Ilsemannite Locality: Mogul Tunnel Occurrence, Boulder County Tungsten District, Boulder County , Colorado , USA ( Locality at mindat.org ) Size: 6.0 x 3.8 x 2.7 cm. A very rich sample of fine blue Ilsemannite on matrix from the Mogul Tunnel in Boulder County, Colorado. There is some question as to whether or not this Molybdenum oxide is a valid species, but I cannot find anything in the current literature to say whether or not it is valid or discredited. Nonetheless, specimens of this material are not seen on the market in any quantity, and this is certainly one of the finest pieces that I know of from Boulder County. Ex. Richard Kosnar Collection. — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Ilsemannite
An uncommon amorphous mixed-valence molybdenum oxide — a grandfathered species known since 1871.
Intro adapted from Wikipedia (CC BY-SA).
Ilsemannite is an IMA-recognized mineral species in the oxides & hydroxide class, with the chemical formula Mo₃O₈·nH₂O.
Key properties: Pale blue streak.
IMA symbol Ils, IMA status: questionable, named in 1871.
Forms secondarily through oxidation of molybdenum-bearing minerals.
It is found associated with Jordisite, molybdenite, molybdite, wulfenite, melanterite, halotrichite, gypsum, oxidized uranium minerals.
The mineral was named to honor Johann Christoph Ilsemann (1727–1822), Mining Commissioner at Clausthal, Germany, for his studies on molybdenite.