Sidwillite is a rare molybdenum oxide mineral that typically forms as a secondary oxidation product. It is easily identified by its distinctive yellow, platy habit and pearly luster, often occurring in oxidized molybdenite deposits.
Is this sidwillite?
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 sidwillite with a known reference. Sidwillite sits at Mohs 2 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Sidwillite leaves a yellow streak.
- 3Read the lusterHold the specimen under a strong light. Sidwillite typically shows a pearly luster.
- 4Match the color rangeCompare against the expected color range: yellow, brownish-yellow.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: platy or micaceous crystals.
Often confused with
Sidwillite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside sidwillite
Minerals reported to co-occur with sidwillite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- MoO₃·2H₂O
- Mohs hardness
- 2
- Density
- 4.21 g/cm³
- Colors
- Streak
- Yellow
- Luster
- Pearly
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Platy or Micaceous Crystals
- Cleavage
- Perfect On {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Oxidized Zones of Molybdenum-bearing Ore Deposits
- Typical price
- $50-300 per specimen
Where rockhounds find sidwillite
Classic worldwide localities
- Glade Creek, Wyoming, USA
- Kyzyl-Tashtyg deposit, Russia
Field-hunting tip
Look in oxidized zones of molybdenum-bearing ore deposits country — that is the host setting where sidwillite typically forms. If you start seeing molybdenite, jarosite, goethite in float, you are in the right ground. Field specimens usually show a platy or micaceous crystals habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find sidwillite on the map
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