Rubid­i­um and cesium belong to the group of rare alka­li met­als char­ac­ter­ized by high chem­i­cal reac­tiv­i­ty and dis­tinc­tive phys­i­cal prop­er­ties. Their con­tent in the Earth’s crust is low; how­ev­er, these ele­ments are present in many min­er­als, main­ly as iso­mor­phic impu­ri­ties sub­sti­tut­ing for potas­si­um and lithi­um. Both met­als are of strate­gic impor­tance due to their rar­i­ty, the com­plex­i­ty of their extrac­tion, and their unique applications—ranging from elec­tro-opti­cal indus­tries and pre­ci­sion mea­sur­ing instru­ments to defense sys­tems and advanced ener­gy tech­nolo­gies.

They are con­sid­ered togeth­er because they belong to the same chem­i­cal group, have sim­i­lar physic­o­chem­i­cal prop­er­ties, often form com­mon min­er­al asso­ci­a­tions, and are enriched under the same geo­log­i­cal con­di­tions. In nature, rubid­i­um and cesium are main­ly con­cen­trat­ed in rare-met­al peg­matites, apogran­ites, greisens, meta­so­matites, salt deposits, brines, and ther­mal waters.

Rubid­i­um and cesium ores is includ­ed in the list of min­er­als of nation­al impor­tance, approved by Res­o­lu­tion of the Cab­i­net of Min­is­ters of Ukraine No. 827 of Decem­ber 12, 1994, as rare met­al ores.

General information

Rubid­i­um (Latin: Rubid­i­um, Rb) is a chem­i­cal ele­ment of Group I of the peri­od­ic table with atom­ic num­ber 37 and atom­ic mass 85.47. It belongs to the alka­li met­als. It was dis­cov­ered in 1861 by R. Bun­sen and G. Kirch­hoff. The name derives from the Latin rubidus (“dark red”). In nature, it occurs as a mix­ture of two sta­ble iso­topes: ^85Rb (72.15%) and ^87Rb (27.85%). The lat­ter is radioac­tive, with a half-life of about 5×10¹⁰ years, which allows the ^87Rb/^87Sr ratio to be used for deter­min­ing the radio­met­ric age of rocks. In addi­tion, 19 arti­fi­cial­ly syn­the­sized iso­topes of rubid­i­um are known.

In its nat­ur­al state, rubid­i­um is a soft, sil­very-white alka­li met­al with a den­si­ty of 1.525 g/cm³ and a melt­ing point of 39.5 °C. Its oxi­da­tion state is +1. The met­al is extreme­ly reac­tive: it ignites eas­i­ly in air, reacts explo­sive­ly with water, and active­ly inter­acts with all inor­gan­ic acids. Its Clarke val­ue is 1.5×10⁻²%. Rubid­i­um belongs to dis­persed ele­ments and typ­i­cal­ly occurs as an iso­mor­phic impu­ri­ty in min­er­als of alka­li met­als, main­ly potas­si­um and lithi­um, such as pol­lu­cite, lep­i­do­lite, zin­nwaldite, ama­zonite, biotite, spo­dumene, and car­nal­lite. The high­est con­cen­tra­tions are found in low-tem­per­a­ture peg­matite veins, where Rb con­tent can reach 1–3%.

Cesium (Latin: Cae­sium, Cs) is also a Group I ele­ment with atom­ic num­ber 55 and atom­ic mass 132.9. It belongs to alka­li met­als as well. Its name comes from the Latin cae­sius (“sky blue”). In nature, it is rep­re­sent­ed by one sta­ble iso­tope, ^133Cs; numer­ous arti­fi­cial radioac­tive iso­topes are also known. Cesium is a low-melt­ing, soft met­al with a yel­low­ish-gold­en hue, a den­si­ty of 1.9 g/cm³, and a melt­ing point of 28.5 °C. Like rubid­i­um, it oxi­dizes instant­ly in air and reacts explo­sive­ly with water. Its Clarke val­ue is 3.7×10⁻³%. It is a rare ele­ment that forms its own mineral—pollucite (Cs[AlSi₂O₆])—and is also found in spo­dumene, lep­i­do­lite, zin­nwaldite, cesium-bear­ing astro­phyl­lite, kuplet­skite, cesium biotite, and oth­er min­er­als.

