Rubidium is the second most electropositive of the alkaline elements and can be a liquid at room temperature. Like other group 1 elements this metal ignites spontaneously in air and reacts violently in water, liberating and sometimes igniting hydrogen. Also like other alkali metals, it forms amalgams with mercury and it can form alloys with gold, caesium, sodium, and potassium. The element gives a yellowishviolet color to a flame.
Rubidium can be easily ionized, and because of this has been considered for use in ion engines[?] for space vehicles[?] (but caesium is more efficient for this purpose). Other potential or current uses:
Rubidium (Lrubidus, deepest red) was discovered in 1861 by Robert Bunsen and Gustav Kirchhoff in the minerallepidolite through the use of a spectroscope. However this element had minimal industrial use until the 1920s. Historically, the most important use for rubidium has been in research and development, primarily in chemical and electronic applications.
This element is considered to be the 16th most abundant element in the earth's crust. It occurs naturally in in the minerals leucite[?], pollucite[?], and zinnwaldite[?], which contains traces of up to 1% of its oxide. Lepidolite contains 1.5% rubidium and this is the commercial source of the element. Some potassium minerals and potassium chlorides also contain the element in commercially significant amounts. One notable source is also in the extensive deposits of pollucite[?] at Bernic Lake[?], Manitoba.
Rubidium metal can be produced by reducing rubidium chloride with calcium among other methods. Rubidium forms four oxides: Rb2O, Rb2O2, Rb2O3, Rb2O4. In 1997 the cost of this metal in small quantities was about US$ 25/gram.
There are 24 isotopes of rubidium known with naturally occurring rubidium being composed of just two isotopes; Rb-85 (72.2%) and the radioactive Rb-87 (27.8%). Normal mixes of rubidium are radioactive enough to expose photographic film in approximetely 30 to 60 days.
Rb-87 has a a half-life of 48.8 x 109 years. It readily substitutes for potassium in minerals, and is therefore fairly widespread. Rb has been used extensively in dating rocks[?]; Rb-87 decays to stable strontium-87 by emission of a negative beta particle. During fractional crystallization[?], Sr tends to become concentrated in plagioclase, leaving Rb in the liquid phase. Hence, the Rb/Sr ratio in residual magma may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation[?]. Highest ratios (10 or higher) occur in pegmatites. If the initial amount of Sr is known or can be extrapolated, the age can be determined by measurement of the Rb and Sr concentrations and the Sr-87/Sr-86 ratio. The dates indicate the true age of the minerals only if the rocks have not been subsequently altered. See Rubidium-Strontium dating for a more detailed discussion.
Rubidium reacts violently with water and can cause fires. To ensure both safety and purity, this element must be kept under a dry mineral oil, in a vacuum or in an inert atmosphere.
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