39
Rubidium [Rb, 37]
39.1 INTRODUCTION
Rubidium (Rb) is a silvery-white soft metal of group 1 in the periodic table of elements,
with properties similar to alkali metals. The Earth’s crust contains Rb within the range
of 90–110 mg/kg. Its higher contents are in acidic igneous rocks, <100–200 mg/kg, and
in sedimentary argillaceous rocks, <120–200 mg/kg. The mean Rb content in coal is
25 mg/kg, but it may be concentrated up to 140 mg/kg.
The common oxidation state of Rb is +1, but may also be from +2 to +6. It is
highly reactive and rapidly oxidized in air, and forms monoxide compounds: Rb 2 O,
Rb 6 O, Rb 9 O 2 , and RbO 2 , in excess of oxygen in media. It also easily forms salts with
halides (e.g., RbBr, RbF, RbI). There are no minerals in which Rb is the predominant metal. It is associated with K minerals and may be concentrated in pegmatites.
Lepidolite (mineral of mica group) is considered the principal ore mineral of Rb and
contains up to 3.5% Rb oxides.
World mining of Rb has been roughly estimated to be about 1 t/yr (USGS 2001).
It is obtained as a by-product from P, Li, and Cs processing, and ferrocyanide products. The most important use of Rb is in various chemical and electronic applications.
Its use in superconductors is increasing. Rb-rich feldspars are used in ceramics. Rb
salts are used in biomedical treatments of some patients.
39.2 SOILS
Rubidium contents in soils are largely inherited from the parent materials, and therefore, its highest contents, 100–120  mg/kg, are in soils derived from granites and
gneisses. Also soils of heavy texture contain its higher amounts (Table 39.1). The
lowest Rb contents are in sandy and organic soils. The significant, positive correlation between Rb contents and clay fraction in soils clearly indicates its sorption by
fine fractions. Especially micaceous clays reveal a great capacity for its sorption. The
range of Rb contents in soils of various countries are given as follows (in mg/kg):
Bulgaria, 63–420; China, 15–140; Japan, 63–100; Poland, mean, 66; Russia, mean,
96; and the United States, 15–140 (Kabata-Pendias 2011).
Sewage sludge may contain Rb up to about 100 mg/kg, whereas P fertilizers have
it at an average value of 5 mg/kg and manure at about 0.006 mg/kg. Groundwaters
contain Rb from 0.05 to 2.5 μg/L.
39.3 WATERS
Rubidium concentrations in seawaters are fairly stable and range from 100 to 200 μg/L
(Table 39.1). In river waters, its amount is much lower, within the range of 0.2–6.5 μg/L.
The global riverine flux of Rb is estimated to be 61 kt/yr (Gaillardet et al. 2003).
267