Vermiculite is a hydrated magnesium aluminum iron silicate mineral that forms through the alteration of phlogopite or biotite. It is well-known for its ability to expand significantly when heated, a property that makes it useful in gardening and industrial insulation, though collectors primarily seek it for its distinctive platy, micaceous crystal habits.
Is this vermiculite?
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 vermiculite with a known reference. Vermiculite sits at Mohs ~1.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: greenish white.
- 3Read the lusterHold the specimen under a strong light. Luster: dull to pearly, bronzy.
- 4Match the color rangeCompare against the expected color range: colorless, green, gray white, yellow brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: lamellar, scaly.
Often confused with
Vermiculite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Biotite is the harder of the two (Mohs 2.5-3 vs. ~1.5); streak differs, so Vermiculite leaves greenish white, Biotite leaves white; luster reads dull to pearly, bronzy on Vermiculite and splendent to submetallic, vitreous, pearly on cleavage on Biotite.
How to tell apart: Streak differs, so Vermiculite leaves greenish white, Phlogopite leaves white; luster reads dull to pearly, bronzy on Vermiculite and pearly to submetallic on cleavage on Phlogopite.
How to tell apart: Streak differs, so Vermiculite leaves greenish white, Chlorite leaves white; luster reads dull to pearly, bronzy on Vermiculite and pearly on Chlorite.
Often found alongside vermiculite
Minerals reported to co-occur with vermiculite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Mg,Fe,Al)₃(Al,Si)₄O₁₀(OH)₂·4H₂O
- Mohs hardness
- ~1.5
- Density
- 2.2-2.6 g/cm³
- Streak
- Greenish white
- Luster
- Dull to pearly, bronzy
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Lamellar, scaly
- Cleavage
- {001}, perfect, typically relic
- Rarity
- Common
- Uses
- Industrial, Horticultural, Insulation
- Host rock
- An alteration product of biotite or phlogopite, by weathering or hydrothermal action
Where rockhounds find vermiculite
4 mapped spotsClassic worldwide localities
- Milbury, near Worcester, Worcester Co., Massachusetts, USA
- Franklin, Macon Co., and at the Rutherford quarry, near Tuxedo, Henderson Co., North Carolina, USA
- Brinton's quarry, Westtown, Chester Co., and on Chestnut Hill, Easton, Northampton Co., Pennsylvania, USA
- Magnet Cove, Hot Spring Co., Arkansas, USA
- Ajmer, Rajasthan, India
- Phalaborwa, Transvaal, South Africa
Field-hunting tip
Where it forms: An alteration product of biotite or phlogopite, by weathering or hydrothermal action. If you start seeing corundum, apatite, serpentine in float, you are in the right ground. Typical habit: lamellar, scaly. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in North Carolina, Georgia, Montana. Start trip planning there.
Common questions
How do you identify vermiculite?+
Where is vermiculite found?+
Can I find vermiculite in the United States?+
What rocks look like vermiculite?+
What minerals are found with vermiculite?+
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Find vermiculite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
