Biotite is a common mica mineral known for its dark, plate-like appearance and perfect basal cleavage, which allows it to be peeled into thin, flexible sheets. It is a major constituent of igneous rocks like granite and is frequently found in metamorphic environments like gneiss and schist.
Is this biotite?
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 biotite with a known reference. Biotite sits at Mohs 2.5-3, 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: splendent to submetallic, vitreous, pearly on cleavage.
- 4Match the color rangeCompare against the expected color range: black, dark brown, dark green, greenish brown, blackish brown, yellow, white.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: lamellar, micaceous, pseudo hexagonal.
Often confused with
Biotite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads splendent to submetallic, vitreous, pearly on cleavage on Biotite and vitreous to pearly or silky on Muscovite.
How to tell apart: Luster reads splendent to submetallic, vitreous, pearly on cleavage on Biotite and pearly to submetallic on cleavage on Phlogopite.
How to tell apart: Luster reads splendent to submetallic, vitreous, pearly on cleavage on Biotite and pearly on Chlorite.
Often found alongside biotite
Minerals reported to co-occur with biotite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- K(Mg,Fe)₃(AlSi₃O₁₀)(OH,F)₂
- Mohs hardness
- 2.5-3
- Density
- 2.7-3.3 g/cm³
- Streak
- White
- Luster
- Splendent to submetallic, vitreous, pearly on cleavage
- Transparency
- Semitransparent
- Crystal system
- Monoclinic
- Crystal habit
- Lamellar, micaceous, pseudo hexagonal
- Cleavage
- {001}, perfect
- Rarity
- Common
- Uses
- Industrial, Collector
- Host rock
- An important rock-forming mineral under a wide range of conditions
Where rockhounds find biotite
30 mapped spotsClassic worldwide localities
- Franklin and Sterling Hill, Ogdensburg, Sussex Co., New Jersey, USA
- Monroe, Orange Co., and Russell, St. Lawrence Co., New York, USA
- Easton, Northampton Co., Pennsylvania, USA
- Pala and Rincon districts, San Diego Co., California, USA
- Vesuvius and Monte Somma, Campania, and in the Pfitschtal and on Mt. Monzoni, Val di Fassa, Trentino-Alto Adige, Italy
- Brevik and Arendal, Norway
U.S. states with biotite
Each link opens a state-specific list of mapped rockhounding spots that produce biotite.
Field-hunting tip
Where it forms: An important rock-forming mineral under a wide range of conditions. If you start seeing quartz, potassic feldspar, plagioclase in float, you are in the right ground. Typical habit: lamellar, micaceous, pseudo hexagonal. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Utah, Maine, Wisconsin. Start trip planning there.
More biotite photos
Common questions
How do you identify biotite?+
Where is biotite found?+
Can I find biotite in the United States?+
What rocks look like biotite?+
What minerals are found with biotite?+
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What is biotite used for?+
Find biotite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
