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. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: white, colorless, violet, yellow, light pink.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: crystalline, prismatic.
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
- 3.11-3.26 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Crystalline, prismatic
- Cleavage
- Distinct on {010}
- Rarity
- Uncommon
- Uses
- Industrial, Refractory material, Collector
- Host rock
- In fused argillaceous inclusions in Tertiary eruptive rocks (Isle of Mull, Scotland); as microscopic inclusions in sillimanite in pelitic inclusions in a tonalite (Val Sissone, Italy)
Where rockhounds find mullite
Classic worldwide localities
- Nuns Pass, Carsaig, and a number of other places on the Isle of Mull, also at Sithean Sluaigh, Strachur, Argyllshire, Scotland
- Tievebulliagh, Co. Antrim, Ireland
- Bergell massif, at Val Sissone, Lombardy, Italy
- Bellerberg, Nickenicher Sattel, and other volcanoes, Eifel district, Germany
- Bushveld complex, at Maandagshoek, Transvaal, and in the Roberts Victor diamond mine, near Kimberley, Cape Province, South Africa
- Buguchan Mountain and near Arkhara Station, Amur region, Russia
Field-hunting tip
Where it forms: In fused argillaceous inclusions in Tertiary eruptive rocks (Isle of Mull, Scotland); as microscopic inclusions in sillimanite in pelitic inclusions in a tonalite (Val Sissone, Italy). If you start seeing corundum, sillimanite, kyanite in float, you are in the right ground. Typical habit: crystalline, prismatic. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify mullite?+
Where is mullite found?+
What rocks look like mullite?+
What minerals are found with mullite?+
What kind of rock does mullite form in?+
What is mullite used for?+
Find mullite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
