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 2.5, 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: resinous to adamantine.
- 4Match the color rangeCompare against the expected color range: yellow, resin yellow, light green.
- 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.
How to tell apart: Streak differs, so Sidwillite leaves pale yellow, Molybdite leaves white; luster reads resinous to adamantine on Sidwillite and adamantine on Molybdite.
How to tell apart: Luster reads resinous to adamantine on Sidwillite and adamantine to silky, earthy on Ferrimolybdite.
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
- 2.5
- Density
- 3.12 g/cm³
- Colors
- Streak
- Pale yellow
- Luster
- Resinous to adamantine
- Transparency
- Semitransparent
- Crystal system
- Monoclinic
- Crystal habit
- Platy or micaceous crystals
- Cleavage
- Perfect on {010}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- An oxidation product of jordisite in a quartz vein
Where rockhounds find sidwillite
Classic worldwide localities
- Lake Como, Hinsdale Co., Colorado, USA (type locality)
Field-hunting tip
Where it forms: An oxidation product of jordisite in a quartz vein. If you start seeing jordisite, quartz in float, you are in the right ground. Typical habit: platy or micaceous crystals. 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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