Rhenium

Rhenium is incorporated into certain superalloys used in aircraft turbines because it helps maintain their mechanical strength at high temperatures. These parts endure intense heat and great stresses while the engine runs. Rhenium is scarce and expensive, so it is reserved for specialized alloys. Its presence can improve certain properties, although the design and cooling of the parts are also essential.

Rhenium-187 slowly decays by emitting beta particles to become osmium-187, with a half-life of 41.2 billion years. Unlike other geological clocks based on silicates, both rhenium and osmium are strongly siderophile and chalcophile, concentrating in sulfide minerals such as molybdenite. By measuring the ¹⁸⁷Re/¹⁸⁷Os ratio in sulfide crystals, geologists can determine with millimeter precision the exact date when the great copper and gold porphyry deposits precipitated.

In the catalytic reforming of oil refineries, heavy low-octane naphthas are converted into branched high-octane aromatic hydrocarbons for unleaded gasolines. Traditional pure-platinum catalysts deactivated quickly because carbon ('coke') deposited on the metal, blocking it. Alloying platinum with rhenium catalytically breaks up the coke precursors and prevents their buildup, allowing the reactor to run without interruption for years at high pressure and temperature.

Most temperature sensors melt or degrade above 1,700 °C. Thermocouples based on tungsten alloys with 5% rhenium paired with tungsten and 26% rhenium exploit the extremely high melting points of both refractory metals. Adding rhenium eliminates the mechanical brittleness typical of recrystallized tungsten, allowing millimeter-faithful measurement of infernal temperatures up to 2,300 °C in nuclear reactors and sintering furnaces.

Unlike other metals that form concentrated, visible minerals, rhenium is one of the scarcest elements in the Earth's crust (barely one part per billion). It also tends to hide, dispersed in microscopic amounts, inside molybdenum and platinum minerals. That is why chemists needed very advanced X-ray analysis techniques in 1925 to detect it for the first time.

Rhenite (ReS₂) was the first pure rhenium mineral confirmed on Earth, found at the Kudriavy volcano in the Kuril Islands. In the deep magma, volcanic gases above 900 °C carry rhenium in the gas phase. As they emerge to the surface through cracks and meet the cold air, rhenium sulfide vapor sublimes directly as fine metallic crystals on the walls of active fumaroles.

Rhenium-187 is radioactive, but its half-life is so long that only a small fraction decays on time scales comparable to the age of the Earth. That is why it remains the most abundant isotope of natural rhenium. It slowly turns into osmium-187, a property that geologists use to study the age and origin of certain rocks and minerals.