Specific gravity: 2.99
Hardness: 1 to 2
Streak: Light yellow
Colour: Yellow, canary yellow, or sulphur yellow, greenish yellow; colourless to canary-yellow in transmitted light
Solubility: Readily soluble in hydrochloric acid and slowly soluble in ammonia with the separation of ferric hydroxide (Mindat).
Ferrimolybdite is a rare secondary mineral occurring in the oxidised
zones of hydrothermal vein and porphyry-type molybdenum-bearing deposits, or in
breccia pipe deposits containing molybdenum.
It is commonly formed from the alteration of molybdenite
Common associates include chalcopyrite, limonite, molybdenite and pyrite (Mindat, HOM).
Near Lowther, 90 miles west of Sydney, New South Wales, Australia, joint fissures in a porphyritic marginal phase of the Bathurst granite batholith are occupied by hydrothermal quartz veins containing small disseminated aggregates of intimately associated molybdenite, pyrite and chalcopyrite which have partly weathered to ferrimolybdite and goethite. In addition to films of dull yellow ferrimolybdite coating fracture surfaces in the quartz, the Lowther specimens show radiating groups of sheaf-like units of bright yellow colour and silky lustre. These units are partly embedded in goethite and project into the cells of goethite box-works or into cavities in the quartz formerly occupied by sulphides (AM 45.1111-1113).
At Mount Moliagul, Moliagul, Central Goldfields Shire, Victoria, Australia, ferrimolybdite occurs in cavities in quartz as tufts of fibres to 5 mm long, associated with goethite and pyrite (AJM 21.1.42).
The type locality is the Alekseevskii Mine, Lake Iktul', Karysh River Basin, Khakassia, Russia.
At the Sherlov mountain, Transbaikalia, Russia, ferrimolybdite occurs with molybdenite (FM 53392).
At Climax, Colorado, USA, ferrimolybdite is essentially microcrystalline and consists of minutely massive material, bundles of fine fibres, and nodules of radial fibres. It is found associated with limonite, goethite and jarosite in surface exposures and drill holes in the vicinity of the large caved area above the Climax mine. Presumably the weathering of pyrite and molybdenite has resulted in the formation of ferrimolybdite, and at the same time jarosite, goethite and limonite were produced (AM 48.14-32).
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