Ahlfeldite

ahlfeldite

penroseite

chalcomenite

olsacherite

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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

Hydrothermal 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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