Applications and raw material requirements

Rubid­i­um and cesium are wide­ly used in high-tech indus­tries. They are applied in the elec­tro-opti­cal indus­try, includ­ing vac­u­um radio tubes, ther­mis­tors, lasers, masers, lumi­nes­cent screens and tubes, for man­u­fac­tur­ing pho­to­cell cath­odes, as well as in spe­cial ceram­ics, glass, and enam­els. Rubid­i­um salts are used in gas-dis­charge tubes and serve as cat­a­lysts in organ­ic syn­the­sis. The iso­tope ^133Cs is used as the basis for quan­tum fre­quen­cy stan­dards.

Cesium is also used in mag­ne­to­hy­dro­dy­nam­ic (MHD) gen­er­a­tors, plas­ma ampli­fiers for ultra-high fre­quen­cies, auto­mat­ic con­trol sys­tems, mis­sile guid­ance sys­tems, long-dis­tance com­mu­ni­ca­tion tech­nolo­gies, and mis­sile defense sys­tems.

Rubid­i­um salts are usu­al­ly obtained as a by-prod­uct dur­ing the pro­duc­tion of lithi­um, mag­ne­sium, and potas­si­um com­pounds. Metal­lic rubid­i­um is pro­duced by reduc­ing its salts with metal­lic cal­ci­um, fol­lowed by purifi­ca­tion through rec­ti­fi­ca­tion and vac­u­um dis­til­la­tion.

Economic overview

Glob­al reserves of rubid­i­um are esti­mat­ed at approx­i­mate­ly 1,150 thou­sand tons, while cesium reserves are about 100 thou­sand tons. Their main con­cen­tra­tion is asso­ci­at­ed with rare-met­al peg­matite deposits in the Unit­ed States, Cana­da, Zim­bab­we, Chi­na, Namib­ia, Brazil, and Argenti­na, as well as with rare-met­al brines of salt lakes in the Unit­ed States.

Cana­da is the largest pro­duc­er of pol­lu­cite con­cen­trate (about 45 t/year), where the Ber­nic Lake deposit is in oper­a­tion. Sig­nif­i­cant min­ing activ­i­ties are also car­ried out in Zim­bab­we (Biki­ta) and Namib­ia (Karibib). Despite hav­ing sub­stan­tial reserves, Rus­sia pri­mar­i­ly extracts cesium from import­ed con­cen­trate. Deposits are con­sid­ered large if reserves exceed 1,000 tons of Rb₂O and 5,000 tons of Cs₂O. How­ev­er, in com­plex deposits, even the first hun­dreds of tons may be of indus­tri­al inter­est.

Genetic and Geological-Industrial Types of Deposits

Rubid­i­um and cesium deposits are divid­ed into endoge­nous and exoge­nous types, which are usu­al­ly close­ly asso­ci­at­ed with lithi­um deposits. Endoge­nous types include:

• rare-met­al gran­ite peg­matites (Kings Moun­tain, Black Hills, Pala-Men — USA; Preis­sac-Lacorne, Ber­nic Lake — Cana­da; Goltsovskoye — Rus­sia; Lalin — Spain; Sencek­ourou — Mali; Fu-Zep — Chi­na; Muna­ki, Karibib — Namib­ia; Darang-Peak — Afghanistan; Biki­ta, Bun — Zim­bab­we);
• rare-met­al meta­so­matites;
• agpaitic nepheline syen­ites;
• rare-met­al vol­canogenic and vol­canogenic-sed­i­men­ta­ry for­ma­tions (Henou-Kamb Hills — USA).

Among exoge­nous types, ele­vat­ed con­cen­tra­tions of Rb and Cs are asso­ci­at­ed with:

• deposits of intra­con­ti­nen­tal salt seas (Wien­burg — Ger­many);
• large marine salt bays;
• mod­ern salt lakes (Sear­les, Great Salt Lake — USA);
• for­ma­tion and arte­sian waters (Pripy­at Depres­sion, Angara-Lena and Tun­gus­ka basins);
• min­er­al­ized waters of moun­tain­ous regions (Azata­van — Less­er Cau­ca­sus; Kara­madon, Karo­bi, Bak­san — Greater Cau­ca­sus; Bakhmir — Pamir);
• ther­mal waters of mod­ern vol­canic zones (Wairakei — New Zealand; Kunashir — Kam­chat­ka; Sal­sko — Italy; Tokanu — New Zealand; Ari­ma — Japan).

