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Formula: MgV2O6.7H2O
Hydrated vanadate
Crystal System: Monoclinic
Specific gravity: 1.96 to 2.09 measured, 2.04 calculated
Hardness: 2½
Streak: White
Colour: Light golden brown
Luminescence: Not fluorescent under UV
Environments
Localities
At the type locality, the Firefly–Pigmay Mine, La Sal Creek Mining District, San Juan County, Utah, USA,
dickthomssenite occurs in a small area as coatings on a sandstone
wall, close to an area that was mined for vanadium and
uranium. Locally, the sandstone
contains organic material, including log fragments, some of which contain uranium and vanadium mineralisation. Mineral deposition is believed to have occurred
since the mining activities ceased, from circulating groundwater, under conditions similar to those that led to
pascoite. In addition to pascoite,
minor amounts of sherwoodite and
native selenium are present in close proximity. Other newly (in 2000) identified
minerals at the mine include bariandite,
devilline, rossite and
hewetitte. Previously identified minerals at the site included
carnotite, clausthalite,
coffinite, tyuyamunite,
uraninite, corvusite,
montroseite, roscoelite,
galena, pyrite and
tennantite.
Orebodies are mostly associated with carbonaceous material that probably provided a reducing environment for
vanadyl and uranyl ions carried in
solution. There is little evidence of any hydrothermal alteration in the mine. Both oxidised and partially oxidised
uranium–vanadium minerals occur. In
the upper workings, principally carnotite and
tyuyamunite are present. In the lower levels of the mine, groundwater
prevailed, and the unoxidised minerals were present, including uraninite,
coffinite, corvusite and
montroseite. Presently, the mine is closed.
Dickthomssenite occurs as translucent crystals ranging from 0.25 mm needle-like to 0.5 x 1.5 mm platy, prismatic
crystals with basal terminations. The crystals occur in fibroradial groups to a length of 5 mm, creating solid
coverage to 1.5 x 2.5 cm
(CM 39.6.1691–1700).
Dickthomssenite from the Firefly–Pigmay Mine -
Image
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