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Niobium [Nb, 41]
30.1 INTRODUCTION
Niobium (Nb) is a soft, gray transition metal of group 4 in the periodic table of elements, with physical and chemical properties similar to those of Ta, the metal of the
same group. It was also formerly called columbium (Cb). Its abundance in the Earth’s
crust varies from 20 to 26 mg/kg. Its highest contents are in acidic magmatic rocks
(15–60 mg/kg) and argillaceous sediments (15–20 mg/kg). The lowest Nb contents
are in limestones (0.05–0.1 mg/kg). In coal, its amounts vary within the range of
5–10 mg/kg, but may also reach the level of 70 mg/kg (Finkelman 1999).
Niobium occurs mainly at +5 oxidation state, and may also have a lower oxidation.
In the nature, Nb and Ta occur together, due to their great similarities in atomic radii
and several properties. Both elements exhibit an affinity to associate with Fe, Ti, and
Zr. It is scattered in several minerals of complex formula, for example, columbite/
niobite/tantalite, (Fe,Mn)(Nb,Ta) 2 O 6 , and pyrochlore, (Na,Ca) 2 (Nb,Ta) 2 O 6 (OH,F).
Niobium is also included in some Ti minerals and Zr compounds. The free Nb element is not found in nature. Columbite–tantalite minerals occur in pegmatite intrusions. Its deposits are also associated with carbonate–silicate igneous rocks.
World mine Nb production (rounded), excluding the United States, in 2010 was
63,000 t, of which 58,000 t was produced in Brazil and 4,400  t was produced in
Canada (USGS 2011). Niobium is used mainly for steel alloys. These alloys are resistant to strength and temperature impacts, and thus are widely used in pipeline construction and in aerial transportation industries. It is used as a precious metal (e.g., in
commemorative coins), often with Ag and Au. Niobium is also added to alloys used
for surgical implants and in the stomatology.
30.2 SOILS
The worldwide Nb content of soils is established at 12.5 mg/kg. Its highest concentrations are in heavy loamy soils (Table 30.1). Its amounts in soils depend mainly on
its contents in mother rocks. Thus, in soils derived from different rocks in the United
Kingdom, the concentrations of Nb vary from 31 to 300 mg/kg (Ure et al. vide KabataPendias 2011). The range of Nb contents of soils in various countries are given as
follows (in mg/kg, for sandy and loamy soils, respectively): Alaska, <4 and 44; China,
9.3 and 37.6; Japan, 6.3 and 14; and Sweden, 5 and 17.
The behavior of Nb during weathering highly depends on host minerals; therefore,
it may be released from some minerals (e.g., amphibolite) or may remain within resistant minerals (e.g., sphene/titanite, zircon). Thus, the accumulation of Nb in certain
residual sediments has often been reported. Its elevated concentrations, >15 mg/kg, also
occur across areas of aeolian sediments (e.g., loess of central Europe) and in residual
soils developed on some carbonate rocks.
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