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Formula: Ni(SeO3).2H2O
Hydrated selenite of nickel,
forms a series with cobaltomenite
Crystal System: Monoclinic
Specific gravity: 3.37 measured, 3.51 calculated
Hardness: 2 to 2½
Streak: Pale green, white
Colour: Apple-green, brown, pink, yellowish; becomes more pink with increasing
cobalt content
Common impurities: Co
Environments
Ahlfeldite is a rare alteration product of nickel-bearing
selenides and sulphides
(HOM).
Localities
At El Dragón mine, Porco Municipality, Antonio Quijarro Province, Potosí, Bolivia,
ahlfeldite is associated with chalcomenite,
olsacherite,
lepidocrocite and
goethite
(HOM).
At the type locality, the Virgen de Surumi mine, Pakajake Canyon, Colquechaca, Colquechaca Municipality,
Chayanta Province, Potosí, Bolivia, the deposit is a vein enclosed in Devonian (419.2 to 358.9 million years
ago) sandstone. The
primary vein minerals are
penrosite, pyrite,
clausthalite,
naumannite, tiemannite,
baryte, siderite,
hematite and calcite. The
secondary minerals include
cerussite, anglesite,
wulfenite, limonite and
the rare selenium minerals,
chalcomenite,
molybdomenite, ahlfeldite and
native selenium.
Ahlfeldite, an alteration product of penrosite, is found
in crystalline crusts up to 1.5 mm thick covering areas of only a few square millimeters, or filling small
solution cavities. Rarely it is present in well formed individual crystals ranging in length from 0.25 mm to
1 mm
(AM 54.448-456).
Associated minerals include penroseite,
chalcomenite,
olsacherite, cerussite
and anglesite
(HOM).
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