Fibrolite is the fibrous, dense variety of the mineral sillimanite, known for its distinct hair-like or acicular crystal habit. It typically forms within high-grade metamorphic rocks like schist and gneiss, often appearing as tangled, straw-like aggregates that are tough and durable.
Is this fibrolite?
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 fibrolite with a known reference. Fibrolite sits at Mohs 6.5-7.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Fibrolite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Fibrolite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, colorless, gray, pale blue, brown.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: fibrous, acicular, radiating masses.
Often confused with
Fibrolite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside fibrolite
Minerals reported to co-occur with fibrolite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Al₂SiO₅
- Mohs hardness
- 6.5-7.5
- Density
- 3.23-3.27 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Fibrous, Acicular, Radiating Masses
- Cleavage
- Perfect in One Direction
- Rarity
- Common
- Uses
- Collector, Industrial, Lapidary
- Host rock
- Metamorphic Rocks
- Typical price
- $10-50 per specimen
Where rockhounds find fibrolite
2 mapped spotsClassic worldwide localities
- France
- USA
- India
- Sri Lanka
- Myanmar
Field-hunting tip
Look in metamorphic rocks country — that is the host setting where fibrolite typically forms. If you start seeing garnet, quartz, mica in float, you are in the right ground. Field specimens usually show a fibrous, acicular, radiating masses habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Idaho, North Carolina — start trip planning there.
Common questions
How do you identify fibrolite?+
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Can I find fibrolite in the United States?+
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Find fibrolite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
