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.

Hardness

6-7

Mohs

Luster

Vitreous

Streak

White

Transparency

Translucent

mineraluncommon

Mullite

Hardness: 6-7
Luster: Vitreous
Streak: White
Transparency: Translucent

Is this mullite?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch mullite with a known reference. Mullite sits at Mohs 6-7 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Mullite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Mullite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: white, gray, colorless, pink.
5
Look at form & habit
Crystal 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
Colors: White Gray Colorless Pink
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 — that is 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

How do you identify mullite?+

Mohs hardness is 6-7. It typically shows a vitreous luster. The streak is white. Common colors include white, gray, colorless, pink.

Where is mullite found?+

Notable localities include Mull, Scotland; Andernach, Germany; Hatzfeld, Germany; Krakow, Poland.

How much is mullite worth?+

Typical asking prices fall in the range of $20-150 for thumbnail to small cabinet specimens. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like mullite?+

Mullite is most often confused with Sillimanite, Kyanite, Andalusite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with mullite?+

Mullite commonly co-occurs with Sillimanite, Corundum, Sanidine, Tridymite, Glass. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does mullite form in?+

Mullite typically forms in thermally metamorphosed argillaceous rocks, fused xenoliths. Working float back to the host body is the standard way to chase a fresh occurrence.

What is mullite used for?+

Mullite is used in industrial, refractory material, collector.

Find mullite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.