Geology Reference
In-Depth Information
B.2.26 Hafnium
Hafnium ores are rare although two are known: hafnon and alvite. However, Hf is
mostly found in quantities of about 2% within the Zr content found in zirconium
ores such as zircon ZrSiO
4
and baddeleyite ZrO
2
.
Hafnium is a transition metal similar to zirconium. It resists corrosion and has
a high melting point. Its major end uses are in nuclear control rods because of its
excellent ability to absorb neutrons. It also finds use in nickel-based superalloys,
nozzles for plasma arc metal cutting and high-temperature ceramics.
B.2.27 Holmium
Holmium has the strongest magnetic moment of any element, even if alloyed in small
amounts it greatly increases the strength of other magnets. Yet its rarity leaves few
opportunities for increasing its commercial application. It is a powerful neutron
absorber and finds application in nuclear control rods. Holmium doped yttrium-
garnets are used in solid state microwave lasers (masers) in ultra-short pulses for
medical surgery and dental rot removal, since their infrared emission is absorbed
rapidly by water in tissues and penetrates less deeply. Holmium doped yttrium
garnets are also used in optical fibre communications. Additionally, holmium oxide
is used as a yellow or red colourant in glass, and provides a dichroic yellow/peach
colour to cubic zirconia which is used in jewellery. Due to its sharp absorption
peaks in the range 200-900 nm, it is also used as a calibration standard for optical
spectrophotometers. For more details see Sec. B.2.50.
B.2.28 Indium
Indium is a rare metal which tends to associate with the similarly sized Zn in its
sulphide minerals and hence it is predominantely produced from residues generated
during zinc and lead sulphide ore processing, mainly from sphalerite ZnS. The
indium metal indite Fe
2+
In
2
S
4
has been found in Siberia.
Indium is used in low-melting fusible alloys, solders and electronics. Large-scale
application for indium was also employed in the protective coating for bearings
and other metal surfaces in high-performance aircraft engines. Nowadays, its main
application is in the manufacture of indium-tin-oxide thin films for Liquid Crystal
Displays (LCD) and touch screens.
B.2.29 Iodine
Iodine is the most electropositive halogen and is considerably less abundant than
the lighter halogens. It can be found naturally in air, water and soil. But the most
important sources are the oceans. It occurs, albeit rarely, as the iodide mineral.
Commercial deposits are usually iodates such as lautarite Ca(IO
3
)
2
and dietzeite
Ca
2
(IO
3
)
2
(CrO
4
).
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