Thallium is said to be observed among sublimates of the Momotombo volcano in Nicaragua. The mineral would be very ephemeral, as metallic thallium is prone to fast oxidation to Tl2O (Mindat).
It is not recognised as a mineral species by the IMA.
Specific gravity: 11.85 at 20oC
Hardness: 1.2 on the mohs scale for metallic elements and alloys
Colour: Silvery grey
Solubility: In the presence of water, the poisonous thallium hydroxide (TlOH) is formed. Thallium dissolves slowly in hydrochloric acid and dilute sulphuric acid and dissolves rapidly in nitric acid (ChC).
Abundance in the Earth’s crust: 850 parts per billion by mass, 80 parts per billion by moles (ChC).
Abundance in the Solar System: 1 part per billion by mass, 10 parts per trillion by moles (ChC).
Melting point: 304oC (ChC).
Boiling point: 1473oC (ChC).
Thallium occurs in the oxidation states 0, 1+, 2+ and 3+; it usually exists in the monovalent state, Tl+, in its compounds.
Thallium and its compounds are highly toxic.
Thallium sulphide is used in photocells because its electrical conductivity increases on exposure to infrared light.
Thallium oxide is used to make glass that has a high index of refraction (ChC).
The main minerals containing thallium are crookesite, hutchinsonite and lorándite, and thallium also occurs in manganese nodules on the ocean floor. Commercially, thallium is recovered as a by-product of sulphuric acid production, as it is also present in pyrite. It can also be obtained from the smelting of lead and zinc ores (ChC).
Thallium-bearing minerals include:
Platinum Group Minerals
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