Molybdenite

The name molybdenite comes from a Greek word that means lead. For most of antiquity and the centuries that followed, the word travelled with the wrong mineral — anything dark, soft, and metallic that left a grey streak was lumped under it, lead ores most of all.

The Greek physician Dioscorides used variants of the name for lead ores in the first century CE. Pliny the Elder repeated the usage in his Natural History of 79 CE. The German mineralogist Agricola was still applying it to lead ores in 1556. One word covered several minerals at once. Galena (the chief lead ore), graphite (the soft black carbon that marks paper), and what we now call molybdenite all answered to it.

The modern usage was fixed in 1747. The Swedish mineralogist Johan Gottschalk Wallerius wrote about molybdenite in his treatise Mineralogia, eller Mineralriket. Even then the name was not unique to one mineral. Graphite and molybdenite, both soft, both grey-black, both greasy to the touch, kept sharing it.

The disambiguation arrived in 1778. The Swedish chemist Carl Wilhelm Scheele showed that molybdenite, in the modern sense, dissolved in acid, while graphite did not. The same year he proposed that molybdenite was the ore of a distinct new element. Three years later, in 1781, the Swedish chemist Peter Jacob Hjelm isolated that element using carbon and linseed oil as reductants. Hjelm worked from a sample Scheele had provided. He called the new metal molybdaenum — the name carried over from the ore that had given it up.

The mineral kept a long shadow in industry. Demand for the metal climbed sharply during the First World War. Allied armies discovered that German tanks owed their toughness to molybdenum-strengthened steel. The Climax mine in Colorado, which began shipping ore in 1915, came at its peak to supply roughly three quarters of the world's molybdenum.

Almost every tonne of molybdenite mined today is fed to a steel mill. The mineral is the most common source of the metal molybdenum, and molybdenum's place in the industrial world is as an alloying agent — a small percentage in the mix that hardens iron and steel and resists corrosion.

About 86 % of the molybdenum produced goes into metallurgy. Structural steels absorb the largest share at 35 %, followed by stainless steel at 25 %, chemicals at 14 %, tool and high-speed steels at 9 %, and cast iron at 6 %. Most high-strength steel alloys contain between 0.25 % and 8 % molybdenum by weight. The same metal is a standard ingredient of superalloys — the nickel- and cobalt-based blends that hold their strength inside jet engines and gas turbines.

Molybdenite reaches the alloy as ferromolybdenum, an iron-rich master alloy stirred directly into molten steel. This is by far the most common end use of the metal.

Production splits across two kinds of deposits. Some mines — notably Climax and Henderson in Colorado — work molybdenite as the primary ore. Others recover it as a by-product of porphyry copper mining. The molybdenum is too dilute to extract on its own, but it rides along once the copper circuit has been built. Bingham Canyon in Utah and Chuquicamata in northern Chile are the two best-known examples. The largest national producers are China, the United States, Chile, Peru, and Mexico.

Beyond the steel mill

Molybdenite earns a second life on its own merits. Its crystal structure is layered — sheets of molybdenum atoms sandwiched between sheets of sulfur — and the sheets slide past one another with very little friction. This makes molybdenum disulfide (MoS₂) a solid lubricant, mixed into greases and applied as a dry film where conventional oils fail. Catalysts and pigments round out the non-metallurgical demand for the metal and its compounds.

A newer line of work treats a single sheet of molybdenite as an electronic material. Peeled down to one atomic layer, the mineral becomes a direct-band-gap semiconductor. That is the property a transistor needs to switch cleanly and a light-emitting device needs to glow. Monolayer molybdenite shows good charge-carrier mobility and can be used to build small, low-voltage transistors, with potential uses in optoelectronics.