Caesium, symbolized as Cs with atomic number 55, is a soft, silvery-golden element belonging to the alkali metal group. It is one of only a few elemental metals that exist in a liquid state at or near room temperature, melting at just \(28.5^\circ\text{C}\). This element holds the distinction of having the lowest ionization energy of all stable elements, making it the most electropositive and chemically reactive metal. Its unique physical and chemical characteristics make it commercially valuable in specialized applications, such as high-precision atomic clocks and in dense drilling fluids used in the petroleum industry.
Caesium’s General Presence in the Earth’s Crust
Caesium is a trace element, with an average concentration in the Earth’s crust estimated to be three to seven parts per million. It is widely dispersed and rarely forms concentrated deposits. This distribution is due to the large radius of the caesium ion. The ion substitutes for the chemically similar potassium ion (\(\text{K}^+\)) in common rock-forming minerals, such as feldspars and micas. This substitution prevents the element from accumulating in high concentrations, meaning the majority of crustal caesium is not economically recoverable.
The Primary Commercial Source: Pollucite
The primary commercial source for caesium is pollucite, a rare zeolite mineral. Pollucite is a hydrated caesium aluminum silicate, found exclusively in highly fractionated Lithium-Caesium-Tantalum (LCT) type pegmatites. The mineral forms during the final stages of magmatic crystallization in these systems. For commercial viability, the ore requires a minimum concentration of around 19\% caesium by weight. Pollucite often appears as massive, colorless to white masses alongside other rare-metal minerals like spodumene and petalite.
Major Global Deposits and Mining Locations
Historically, the global supply of naturally occurring caesium has been dominated by a few large pollucite deposits. The Tanco Mine in Manitoba, Canada, was the most significant historical producer, holding substantial world reserves for decades. Located at Bernic Lake, this mine provided raw material for most caesium compounds, particularly for oil and gas drilling fluids.
Another historically important source is the Bikita Pegmatite in Zimbabwe. More recently, the Sinclair Mine in Western Australia also commercially extracted pollucite. However, these older mines are now reported to be nearing exhaustion or have ceased primary pollucite production.
The current landscape is shifting with the discovery of the Shaakichiuwaanaan Property in Quebec, Canada, which includes the Rigel and Vega zones. This site hosts the world’s largest reported pollucite-hosted caesium pegmatite resource. Exploration continues in regions like China and Kazakhstan, as well as at smaller deposits in Australia and Namibia, to secure future supplies.
Where Radioactive Caesium Isotopes are Found
Naturally occurring caesium is the stable isotope \(\text{Cs}-133\). Radioactive Caesium-137 (\(\text{Cs}-137\)) is synthetic, generated through the nuclear fission of uranium-235 in reactors and weapons. It is a fission byproduct with a half-life of about 30 years.
The largest concentrations of \(\text{Cs}-137\) are found in controlled environments, such as high-level nuclear waste storage facilities and repositories. Environmentally, the isotope is dispersed in soil, water, and vegetation in zones affected by major nuclear accidents. Chernobyl and Fukushima Daiichi are the most well-known examples of widespread environmental contamination.
Smaller amounts of \(\text{Cs}-137\) are contained in sealed sources used for industrial and medical applications, including radiotherapy equipment. These sources are stored securely in hospitals, laboratories, and industrial settings. Due to its volatility and water solubility, any release can lead to its dispersal over wide areas as radioactive fallout.