Beidellite is a member of the smectite clay group known for its high swelling capacity when hydrated. It typically appears as massive, waxy, or earthy clay accumulations found in altered volcanic tuff or hydrothermal vein environments. Collectors usually identify it by its clay-like texture and its tendency to adhere to the tongue when dry.
Is this beidellite?
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 beidellite with a known reference. Beidellite 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: waxy to vitreous.
- 4Match the color rangeCompare against the expected color range: white, gray, reddish white, brownish white, pale yellow.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: earthy.
Often confused with
Beidellite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads waxy to vitreous on Beidellite and dull, earthy on Montmorillonite.
How to tell apart: Luster reads waxy to vitreous on Beidellite and pearly to dull earthy on Kaolinite.
Often found alongside beidellite
Minerals reported to co-occur with beidellite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Na,Ca)₀.₅Al₄(Si,Al)₈O₂₀(OH)₄·nH₂O
- Mohs hardness
- 1-2
- Density
- 2-2.3 g/cm³
- Streak
- White
- Luster
- Waxy to vitreous
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Earthy
- Cleavage
- Perfect on {001}
- Rarity
- Common
- Uses
- Industrial, Scientific, Collector
- Host rock
- A constituent of bentonitic clays; an alteration product in hydrothermal mineral deposits, especially porphyry Cu-Mo systems; in soils derived from mafic rocks
Where rockhounds find beidellite
1 mapped spotsClassic worldwide localities
- Beidell, Saguache Co., and at Wagon Wheel Gap, Mineral Co., Colorado, USA
- Black Jack mine, Carson district, Owyhee Co., Idaho, USA
- Morenci, Greenlee Co., Ajo, Pima Co., in the San Manuel mine, Pinal Co., and in the Globe-Miami district, Gila Co., Arizona, USA
- Boron, Kern Co., California, USA
- Princess mine, Namiquipa, Chihuahua, Mexico
- Sibert, Rhône, France
Field-hunting tip
Where it forms: A constituent of bentonitic clays; an alteration product in hydrothermal mineral deposits, especially porphyry Cu-Mo systems; in soils derived from mafic rocks. If you start seeing plagioclase, quartz, orthoclase in float, you are in the right ground. Typical habit: earthy. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Utah. Start trip planning there.
Common questions
How do you identify beidellite?+
Where is beidellite found?+
Can I find beidellite in the United States?+
What rocks look like beidellite?+
What minerals are found with beidellite?+
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What is beidellite used for?+
Find beidellite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
