Loudounite is an extremely rare zirconium silicate mineral first discovered in the diabase quarries of Loudoun County, Virginia. It typically forms delicate, radiating clusters of colorless to white acicular needles inside cavities of metamorphosed basaltic rock.
Is this loudounite?
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 loudounite with a known reference. Loudounite sits at Mohs ~5, 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: vitreous.
- 4Match the color rangeCompare against the expected color range: white, colorless, green, light green.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: fibrous, radial, spherical.
Often confused with
Loudounite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside loudounite
Minerals reported to co-occur with loudounite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- NaCa₅Zr₄Si₁₆O₄₀(OH)₁₁·8H₂O
- Mohs hardness
- ~5
- Density
- 2.48 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Fibrous, radial, spherical
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A late-stage mineral in the alteration of diabase
Where rockhounds find loudounite
Classic worldwide localities
- Goose Creek quarry, Leesburg, Loudoun Co., and the Fairfax quarry, Centreville, Fairfax Co., Virginia, USA
Field-hunting tip
Where it forms: A late-stage mineral in the alteration of diabase. If you start seeing ancylite, epidote, prehnite in float, you are in the right ground. Typical habit: fibrous, radial, spherical. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify loudounite?+
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What rocks look like loudounite?+
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Find loudounite on the map
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