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 4-5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Loudounite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Loudounite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, colorless.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: acicular or prismatic clusters.
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
- Ca₄Zr₂Si₁₀O₂₄·12H₂O
- Mohs hardness
- 4-5
- Density
- 2.47 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Acicular or Prismatic Clusters
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Diabase
- Typical price
- $50-300 per specimen
Where rockhounds find loudounite
Classic worldwide localities
- Fairfax Quarry, Virginia, USA
- Goose Creek Quarry, Virginia, USA
Field-hunting tip
Look in diabase country — that is the host setting where loudounite typically forms. If you start seeing prehnite, apophyllite, laumontite in float, you are in the right ground. Field specimens usually show a acicular or prismatic clusters habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find loudounite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
