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Formula: Ca(MoO4)
Anhydrous molybdate,
scheelite group, forms a series with
scheelite
Crystal System: Tetragonal
Specific gravity: 4.26 measured, 4.255 calculated
Hardness: 3½ to 4
Streak: Light yellow
Colour: Yellow, brown, greenish yellow, grey, blue or black, colourless; black material is
deep blue in transmitted light.
Luminescence: Creamy white or yellow to golden yellow under short wave UV
Solubility: Decomposed by hydrochloric and nitric acid
Common impurities: W
Environments:
Pegmatites
Metamorphic environments
Hydrothermal environments typical
Basaltic cavities
Powellite is usually a secondary mineral, often
formed by the alteration of molybdenite. Also sometimes
formed in low-temperature hydrothermal environments, such as with
zeolites in vugs in
basalt or in some low-temperature
copper or mercury mines. More rarely formed in
skarn and
contact metamorphic environments with
scheelite
(Dana).
It is also occasionally found in granite
pegmatites
(Mindat, HOM).
Powellite is associated with molybdenite,
ferrimolybdite,
stilbite, laumontite
and apophyllite
(HOM).
Localities
At Mount Moliagul, Moliagul, Central Goldfields Shire, Victoria, Australia, powellite is often found mixed with
or forming between the leaves of molybdenite crystals. It sometimes
appears to replace small rosettes of molybdenite in pegmatite cavities,
suggesting that it
is an alteration product of molybdenite rather than a
primary mineral
(AJM 21.1.44).
Near Nasik, Maharashtra, India, powellite occurs in vugs, grown partly on
quartz and partly on
laumontite,
embedded in stilbite and
apophyllite, and at Jalgaon it is associated with
apophyllite
(Dana, MinRec 34.1.55-56).
From North Cannaver Island, Lough Corrib, Galway County, Connacht, Ireland, powellite was found on a specimen
from the Russell collection. The specimen consists of a cinnamon coloured
garnet with dark greyish green fibrous
amphibole, and quartz, and a small
eyeof bright foliated molybdenite surrounded by a film ring of brownish
yellow molybdite. Both in contact with the
molybdenite and as an outer ring there is a pearly-white mineral,
undetermined by Russell, but now identified as powellite by EDS. It appears to have formed as a result of
weathering of a molybdenite crystal, which is still largely intact
(JRS 23.116).
From the Murvey Opencut, Roundstone, County Galway, Ireland, a specimen of massive poorly crystalline
molybdenite from the Russell collection has a broad pale yellow
fluorescent rim extending up to 1 cm into the surrounding rock. The only reasonable interpretation of the results of
EDS analysis is that the rim consists of extremely thin films of powellite, that are optically invisible
(JRS 23.116).
From Ballycummisk Mine, Audley Mines, Ballydehob, Mizen Peninsula, Cork County, Munster, Ireland, a museum specimen
of fine-grained molybdenite is coated by a thin dirty yellow crust. The
crust originally was thought to be ferrimolybdite, but EDS shows that
it contains powellite, clay minerals and possible
ferrimolybdite
(JRS 23.116).
At Bou Azzer, Morocco, powellite occurs is several locations. At Ightem it is found in
dolomite boulders or resting on
dolomite,
conichalcite,
malachite or
olivenite.
At Tandrost-West it is associated with annabergite and at Aghbar it occurs in
quartz cavities with
conichalcite
(MinRec 38.5.388).
At Carrock Mine, Mungrisdale, Eden, Cumbria, England, UK, powellite, identified by its characteristic
fluorescence, is reported as fine, almost invisible, grains in fractures and joints associated with
apatite, carbonates, scheelite
and flaky sericite mica. most of the fluorescent material occurs as a thin
coating in mica-rich joints and fractures around
molybdenite, and it is occasionally also present on the surface of
molybdenite grains. powellite was confirmed by EDS in museum
specimens. On one specimen powellite occurs as a white to cream pearly crust in a cavity created by the
apparent removal of a molybdenite crystal. It seems likely that
powellite formed at the interface between a molybdenite crystal
and the quartz matrix, and that the crystal subsequently fell out leaving a
cavity
(JRS 23.113-114).
