, and increasingly important, geological setting is evaporite — the mineral residues left behind when ancient seas or lakes dried out. Carnallite and bischofite are hydrated magnesium chloride salts that accumulate in the deepest parts of evaporite sequences, beneath potassium and sodium salts that crystallise first. The Dead Sea basin in Israel and Jordan, and the Zechstein evaporites beneath parts of Europe, are well-known examples. Similarly, brines trapped in subsurface aquifers or concentrated in modern terminal lakes — including the Great Salt Lake in Utah and the Salar de Atacama in Chile — carry dissolved magnesium in concentrations that can make extraction economic. A third geological source is the olivine and serpentine minerals in ultramafic igneous rocks, though these are less widely exploited for metal production.

The geographical spread of deposits is genuinely broad, which means scarcity of the raw feedstock is not the binding constraint on supply. The binding constraint, as the production table on this page makes plain, is the concentration of metal-making capacity in one country. Dolomite and magnesite are found on every continent; the process that turns them into metal is what determines where production happens.

Getting it out

Because the principal feedstocks for magnesium metal — dolomite and magnesite — are sedimentary or altered carbonate rocks that occur in thick, near-horizontal beds close to the surface, open-pit quarrying is the standard extraction method. The rock is drilled, blasted, loaded and hauled in the same way as any large hard-rock quarry. The ore grades that matter here are not like the grades of a gold or copper mine, where a fraction of a percent defines whether a deposit is worth touching. Carbonate rocks used for magnesium extraction contain magnesium as a major constituent of the mineral itself, so the chemistry of the rock type matters more than a trace concentration. What the operator is managing is the purity of the carbonate — its silica content, iron content, and the ratio of magnesium to calcium — rather than recovering a rare element from a host rock.

Brine operations work differently. At the Dead Sea in Israel, magnesium-rich brines are pumped or allowed to flow into large evaporation ponds, where solar energy concentrates the dissolved salts. The resulting magnesium chloride liquor is then processed chemically rather than crushed and sorted. This approach produces essentially no solid waste in the quarrying sense, but it consumes large areas of shallow water and generates significant volumes of byproduct salts. The strip ratio — the amount of waste material moved per tonne of usable material — is very low in good dolomite quarries, because the rock itself is the feedstock and rejection rates are modest. In brine operations the concept of strip ratio does not apply in the same way; instead, energy and pond area become the governing constraints.

The United States, which the production table shows as having no reported domestic primary magnesium output in the most recent year, historically produced metal from both brine sources along the Gulf Coast and from dolomite. The absence of current U.S. production is not a consequence of depleted geology — the raw materials exist — but reflects the economics of the conversion processes relative to imported metal.

What pulls on it

Magnesium metal reaches consumers through two main channels: as an alloying addition to aluminium, and as the base metal in its own right for die-cast components. The aluminium alloying use is by far the larger of the two. Most of the aluminium alloys used in automotive body sheet, beverage cans, and structural applications carry some magnesium; it increases strength through a mechanism called solid-solution hardening and precipitation hardening, and it controls corrosion behaviour. The amount added per tonne of aluminium alloy is small in percentage terms, but the total volume of aluminium produced globally is enormous, which makes the aggregate demand for magnesium as an alloying addition substantial. This structural link means that magnesium demand tracks aluminium demand fairly closely, and a shortage in magnesium supply becomes a production problem for aluminium alloy makers within a matter of weeks, because stocks are not typically held in large quantity at the point of use.

The second channel — magnesium die castings used directly — matters most in the automotive industry, where thin-walled magnesium castings appear in steering columns, instrument panel frames, seat structures and transmission cases. Magnesium's low density, at 1.738 grams per cubic centimetre, makes it attractive wherever mass saving matters: a component that would weigh a given amount in steel can be replaced by a much lighter magnesium casting. Growth in this use is connected to vehicle electrification, because battery-electric vehicles carry heavy battery packs that create pressure to reduce mass elsewhere in the vehicle. Steel desulfurisation — adding small quantities of magnesium to molten steel to remove sulfur and improve steel quality — is a further industrial use that draws on a modest but steady tonnage.

What would have to change for demand to shift sharply? A decline in automotive production or a sustained shift away from aluminium alloys containing magnesium would reduce consumption. On the other side, a faster-than-expected growth in lightweight die-cast components for electric vehicles, or expanded use in aerospace where strict weight limits apply, would pull demand upward. The connection to aluminium means that demand is partly insulated from substitution at the magnesium level — a buyer cannot easily stop using aluminium alloys in the short term — but it also means that any disruption to magnesium supply is amplified by the importance of the downstream product.