16
Gallium [Ga, 31]
16.1 INTRODUCTION
Gallium (Ga), a metal of the group 13 in the periodic table of elements, reveals
chalcophillic properties and resembles the geochemistry of Al. Its content on the
Earth’s crust is within the range 1–25 mg/kg. Rocks contain Ga from 1 to 25 mg/kg;
its lowest amounts are in ultramafic and calcareous rocks. In argillaceous sediments,
its distribution is associated with clay minerals. Its concentration in both coal and fly
ash may be up to about 45 mg/kg.
Gallium occurs mainly at +3 oxidation state, but also at +1 and +2. Its
concentrations in ores of other metals are very small. It forms only few minerals.
Those, containing about 30% of Ga are gallite, CuGaS 2 ; sohngeite, Ga(OH) 3 ; and
tsungeite, GaO(OH). However, it may be associated with several minerals, such as
feldspars, amphibolites, and micas. The electro-negativities and low ionic potential
suggest that Ga may be amphoteric, and may form different complexes with some
anions (e.g., Ga 2 O, Ga 2 S, Ga[OH]) easily.
World primary Ga production capacity in 2010 was estimated to be 184 t; scrap
refinery capacity, 177  t; and recycling capacity, 141  t (USGS 2011). Most gallium
is produced as a by-product of Al production from bauxite, and the remainder is
produced from zinc-processing residues. Its concentration in bauxite varies, and
the highest (0.08%) is reported to be in the bauxites from tropic regions. It is also
obtained at sphalerite (ZnS) and other sulfide metals explorations.
The melting temperature of Gallium is very low, at about 30°C, and therefore it
has been primarily used as an agent to make low-melting alloys. The alloy galinstan,
containing 68.5% of Ga, has a melting point at –19°C, and is used instead of Hg for
some thermometers. Nowadays, almost all forms of Ga are used in the field of microelectronics, due to its semiconducting properties. GaAs, GaN, and InGaN compounds are used in semiconductors, solar cells, photodetectors, and light-emitting
diodes.
Gallium and ferric salts behave similarly in biological systems, and therefore it
is used to mimic iron ions in medical applications. Productions of Ga-containing
pharmaceuticals and radiopharmaceuticals have been developed.
16.2 SOILS
Average Ga content of soils is estimated at 15.2  mg/kg, and its abundance varies
from <3 to 70 mg/kg (Table 16.1). Its concentrations in soils of various countries
are reported as follows (in mg/kg): United States, San Joachim Valley, 16–35; Japan,
18–23; Russia, 6–48; and Sweden, 3.4–16 (Kabata-Pendias 2011).
Gallium, present in soils mainly as Ga(OH) 3 , is relatively mobile. However, it
is mobilized during weathering processes, and by complexing with organic matter
119