Ferrimolybdite is a secondary oxidation product that typically forms as bright yellow, powdery, or fibrous crusts on weathering molybdenite. It is easily identified by its distinctive canary-yellow color and its common occurrence as a soil or vein coating in ore deposits.
Is this ferrimolybdite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch ferrimolybdite with a known reference. Ferrimolybdite sits at Mohs 1-2, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: pale yellow.
- 3Read the lusterHold the specimen under a strong light. Luster: adamantine to silky, earthy.
- 4Match the color rangeCompare against the expected color range: yellow, canary yellow, sulfur yellow.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: earthy, fibrous, scaly.
Often confused with
Ferrimolybdite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Molybdite is the harder of the two (Mohs 3-4 vs. 1-2); streak differs, so Ferrimolybdite leaves pale yellow, Molybdite leaves white.
How to tell apart: Luster reads adamantine to silky, earthy on Ferrimolybdite and subadamantine to vitreous, resinous on fractures on Jarosite.
How to tell apart: Luster reads adamantine to silky, earthy on Ferrimolybdite and vitreous, pearly on {001} on Autunite.
Often found alongside ferrimolybdite
Minerals reported to co-occur with ferrimolybdite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Fe₂(MoO₄)₃·7H₂O
- Mohs hardness
- 1-2
- Density
- 2.99 g/cm³
- Colors
- Streak
- Pale yellow
- Luster
- Adamantine to silky, earthy
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Earthy, fibrous, scaly
- Cleavage
- Distinct on {001}
- Rarity
- Common
- Uses
- Collector, Indicator mineral
- Host rock
- An uncommon secondary mineral in the oxidized portions of hydrothermal vein and porphyry-type molybdenum-bearing deposits
Where rockhounds find ferrimolybdite
Classic worldwide localities
- Climax mine, Lake Co., Colorado, USA
- Sulfur Gulch, Questa, Taos Co., New Mexico, USA
- Mineral Park, Mohave Co., Arizona, USA
- Little Cottonwood district, Salt Lake Co., Utah, USA
- Alekseevskii mine, Khakassia district, Siberia, Russia
- Vielsalm, Belgium
Field-hunting tip
Where it forms: An uncommon secondary mineral in the oxidized portions of hydrothermal vein and porphyry-type molybdenum-bearing deposits. If you start seeing molybdenite, pyrite, chalcopyrite in float, you are in the right ground. Typical habit: earthy, fibrous, scaly. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify ferrimolybdite?+
Where is ferrimolybdite found?+
What rocks look like ferrimolybdite?+
What minerals are found with ferrimolybdite?+
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Find ferrimolybdite on the map
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