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.
Is this montmorillonite?
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 montmorillonite with a known reference. Montmorillonite sits at Mohs 1-2, 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: dull, earthy.
- 4Match the color rangeCompare against the expected color range: white, gray white, yellow, brownish yellow, greenish yellow.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: earthy.
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, earthy on Montmorillonite and pearly to dull earthy on Kaolinite.
How to tell apart: Luster reads dull, earthy on Montmorillonite and earthy on Illite.
How to tell apart: Luster reads dull, earthy on Montmorillonite and waxy, resinous, dull on Nontronite.
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
- Streak
- White
- Luster
- Dull, earthy
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Earthy
- Cleavage
- {001}, perfect
- Rarity
- Common
- Uses
- Industrial, Scientific, Cosmetic, Drilling mud
- Host rock
- An alteration product of volcanic tu® and ash, forming bentonite beds, and of pegmatite dikes and wall rocks bordering hydrothermal mineral deposits
Where rockhounds find montmorillonite
Classic worldwide localities
- Bayard and Santa Rita, Grant Co., New Mexico, USA
- Chambers, Apache Co., Arizona, USA
- Belle Fourche, Butte Co., South Dakota, USA
- John C. Lane tract, Upton, Weston Co., at Clay Spur, near Newcastle, Crook Co., Wyoming, USA
- Montmorillon, Vienne, France
Field-hunting tip
Where it forms: An alteration product of volcanic tu® and ash, forming bentonite beds, and of pegmatite dikes and wall rocks bordering hydrothermal mineral deposits. If you start seeing cristobalite, zeolites, biotite in float, you are in the right ground. Typical habit: earthy. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify montmorillonite?+
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What rocks look like montmorillonite?+
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Find montmorillonite on the map
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