Hydrobiotite is an interstratified mineral consisting of alternating layers of biotite and vermiculite, typically formed by the hydrothermal alteration of biotite. It is characterized by its soft, flexible, and micaceous nature, often appearing as weathered-looking platy masses.
Is this hydrobiotite?
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 hydrobiotite with a known reference. Hydrobiotite 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: dull.
- 4Match the color rangeCompare against the expected color range: golden yellow, black.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: platy masses, micaceous foliated aggregates.
Often confused with
Hydrobiotite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads dull on Hydrobiotite and splendent to submetallic, vitreous, pearly on cleavage on Biotite.
How to tell apart: Hydrobiotite is noticeably harder (Mohs 2.5-3 vs. ~1.5); streak differs, so Hydrobiotite leaves white, Vermiculite leaves greenish white.
How to tell apart: Luster reads dull on Hydrobiotite and pearly to submetallic on cleavage on Phlogopite.
Often found alongside hydrobiotite
Minerals reported to co-occur with hydrobiotite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- K(Mg,Fe²⁺)₃(Al,Fe³⁺,Si)₄O₁₀(OH)₂·nH₂O
- Mohs hardness
- 2.5-3
- Density
- 2.49-2.64 g/cm³
- Colors
- Streak
- White
- Luster
- Dull
- Transparency
- Semitransparent
- Crystal system
- Monoclinic
- Crystal habit
- Platy masses, micaceous foliated aggregates
- Cleavage
- {001}, perfect
- Rarity
- Common
- Uses
- Collector, Scientific research
- Host rock
- As an alteration product of other micas
Where rockhounds find hydrobiotite
Classic worldwide localities
- Rainy Creek complex, near Libby, Lincoln Co., Montana, and from the Enoree area, Spartanburg Co., South Carolina, USA
- Perhaps at Křemže, Czech Republic
- Phalaborwa, Transvaal, South Africa
Field-hunting tip
Where it forms: As an alteration product of other micas. If you start seeing vermiculite, biotite, apatite in float, you are in the right ground. Typical habit: platy masses, micaceous foliated aggregates. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify hydrobiotite?+
Where is hydrobiotite found?+
What rocks look like hydrobiotite?+
What minerals are found with hydrobiotite?+
What kind of rock does hydrobiotite form in?+
What is hydrobiotite used for?+
Find hydrobiotite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
