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. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 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
Where rockhounds find fibrolite
2 mapped spotsClassic worldwide localities
- France
- USA
- India
- Sri Lanka
- Myanmar
Field-hunting tip
Where it forms: metamorphic rocks. If you start seeing garnet, quartz, mica in float, you are in the right ground. Typical habit: fibrous, acicular, radiating masses. 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.
More fibrolite photos
Common questions
How do you identify fibrolite?+
Where is fibrolite found?+
Can I find fibrolite in the United States?+
What rocks look like fibrolite?+
What minerals are found with fibrolite?+
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What is fibrolite used for?+
Find fibrolite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
