Sillimanite is a high-temperature aluminum silicate often found in regional metamorphic rocks like schist and gneiss. It is best known for its fibrous, splintery habit, which earned it the synonym 'fibrolite', though it also occurs as slender, long prismatic crystals.
Is this sillimanite?
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 sillimanite with a known reference. Sillimanite 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 to subadamantine.
- 4Match the color rangeCompare against the expected color range: white, colorless, gray, brown, pale blue, yellowish, bluish, brownish greenish, gray, gray green.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: acicular, fibrous, prismatic.
Often confused with
Sillimanite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside sillimanite
Minerals reported to co-occur with sillimanite. 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
- 3.23-3.24 g/cm³
- Streak
- White
- Luster
- Vitreous to subadamantine
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Acicular, fibrous, prismatic
- Cleavage
- Perfect on {010}
- Rarity
- Common
- Uses
- Collector, Refractory materials, Gemstone
- Host rock
- In high-grade metamorphic schists, gneisses, and hornfels; more rarely in pegmatites
Where rockhounds find sillimanite
17 mapped spotsClassic worldwide localities
- Chester, near Saybrook, Middlesex Co., and near Norwich, New London Co., Connecticut, USA
- Chester, Delaware Co., Pennsylvania, USA
- Monroe, Orange Co., New York, USA
- Lisens Alp, Selraintal, Tirol, Austria
- Maršová (Marschendorf), Czech Republic
- Bodenmais, Bavaria, Germany
Field-hunting tip
Where it forms: In high-grade metamorphic schists, gneisses, and hornfels; more rarely in pegmatites. If you start seeing andalusite, kyanite, potassic feldspar in float, you are in the right ground. Typical habit: acicular, fibrous, prismatic. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Georgia, South Carolina, Alabama. Start trip planning there.
Common questions
How do you identify sillimanite?+
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Can I find sillimanite in the United States?+
What rocks look like sillimanite?+
What minerals are found with sillimanite?+
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Find sillimanite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
