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.

Hardness

1-2

Mohs

Luster

Pearly

Streak

White

Transparency

Translucent

mineralcommonMica Group family

Hydrobiotite

Hardness: 1-2
Luster: Pearly
Streak: White
Transparency: Translucent

No image

Is this hydrobiotite?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch hydrobiotite with a known reference. Hydrobiotite sits at Mohs 1-2 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Hydrobiotite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Hydrobiotite typically shows a pearly luster.
4
Match the color range
Compare against the expected color range: brown, yellowish-brown, greenish-brown.
5
Look at form & habit
Crystal 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.

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: 1-2
Density: 2.3-2.5 g/cm³
Colors: Brown Yellowish-brown Greenish-brown
Streak: White
Luster: Pearly
Transparency: Translucent
Crystal system: Monoclinic
Crystal habit: Platy Masses, Micaceous Foliated Aggregates
Cleavage: Perfect Basal
Rarity: Common
Uses: Collector, Scientific Research
Host rock: Ultramafic Rocks, Carbonatites
Typical price: $5-30 for specimens

Where rockhounds find hydrobiotite

Classic worldwide localities

Libby, Montana, USA
Palabora, South Africa
Kola Peninsula, Russia

Field-hunting tip

Look in ultramafic rocks, carbonatites country — that is the host setting where hydrobiotite typically forms. If you start seeing biotite, vermiculite, magnetite in float, you are in the right ground. Field specimens usually show a platy masses, micaceous foliated aggregates habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify hydrobiotite?+

Mohs hardness is 1-2. It typically shows a pearly luster. The streak is white. Common colors include brown, yellowish-brown, greenish-brown.

Where is hydrobiotite found?+

Notable localities include Libby, Montana, USA; Palabora, South Africa; Kola Peninsula, Russia.

How much is hydrobiotite worth?+

Typical asking prices fall in the range of $5-30 for specimens. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like hydrobiotite?+

Hydrobiotite is most often confused with Biotite, Vermiculite, Phlogopite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with hydrobiotite?+

Hydrobiotite commonly co-occurs with Biotite, Vermiculite, Magnetite, Apatite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does hydrobiotite form in?+

Hydrobiotite typically forms in ultramafic rocks, carbonatites. Working float back to the host body is the standard way to chase a fresh occurrence.

What is hydrobiotite used for?+

Hydrobiotite is used in collector, scientific research.

Find hydrobiotite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.