Geology Reference
In-Depth Information
gadolinium-silicon-germanium alloys (Gd
5
Ge
2
Si
2
) show a high magnetocaloric ef-
fect and have important potential in new e
cient clean cooling and refrigeration
technologies. Magnesium alloys with gadolinium show strength as well as ductility
meaning they deform under tensile stress, something which becomes quite useful in
metalworking. In general, gadolinium alloys are temperature tolerant and resistant
to corrosion. In medical applications gadolinium enhances the contrast of magnetic
resonance imaging, (MRI) and X-Rays. Despite the fact that superconductivity and
ferromagnetism are antagonistic phenomena, it has been found that gadolinium-
arsenide oxides of the type GdOFFeAs, a similar compound to samarium-doped
arsenides, show superconductivity at 36 K. This opens up promise in high tempe-
rature superconductors. Gadolinium is expected to have a 2% share of world REE
supply by 2015. For more details see Sec. B.2.50.
B.2.23 Gallium
Gallium is a rare element which had little in the way of use until its properties
as a semiconductor were discovered. Analog integrated circuits are the largest
application for gallium with optoelectronic devices (mostly laser diodes and light-
emitting diodes) providing the second largest end use.
The highest concentrations (0.1-1%) are found in the rare mineral germanite
(Cu
26
Fe
4
Ge
4
S
32
); concentrations in sphalerite (ZnS), bauxite or coal are a hun-
dredfold less. It was formerly recovered from flue dusts emitted during sulphide
roasting or coal burning (up to 1.5% Ga) but is now obtained as a byproduct of the
Al industry.
B.2.24 Germanium
Germanium minerals are extremely rare but the element is widely distributed in
trace amounts among silicates, igneous as well as sedimentary and metamorphic
rocks. Recovery is normally achieved from the flue dusts of smelters processing Zn
ores.
It is a semiconductor and was used for transistors, diodes and rectifiers until it
was replaced by pure silicon in the early 1970's. It is now used in fiber optics com-
munication networks, infrared night vision systems and polymerisation catalysts.
B.2.25 Gold
Gold is widely but sparsely distributed both in its native state and in tellurides.
It is is predominately used in the manufacture of jewellery. However, because of
its superior electrical conductivity and resistance to corrosion, it has also emerged in
the late 20th century as an essential industrial metal in computers, communication
equipment, spacecraft, etc. Historically, gold has been considered as a symbol of
personal and national wealth. It has been used for coinage and has always been a
secure medium of payment.
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