Geology Reference
In-Depth Information
B.2.66 Thallium
Thallium is a soft and malleable heavy metal and although reasonably abundant in
the crust, it mostly exists in association with potassium minerals such as sylvite and
pollucite. It is not however generally considered to be commercially recoverable from
these forms. Very rare minerals of thallium, such as hutchingsonite PbTlAs 5 S 9 ,
occur in Nature as sulphide or selenide complexes with antimony, arsenic, copper,
lead and silver but these are of no commercial importance either. Instead, thallium
is commercially recovered as a byproduct from the flue dust and residues generated
during the roasting and smelting of Zn and Pb sulphide ores.
It is used in a wide variety of applications. Some of them include: as a semicon-
ductor material for selenium rectifiers, in gamma radiation detection equipment, in
infrared radiation detection and transmission equipment, in crystalline filters for
light diffraction and in medical diagnostic tests to detect heart disease.
B.2.67 Thorium
Thorium is a silver-grey heavy metallic element of the actinide series. It is very
abundant in the Earth's crust (three times more abundant than uranium). Thorium
occurs naturally in the minerals thorite, uranothorite, thorianite and is a major
component of monazite, where it is usually commercially extracted as a byproduct.
It is also present in significant amounts as a diodochic element in the minerals
zircon, titanite, gadolinite and blomstrandite.
Unfortunately, despite its abundance, worldwide demand for this element is
relatively small. Some of its applications are as an alloying element in magnesium,
as a coating for wolfram wire used in electronic equipment, to control the grain size
of plutonium used for electric lamps, as a catalyst for the manufacture of refractory
materials and for the metallurgical industries or as a fertile material for producing
nuclear fuel.
B.2.68 Thulium
Thulium and ytterbium, like holmium and erbium, may be used as dopants for high
power and wavelength-tunable solid-state lasers. They are used in crystals such as
yttrium-aluminium-garnet (Y b : Y AG), (Tm : Y AG), strontium-yttrium-silicate
(Y b : SY S), typically operating at around 1020-1050 nm. They are very e cient
and extreme ultra-short pulses can be achieved. Both thulium and ytterbium are
also used as a radiation source in portable X-ray devices once the metal has been
bombarded in a nuclear reactor (Paschotta, 2008). Thulium is so rare that its po-
tential applications are quite limited, especially as they highly exceed any optimistic
forecast of world supply. For more details see Sec. B.2.50.
 
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