Mullite is a rare high-temperature mineral typically formed by the thermal decomposition of aluminosilicates in fused xenoliths. It usually presents as thin, needle-like acicular crystals or fibrous masses embedded in glass. It is most famously found in the Tertiary igneous rocks of the Isle of Mull, Scotland.
Is this mullite?
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 mullite with a known reference. Mullite sits at Mohs 6-7, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Mullite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Mullite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, gray, colorless, pink.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: acicular crystals, prismatic, fibrous, massive.
Often confused with
Mullite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside mullite
Minerals reported to co-occur with mullite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Al₄Si₂O₁₀
- Mohs hardness
- 6-7
- Density
- 3.1-3.2 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Acicular Crystals, Prismatic, Fibrous, Massive
- Cleavage
- Distinct On {110}
- Rarity
- Uncommon
- Uses
- Industrial, Refractory Material, Collector
- Host rock
- Thermally Metamorphosed Argillaceous Rocks, Fused Xenoliths
- Typical price
- $20-150 for thumbnail to small cabinet specimens
Where rockhounds find mullite
Classic worldwide localities
- Mull, Scotland
- Andernach, Germany
- Hatzfeld, Germany
- Krakow, Poland
Field-hunting tip
Look in thermally metamorphosed argillaceous rocks, fused xenoliths country, the host setting where mullite typically forms. If you start seeing sillimanite, corundum, sanidine in float, you are in the right ground. Field specimens usually show a acicular crystals, prismatic, fibrous, massive habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find mullite on the map
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