Cesium is a relatively rare element, about 3 ppm in the Earth’s crust. It is the 45th most abundant element and the 36th among the metals. It is 3.3% as abundant as Rb, with which it is closely associated, chemically. Due to its large ionic radius, cesium is one of the “incompatible elements.” During magma cooling and crystallization, Cs is concentrated in the remaining liquid phase and crystallizes last. Therefore the largest deposits of cesium are found in pegmatite ore bodies formed by this enrichment process.

Discovery

In 1860 German chemist Robert Wilhelm Eberhard Bunsen (March 30, 1811 to August 16, 1899) and physicist Gustav Robert Kirchhoff (March 12, 1824 to October 17, 1887) discovered cesium in the mineral water from the spa town of Bad Du¨rkheim, Rhineland-Palatinate, Germany (Kirchhoff and Bunsen, 1861) . Due to the bright blue emission lines in the emission spectrum, they derived the element’s name from the Latin word caesius, meaning sky blue.

Major minerals

Cesium is found in only a limited number of minerals. Small amounts of cesium may be found in beryl (Be3Al2(SiO3)6) and avogadrite ((K, Cs)BF4), up to 15 wt.% Cs2O in the closely to beryl related mineral pezzottaite (Cs(Be2Li)Al2Si6O18), up to 8.4 wt.% Cs2O in the rare mineral londonite ((Cs,K)Al4Be4(B,Be)12O28). The only commercially significant ore for cesium is pollucite Cs(AlSi2O6), which is found in a few places around the world in zoned pegmatites, together with the more economically important lithium minerals, lepidolite, and petalite.

Fig.1  galkhaite

Only 27 minerals are known in nature to have structural Cs. Two sulfides, galkhaite ((Cs,Tl)(Hg,Cu,Zn)6(As,Sb)4S12) (Fig.1) and pautovite (CsFe2S3), contain Cs. In the oxide class three minerals are known, cesiokenopyrochlore (Nb2(O, OH)6Cs12x (xB0.20)), cesplumtantite ((Cs, Na)2(Pb, Sb31)3Ta8O24), and margaritasite((Cs, K, H3O)2(UO2)2(VO4)2?H2O). Two borates, londonite ((Cs,K,Rb)Al4Be4(B,Be)12O28) (Fig.2) and ramanite-(Cs) (Cs[B5O6(OH)4]?2H2O), have structural Cs.

Fig.2 londonite

Chemistry

Cesium is a soft, golden-yellow metal with a melting point of 28.5
C, which makes it a liquid at temperatures close to room temperature. It has 55 electrons arranged in an electronic configuration of [Xe] 6s1. It is a member of the alkali metals and exhibits physical and chemical properties similar to other members of the group especially of rubidium and potassium above it. The ease of abstracting the lone electron from the outer shell dominates most of the chemistry of the alkali metal group and as such their oxidation states in isolable compounds are always11. Following the periodic trend of chemical behavior, Cs are more reactive than lighter alkali metals and are regarded as the most reactive of all metals. It is so reactive that it reacts with solid water even at 2116℃. It explodes upon contact with liquid water.

Uses

Cesium is used as a getter in electron tubes. Other applications are in photoelectric cells; ion propulsion systems; heat transfer fluid in power generators; and atomic clocks. The radioactive Cs-37 has prospective applications in the sterilization of wheat, flour, and potatoes.

Hazards

Nonradioactive cesium compounds are only mildly toxic, and nonradioactive cesium is not a significant environmental hazard. The median lethal dose (LD50) for cesium chloride in mice is 2.3 g per kilogram, which is comparable to the LD50 values of potassium chloride and sodium chloride.

Cesium metal is one of the most reactive elements and is highly explosive in the presence of water. The hydrogen gas produced by the reaction is heated by the thermal energy released at the same time, causing ignition and an explosion.