Rubidium (Rb)

Rubidium is quietly radioactive and almost nobody thinks of it that way. Just over a quarter of every natural sample is rubidium-87, which decays by beta emission with a half-life near 49 billion years — some ten times the age of the Earth, slow enough that the element is treated as stable in every practical sense and fast enough that the products have accumulated measurably in every rock on the planet. That decay is the basis of one of geology's most important dating methods, and it makes rubidium unusual among the light elements: a common, unremarkable metal that has been running a clock since the solar system condensed.

Deep red lines in Dürkheim water

Robert Bunsen and Gustav Kirchhoff had just invented spectrum analysis, and in 1860 they used it to pull cesium out of mineral water from Bad Dürkheim. In 1861, working through the same source, they found a second unknown — a pair of lines at the far red end of the spectrum, beyond anything potassium produced.

Getting enough of it to weigh was brutal. The concentration in the spring water was so low that Bunsen worked through something on the order of forty thousand litres to accumulate a few grams of rubidium salts. He named it from Latin rubidus, meaning deep red, describing the lines rather than the metal, which is a soft silvery thing with no red about it at all. The naming convention here is worth noticing: cesium, rubidium, thallium and indium were all named for the colours of their emission lines, and all four were found in the first years after the spectroscope made that possible.

Rubidium in, strontium out

Rubidium-87 becomes strontium-87. Rubidium substitutes readily for potassium in minerals, so it concentrates in micas, potassium feldspars and the granites built from them; strontium substitutes for calcium and goes elsewhere. A rock that crystallises therefore starts with rubidium and strontium separated into different minerals, and from that moment each mineral's strontium-87 content grows in proportion to the rubidium it trapped.

Plotting strontium-87 against rubidium-87 across several minerals from the same rock gives a straight line — an isochron — whose slope gives the age and whose intercept gives the initial strontium composition, so no assumption about starting conditions is needed. The method dated the oldest terrestrial rocks and the chondritic meteorites, and it produced one of the early independent confirmations that the solar system is about 4.5 billion years old.

Its weakness is the same as its strength. Rubidium and strontium are both mobile in hot fluids, so metamorphism can reset the clock or scatter the points, and modern geochronology leans harder on uranium–lead in zircon. Rubidium–strontium remains the standard tool for whole-rock ages and for tracing where crustal material came from, because the strontium ratio a rock inherits is a fingerprint of its source.

Why physicists chose rubidium-87

In June 1995, at JILA in Boulder, Eric Cornell and Carl Wieman cooled about two thousand rubidium-87 atoms to 170 nanokelvin and watched them collapse into a single quantum state — the first Bose-Einstein condensate, seventy years after Bose and Einstein predicted one. It shared the 2001 Nobel Prize in Physics with Wolfgang Ketterle's sodium work at MIT.

Rubidium was not an arbitrary choice. Laser cooling needs a transition that a laser can reliably drive, and rubidium-87's sits at 780 nanometres, right in the range of inexpensive, stable diode lasers of the kind mass-produced for compact disc players. Its collision properties also happen to be favourable for evaporative cooling, the final stage that gets a cloud from microkelvin to nanokelvin. The result is that rubidium became the default atom of the entire cold-atom field: atom interferometers, quantum simulators, optical lattice experiments and Rydberg-atom quantum computers overwhelmingly run on it, largely because the hardware is cheap and the recipe is known.

The clock that is good enough

Cesium defines the second, but cesium clocks are large and expensive. A rubidium standard uses a small glass cell of rubidium vapour with a buffer gas, locks a quartz oscillator to the hyperfine transition, and fits in a box the size of a paperback for a small fraction of the price. It drifts, so it needs occasional discipline from something better, but its short-term stability is excellent.

That combination has made rubidium the most widely deployed atomic clock in the world. Mobile network base stations, broadcast timing, laboratory frequency references and the navigation satellites of GPS, Galileo and BeiDou all rely on rubidium oscillators, in most cases with several aboard each spacecraft. When timekeeping matters commercially rather than metrologically, it is nearly always rubidium doing it.

The rest of the market is very small

Outside physics, rubidium has almost no industrial life. It appears in some photocathodes and night-vision components, in a few specialty glasses, and in fireworks where its salts give a red-violet flame. Its most substantial applied use is medical: rubidium-82, a positron emitter with a half-life of about 75 seconds, is eluted from a strontium-82 generator directly at the scanner and used for cardiac perfusion PET. The half-life is so short that the patient must be positioned before the dose is drawn, which is exactly why it delivers a low radiation burden.

Biologically, rubidium behaves as a slightly clumsy potassium. Cells take it up through potassium channels and distribute it around the body, and an adult carries a few tenths of a gram of it with no known function whatever — probably the most abundant element in human tissue that does nothing at all.

Nobody mines it

Rubidium has no ore of its own; it is too similar to potassium to concentrate separately in most settings. It is recovered in small quantities from lepidolite and from pollucite processed primarily for cesium, principally at Bernic Lake in Manitoba, and world production is measured in tonnes rather than thousands of tonnes. For an element that is more abundant in the crust than copper, that is a striking gap between how much exists and how much anyone can buy.