Montmorillonite is a soft clay mineral formed by the weathering of volcanic ash. It is widely known for its significant swelling properties when exposed to water, making it a primary component of bentonite clay.

Hardness

1-2

Mohs

Luster

Dull

Streak

White

Transparency

Opaque

mineralcommonSmectite Group family

Montmorillonite

Hardness: 1-2
Luster: Dull
Streak: White
Transparency: Opaque

Is this montmorillonite?

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 montmorillonite with a known reference. Montmorillonite sits at Mohs 1-2 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Montmorillonite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Montmorillonite typically shows a dull luster.
4
Match the color range
Compare against the expected color range: white, gray, yellow, pink, blue.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: massive.

Often confused with

Montmorillonite vs. its common look-alikes — and how to tell them apart in the field.

How to tell apart: Luster reads dull on Montmorillonite and pearly on Illite.

How to tell apart: Streak differs — Montmorillonite leaves white, Nontronite leaves yellow.

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Often found alongside montmorillonite

Minerals reported to co-occur with montmorillonite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: (Na,Ca)₀.₃₃(Al,Mg)₂Si₄O₁₀(OH)₂·nH₂O
Mohs hardness: 1-2
Density: 2.0-2.7 g/cm³
Colors: White Gray Yellow Pink Blue
Streak: White
Luster: Dull
Transparency: Opaque
Crystal system: Monoclinic
Crystal habit: Massive
Cleavage: Perfect
Rarity: Common
Uses: Industrial, Scientific, Cosmetic, Drilling Mud
Host rock: Altered Volcanic Ash Beds
Typical price: $5-20 per specimen

Where rockhounds find montmorillonite

Classic worldwide localities

France
USA
Italy
Greece
Japan

Field-hunting tip

Look in altered volcanic ash beds country — that is the host setting where montmorillonite typically forms. If you start seeing calcite, gypsum, quartz in float, you are in the right ground. Field specimens usually show a massive habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify montmorillonite?+

Mohs hardness is 1-2. It typically shows a dull luster. The streak is white. Common colors include white, gray, yellow, pink.

Where is montmorillonite found?+

Notable localities include France; USA; Italy; Greece; Japan.

How much is montmorillonite worth?+

Typical asking prices fall in the range of $5-20 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like montmorillonite?+

Montmorillonite is most often confused with Kaolinite, Illite, Nontronite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with montmorillonite?+

Montmorillonite commonly co-occurs with Calcite, Gypsum, Quartz, Cristobalite, Zeolites. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does montmorillonite form in?+

Montmorillonite typically forms in altered volcanic ash beds. Working float back to the host body is the standard way to chase a fresh occurrence.

What is montmorillonite used for?+

Montmorillonite is used in industrial, scientific, cosmetic, drilling mud.

Find montmorillonite on the map

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