Powellite

minerals

ferrimolybdite

scheelite

molybdenite

Images
Formula: Ca(MoO4)
Anhydrous molybdate, scheelite group
Forms a series with scheelite
Specific gravity: 4.26
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.
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).

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 the South Hecla and Isle Royale mines, Houghton county, Michigan, USA, powellite is associated with native copper and epidote (Dana).

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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