Images
Formula: Mo3O8.nH2O (?)
Simple oxide, molybdenum-bearing mineral, questionable status
Crystal System: Amorphous
Hardness: 5½ to 6
Streak: Pale blue
Colour: Black, blue-black, blue; turns blue on exposure
Solubility: Forms a deep molybdenum-blue colloidal solution in water
Environments
Sedimentary environments
Hydrothermal environments
Ilsemannite is a secondary mineral formed by the
oxidation of molybdenum-bearing minerals. Associated minerals include
jordisite, molybdenite,
molybdite, wulfenite,
melanterite, halotrichite,
gypsum and oxidised uranium minerals
(HOM). It is an alteration product of molybdenite
(Webmin).
Localities
At the type locality, Bad Bleiberg, Villach-Land District, Carinthia, Austria, ilsemannite is associated with
wulfenite, baryte and
gypsum
(Dana).
At the Boulder Creek mine, Gibson, Klamath Mountains, Shasta county, California, USA, ilsemannite occurs as an
alteration product of molybdenite
(Dana).
At Kiggins, Oak Grove Fork, Clackamas county, Oregon, USA, andesite
forms the country rock in the region of the mine. It is highly fractured, and veins filling the fractures and related
fissures contain cinnabar in commercial amounts.
Calcite, pyrite and
stilbite are the principal gangue
minerals. Ilsemannite is found as veinlets and irregular lenses or pods closely associated with the
cinnabar.
Stilbite and cinnabar are
characteristic of low temperature, epithermal deposits. The ilsemannite when first found has a dark blue-black
colour and often stains the calcite light blue. On exposure to sunlight the
ilsemannite turns blue-green and fades
(AM 36.609-614).
Near Ouray, Uteland Knoll District, Uintah county, Utah, USA, ilsemannite is associated with
halotrichite and molybdite
in sandstone
(Dana).
Back to Minerals