Downeyite is an extremely rare selenium oxide mineral that forms as a sublimate in areas of coal seam fires. It is typically found as fragile, needle-like acicular crystals or thin crusts associated with burning coal waste environments.
Is this downeyite?
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 downeyite with a known reference. Downeyite 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. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: adamantine.
- 4Match the color rangeCompare against the expected color range: colorless.
- 5Look at form & habitCrystal system: tetragonal. Typical habit: acicular crystals, crusts.
Often confused with
Downeyite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside downeyite
Minerals reported to co-occur with downeyite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- SeO₂
- Mohs hardness
- 2
- Density
- 4.146 g/cm³
- Colors
- Streak
- White
- Luster
- Adamantine
- Transparency
- Transparent
- Crystal system
- Tetragonal
- Crystal habit
- Acicular crystals, crusts
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- As a rare sublimation product of gases escaping through vents in actively burning culm banks in anthracite coal deposits; forms at about 200°C near the vents
Where rockhounds find downeyite
Classic worldwide localities
- Coal waste piles at Glen Lyon, Luzerne Co., Williamstown, Dauphin Co., Forestville, Schuylkill Co., and Burnside, Northumberland Co., Pennsylvania, USA
Field-hunting tip
Where it forms: As a rare sublimation product of gases escaping through vents in actively burning culm banks in anthracite coal deposits; forms at about 200°C near the vents. If you start seeing selenium, rosickýite, mascagnite in float, you are in the right ground. Typical habit: acicular crystals, crusts. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify downeyite?+
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Find downeyite on the map
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