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Formula: Fe3+2(Mo6+O4)3.7H2O
Hydrated molybdate
Crystal System: Orthorhombic
Specific gravity: 2.99 measured, 3.085 calculated
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).
Environments
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
(Mindat, HOM).
Common associates include chalcopyrite,
limonite, molybdenite and
pyrite
(Mindat, HOM).
Localities
The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia,
are skarn deposits at the contact between
granodiorite and volcanic rocks.
ferrimolybdite is a secondary oxide that has been detected
in yellow powdery crusts in the walls of the adit at the Two Mile deposit, and in nearby
actinolite-magnetite rocks as
yellow fibres
(AJM 22.1.41).
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).
At the Childs-Adwinkle Mine, Copper Creek, Bunker Hill District, Pinal county, Arizona, USA, ferrimolybdite is found as an
alteration product of molybdenite
(R&M 96.2.154).
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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