Selenium

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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: 220^oC (ChC)
Boiling point: 685^oC (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:

Sulphides - the sulphide anion is S^2-
aguilarite
laitakarite
nevskite

Sulphosalts - sulphide minerals with the general formula A_mB_nX_p, where A represents a metal , B usually represents a semi-metal and X is sulphur or rarely selenium and/or tellurium
chaméanite
eldragónite
hakite
hakite-(Cd)
hakite-(Fe)
hakite-(Hg)
hakite-(Zn)
wittite

Sulpharsenites
arsenoústalečite

Selenides - the selenide anion is Se^2-

Selenates - the selenate anion is (Se⁶⁺O₄)^2-
franksousaite
olsacherite
schmiederite

Selenites - the selenite anion is (Se⁴⁺O₃)^2-

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