The most impor­tant ore types for endoge­nous deposits include spo­dumene ores, spo­dumene-lep­i­do­lite with pol­lu­cite, lep­i­do­lite with petal­ite, pure pol­lu­cite ores, holmquistite–cesium-biotite, lepidolite–microcline–albite, astro­phyl­lite fen­ites, cesium-bear­ing vol­canic glass­es, etc. For exoge­nous deposits, sig­nif­i­cant mate­ri­als include car­nal­lites, var­i­ous types of brines (sodi­um chlo­ride, car­bon­ate-chlo­ride sodi­um, sul­fate-chlo­ride sodi­um-mag­ne­sium), as well as chlo­ride, bicar­bon­ate-chlo­ride, and oth­er min­er­al­ized waters.

Deposits in Ukraine

On the Ukrain­ian Shield, rare-met­al peg­matites with pol­lu­cite are known only on the south­ern flank of the Zhov­torichenske deposit in the Dnipropetro­vsk region. They were iden­ti­fied dur­ing ura­ni­um ore explo­ration between 1960 and 1991. The peg­matite bod­ies are con­fined to meta­so­mat­ic diop­side quartzites of the Hdantsiv­ka suite.

Series of veins with a thick­ness of 0.1–0.5 m have been iden­ti­fied, exhibit­ing a zon­al struc­ture enriched in pol­lu­cite. The cen­tral zones con­sist of coarse-grained aggre­gates of micro­cline, spo­dumene, and pol­lu­cite; the endo­con­tacts are com­posed of fine-grained for­ma­tions with apatite and rubel­lite. Apophy­ses and branch­es (up to 5 cm thick and 1.5 m long) com­posed of albite-polu­cite aggre­gates with micro­cline and apatite are com­mon.

The peg­matites con­tain quartz, micro­cline, albite, grains of spo­dumene, pol­lu­cite, rubel­lite, apatite, columbite, arsenopy­rite, and mus­covite. Spo­dumene occurs as short pris­mat­ic crys­tals up to 4×1 cm; pol­lu­cite is fine-grained, white or bluish with a metal­lic lus­ter. Apatite and rubel­lite reach up to 1 cm, while columbite and arsenopy­rite are up to 3 mm.

At the Stanku­vatske deposit, Rb and Cs are asso­ci­at­ed with zones of bioti­ti­za­tion and phl­o­go­pi­ti­za­tion at the con­tacts of peg­matites with amphi­bo­lites and ultra­ba­sic rocks. The thick­ness of these zones ranges from a few cen­time­ters to 8 m. Con­tents: up to 1.80% Rb₂O and up to 0.50% Cs₂O. The main con­cen­tra­tor is phl­o­go­pite (on aver­age 0.76% Rb₂O and 0.83% Cs₂O).

At the Polokhivske deposit, indus­tri­al con­cen­tra­tions are record­ed in recrys­tal­lized gar­net-biotite and gar­net-biotite-cordierite gneiss­es: aver­age con­tents are 0.11% Rb₂O and 0.04% Cs₂O. The main car­ri­er of rare alka­lis is biotite.

Addi­tion­al­ly, ele­vat­ed con­cen­tra­tions have been iden­ti­fied:

• in zones of phl­o­go­pi­ti­za­tion at the con­tact of the Novo­moskovsk gran­ite mas­sif with ultra­ba­sic rocks;
• in small gran­ite intru­sions of the Volyn and Azov blocks;
• in hydrother­mal­ly altered gran­ites of the Perzhanske ore field (Zhy­to­myr region), up to 2000 g/t Rb₂O;
• in ground­wa­ter of Don­bas (Rb up to 1.25 mg/L, Cs up to 0.08 mg/L);
• in ground­wa­ter of the Kerch–Taman region (Rb up to 4 mg/L).

Rubid­i­um and cesium, as alka­li met­als, have sim­i­lar physic­o­chem­i­cal prop­er­ties, which deter­mines their com­bined study and extrac­tion. They are main­ly asso­ci­at­ed with rare-met­al peg­matite deposits and often occur as iso­mor­phic impu­ri­ties in lithi­um- and potas­si­um-bear­ing min­er­als. Both ele­ments are wide­ly used in high-tech industries—from elec­tro-opti­cal and laser tech­nolo­gies to com­mu­ni­ca­tion sys­tems, quan­tum fre­quen­cy stan­dards, and advanced mate­ri­als. Despite rel­a­tive­ly lim­it­ed reserves, their strate­gic impor­tance is increas­ing due to the devel­op­ment of high-tech indus­tries, stim­u­lat­ing the search for new sources and improve­ment of pro­cess­ing meth­ods for com­plex raw mate­ri­als.