Selenium

minerals

tellurium

ferroselite

zippeite

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Formula: Se
Native element, selenium group
Specific gravity: 4.8
Hardness: 2
Streak: Red
Colour: Grey to greyish black, reddish grey, red
Melting point: 220oC (ChC)
Boiling point: 685oC (ChC)
Abundance in the Earth’s crust: 50 parts per billion by weight, 10 parts per billion by moles (ChC)
Solubility: Insoluble in hydrochloric acid, mildly soluble in nitric acid (ChC)
Reaction with air: Vigorous, with the emission of heat, forming SeO2 (ChC)
Common impurities: S
Environments:

Hydrothermal environments
Sublimates

Selenium occasionally occurs free in nature, but more often as selenides of iron, lead, silver or copper (ChC). It occurs as a low temperature mineral due to sublimation of fumarolic vapours, and from the oxidation of selenium-bearing organic compounds in sandstone-hosted uranium and uranium-vanadium deposits (Webmin, Dana, HOM). It also forms from burning coal and pyritic ores (HOM).
Selenium is often associated with tellurium (Dana), and also with pyrite, ferroselite, zippeite, metatyuyamunite, metarossite, montroseite and corvusite (HOM).
Many selenium compounds, such as selenates and selenites, are highly toxic although elemental selenium is not (ChC).
Selenium can have oxidation states -2, +2, +4 or +6. For the selenides it is -2 (Wiki).

Localities

At the Mount Deverell variscite deposit, Milgun Station, Western Australia, rare, very small grains of selenium occur in alunite that has pseudomorphed pyrite in variscite. The variscite deposits are hosted by marine sedimentary rocks (AJM 20.2.).

At Colquechaca, Chile, selenium occurs with selenide minerals (Dana).

At Kladno, Czech Republic, selenium occurs on burning heaps of pyritic sediments (Dana).

At Glen Lyon, Luzerne county, Pennsylvania, USA, selenium occurs in old burning coal-waste heaps, as a surface fumarolic product (Dana).

Alteration

Selenium reacts with nitric acid to form selenious acid, with the evolution of NO2
Se + 4HNO3 → H2SeO3 + 4NO2 + H2O

Selenium-bearing minerals include:

Selenides
aguilarite
athabascaite
bellidoite
berzelianite
clausthalite
crookesite
eucairite
klockmannite
laitakarite
naumannite
penroseite
sederholmite
tiemannite
trüstedtite
umangite

Selenates
schmiederite

Selenites
burnsite
chalcomenite
ilinskite
sofiite

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