Hi @MusCabout. Here are 2 excerpts from the US Geological Survey's Mineral Commodity Summaries 2022. All 4 pages are a fascinating read, but the following may assist in understanding why caesium and rubidium are on the US government's List of Critical Minerals. They are classified as "critical" because technologically and medically, we basically can't do without them. They are "strategic" for their military and security uses.
"The primary application for cesium, by gross weight, is incesium formate brines used for high-pressure, high-temperature well drilling for oil and gas exploration andproduction. With the exception of cesium formate, cesium is used in relatively small-scale applications, using only afew grams for most applications. Owing to the lack of global availability of cesium, many applications have usedmineral substitutes and the use of cesium in any particular application may no longer be viable.
Cesium metal is used in the production of cesium compounds and potentially in photoelectric cells. Cesium bromide isused in infrared detectors, optics, photoelectric cells, scintillation counters, and spectrophotometers. Cesiumcarbonate is used in the alkylation of organic compounds and in energy conversion devices, such as fuel cells,magneto-hydrodynamic generators, and polymer solar cells. Cesium chloride is used in analytical chemistryapplications as a reagent, in high-temperature solders, as an intermediate in cesium metal production, in isopycniccentrifugation, as a radioisotope in nuclear medicine, as an insect repellent in agricultural applications, and inspecialty glasses. Cesium hydroxide is used as an electrolyte in alkaline storage batteries. Cesium iodide is used influoroscopy equipment—Fourier-transform infrared spectrometers—as the input phosphor of X-ray image intensifiertubes, and in scintillators. Cesium nitrate is used as a colorant and oxidizer in the pyrotechnic industry, in petroleumcracking, in scintillation counters, and in X-ray phosphors. Cesium sulfates are soluble in water and are thought to beused primarily in water treatment, fuel cells, and to improve optical quality for scientific instruments.
Cesium isotopes, which are obtained as a byproduct in nuclear fission or formed from other isotopes, such asbarium-131, are used in electronic, medical, metallurgical, and research applications. Cesium isotopes are used as anatomic resonance frequency standard in atomic clocks, playing a vital role in aircraft guidance systems, globalpositioning satellites, and internet and cellular telephone transmissions. Cesium clocks monitor the cycles ofmicrowave radiation emitted by cesium’s electrons and use these cycles as a time reference. Owing to the highaccuracy of the cesium atomic clock, the international definition of 1 second is based on the cesium atom. The U.S.civilian time and frequency standard is based on a cesium fountain clock at the National Institute of Standards andTechnology in Boulder, CO. The U.S. military frequency standard, the United States Naval Observatory (USNO)timescale, is based on 48 weighted atomic clocks, including 25 USNO cesium fountain clocks.
A company in Richland, WA, produced a range of cesium-131 medical products for treatment of various cancers.Cesium-137 may be used in industrial gauges, in mining and geophysical instruments, and for sterilization of food,sewage, and surgical equipment. Because of the danger posed by the radiological properties of cesium-137, efforts tofind substitutes in its applications continued."
"Applications for rubidium and its compounds include biomedical research, electronics, specialty glass, andpyrotechnics. Specialty glasses are the leading market for rubidium; rubidium carbonate is used to reduce electricalconductivity, which improves stability and durability in fiber-optic telecommunications networks. Biomedicalapplications include rubidium salts used in antishock agents and the treatment of epilepsy and thyroid disorder;rubidium-82, a radioactive isotope used as a blood-flow tracer in positron emission tomographic imaging; andrubidium chloride, used as an antidepressant. Rubidium atoms are used in academic research, including thedevelopment of quantum-mechanics-based computing devices, a future application with potential for relatively highconsumption of rubidium. Quantum computing research uses ultracold rubidium atoms in a variety of applications.Quantum computers, which have the ability to perform more complex computational tasks than traditional computersby calculating in two quantum states simultaneously, were expected to be in prototype phase within 10 years.
Rubidium’s photoemissive properties make it useful for electrical-signal generators in motion-sensor devices, nightvision devices, photoelectric cells (solar panels), and photomultiplier tubes. Rubidium is used as an atomicresonance-frequency-reference oscillator for telecommunications network synchronization, playing a vital role inglobal positioning systems. Rubidium-rich feldspars are used in ceramic applications for spark plugs and electricalinsulators because of their high dielectric constant. Rubidium hydroxide is used in fireworks to oxidize mixtures ofother elements and produce violet hues. The U.S. military frequency standard, the United States Naval Observatory(USNO) timescale, is based on 48 weighted atomic clocks, including 4 USNO rubidium fountain clocks."
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