At the Castle Hill quarry, Mountsorrel, Charnwood, Leicestershire, England, UK, masses of powellite were noted
in the granodiorite during a routine examination of specimens from
this locality. Additional confirmation was forthcoming in the finding of a patch of powellite in
incomplete pseudomorphism of a small mass of
molybdenite. The
pseudomorphism suggests a significant change in the chemical environment
during crystallisation
(JRS 23.115-116 ).
At the Upper Middleton Farm Quarry, Inverurie, Aberdeenshire, Scotland, UK,
molybdenite occurs in cavities with grey-brown
scheelite and bright yellow sheaves of
ferrimolybdite in a quartz-rich
gneiss. Examination in shortwave UV revealed minute sub-millimetre
regions with the characteristic pale yellow fluorescence of powellite; the identification was confirmed by EDS.
The occurrence is unusual in that powellite occurs in small cavities in the
quartz groundmass near to the
molybdenite, rather than as rims, crusts or films surrounding the crystals
(JRS 23.116).
At the Ballachulish Igneous Intrusion in Gleann a’Chaolais, Argyll and Bute, Scotland, UK,
molybdenite is found sparingly throughout the intrusion. A
quartz vein, exposed along a stream in the remote Coire Dearg, contains small
flakes of molybdenite with pyrite
and chalcopyrite altering to
malachite and limonite.
Occasionally, the molybdenite is entirely or partially removed from the
vein leaving distinctive cavities. These are rarely coated in a fine dusting of powellite. The powellite
crusts are almost invisible to the naked eye but are clearly revealed by their pale yellow fluorescence under
shortwave UV. The identification was confirmed by EDS
(JRS 23.116-117).
At Coire Buidhe, Glen Creran, Argyll and Bute, Scotland, UK, powellite occurs in a small
skarn-type occurrence. It was first reported as squat bipyramids in
cavities; subsequent analysis showed that these crystals were scheelite,
but powellite was later identified as yellowish buff poorly crystalline crusts when a very thin pale yellow
coating on molybdenite was confirmed as powellite by EDS; the
powellite appears to be an alteration product of molybdenite. In
contrast, very pale blue-green polycrystalline scheelite is a
primary rock-forming component of the
skarn
(JRS 23.115).
At Traprain Law, Haddington, East Lothian, Scotland, UK, A single subhedral powellite crystal, about 1 cm
across, was reported with analcime,
apophyllite and calcite in a
cavity in phonolite. The powellite appears to have formed in
a late-stage hydrothermal fluid containing high concentrations of
molybdenum which accumulated in gas cavities in the
phonolite as the intrusion cooled
(JRS 23.114-115).
At the Benallt Mine, Rhiw, Aberdaron, Gwynedd, Wales, UK, pale yellow bipyramidal crystals of powellite up to
about 0.4 mm across were identified in a small cavity in a medium-grained basic igneous rock in 2006. The occurrence
appears to be related to latestage and localised molybdenum-rich
hydrothermal fluids accumulating in vesicles in igneous rock
(JRS 23.115).
The Nelly James Mine, Miller Canyon, Miller Peak, Cochise County, Arizona, USA, is a former small surface
lead, copper,
zinc, gold and
silver mine located at an altitude of 7250 feet. Mineralisation is a vein
deposit
Mindat).
The mine is now famous for fluorescent minerals collected from the dumps, including
calcite (fluoresces red),
hydrozincite (sky blue),
powellite (creamy-yellow),
smithsonite (crimson red),
sphalerite (yellow-orange) and
willemite (green).
Powellite is apparently the least abundant fluorescent mineral at the mine. It is usually white or pale grey in
daylight. Under shortwave and medium range UV light the response is a bright creamy yellow. Under longwave UV light the
response is also a creamy yellow but with only medium brightness
(R&M 97.1.48-56).
At the South Hecla and Isle Royale mines, Houghton county, Michigan, USA, powellite is associated
with native copper and
epidote
(Dana).
At the Minesota Mine, Rockland, Ontonagon County, Michigan, USA, powellite has been found with
quartz and calcite
(Mindat photo).
Powellite from the Minesota Mine -
Image
At the Tonopah Divide mine, Nye county, Nevada, USA, powellite occurs in vugs in altered
rhyolite
(Dana).
In the Nuratinsky range, Uzbekistan, powellite occurs in metasomatic deposits with
scheelite and
molybdenite
(Dana).
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