20
Indium [In, 49]
20.1 INTRODUCTION
Indium (In) is a very soft, malleable, and easily fusible metal of the group 13 in the
periodic table of elements and is chemically similar to Ga and Th, elements of the
same group. Its terrestrial abundance ranges between 0.11 and 0.25  mg/kg, and is
likely to be more concentrated in ultramafic igneous rocks (<0.2 mg/kg) than in sedimentary rocks (<0.07 mg/kg). Hard coal contains In within the range of 0.1–0.2 mg/kg),
and sewage sludge, up to about 0.15 mg/kg.
The main oxidation state of In is +3, and may be +1 and +2. It is likely to precipitate under conditions that form Fe and Mn hydroxides, and therefore it is associated
with Fe-bearing minerals. Due to its chalcophilic properties, In forms sulfide minerals such as indite, FeIn 2 S 4 , and roquésite, CuInS 2 . It is also associated with other sulfide minerals such as sphaleryt, ZnS; galena, PbS; and cahlcopyrite, CuFeS 2 . Indium
is very rarely found as grains of native metals, and its primary sources are Zn ores.
Some amounts of In may also be recovered from slags, during the production of
Pb and Sn.
In 2010, world mine production of In was 574 t, of which China produced 300;
Korea produced 80; and Japan produced 70 (USGS 2011). A large amount of In is
obtained from scraps recycling. Its current primary applications are to form transparent electrodes indium tin oxide and in touchscreens. It is applied in thin films to
form lubricated layers, for alloys of a low melting point, and to some Pb-free solders.
Alloys containing In are also used to replace Hg in alkaline batteries, and in absorbers for nuclear reactors.
Some tests have been made to use the radionuclide, 111 In, for the radioimmunoassay and as an antibody in cancer patients.
20.2 SOILS
Worldwide mean In contents in mineral soils are reported to range from 0.01 to
0.5 mg/kg, with the mean value of 0.06 mg/kg. Its higher concentrations, up to
2.6 mg/kg are found in organic soils (Table 20.1). During weathering, In oxidizes
to In 3+ , and behaves similarly to other cations, mainly to Fe 3+ , and easily precipitates with Fe hydroxides. Because In is associated with organic matter (OM), it
is likely to be concentrated in surface-soil horizons. Increased level of In in the
forest litter, as compared with its contents of leaves, also indicate its sorption by
OM (Rühling and Tyler 2004). Mobile In species, at acidic and neutral pH range,
are InCl 2+ and In(OH) 2+ . Above pH 9.5, the anion In OH ) 4
is likely to occur. Its
(
distribution in soils indicates the association with clay fraction. Microorganisms
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