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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:
Sulphides - the sulphide anion is S2-
aguilarite
laitakarite
nevskite
Selenides - the selenide anion is Se2-
antimonselite
athabascaite
auroselenide
berzelianite
bukovite
cadmoselite
clausthalite
crookesite
dzharkenite
eskebornite
eucairite
ferroselite
gachingite
geffroyite
kalungaite
kawazulite
kitkaite
klockmannite
krut'aite
kullerudite
mäkinenite
naumannite
okruginite
palladseite
paraguanajuatite
penroseite
sabatierite
sederholmite
svetlanaite
tiemannite
trüstedtite
tyrrellite
umangite
zaykovite
Sulphosalts - sulphide minerals with the general formula
AmBnXp, 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
Selenates - the selenate anion is (Se6+O4)2-
franksousaite
olsacherite
schmiederite
Selenites - the selenite anion is (Se4+O3)2-
ahlfeldite
alfredopetrovite
bernardevansite
burnsite
chalcomenite
chloromenite
clinochalcomenite
cobaltomenite
demesmaekerite
derriksite
favreauite
guangyuanite
guilleminite
haynesite
ilinskite
mandarinoite
marthozite
molybdomenite
parageorgbokiite
petermegawite
sofiite
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