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, so Montmorillonite leaves white, Nontronite leaves yellow.

Compare →

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, 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.