Fougèrite is a member of the hydrotalcite supergroup often referred to as 'green rust' due to its appearance in soil chemistry and anaerobic environments. It occurs as microscopic, hexagonal, platy crystals that are highly sensitive to oxidation, making it difficult to collect and preserve in its primary form.

Hardness

Mohs

Transparency

Translucent

mineralrareHydrotalcite Group family

Fougèrite

Also: green rust

Hardness: 2
Transparency: Translucent

Is this fougèrite?

3-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 fougèrite with a known reference. Fougèrite sits at Mohs 2, softer than the next harder reference, harder than the previous one.
2
Match the color range
Compare against the expected color range: bluish gray.
3
Look at form & habit
Crystal system: trigonal. Typical habit: platy crystals, fine-grained aggregates, crusts.

Often confused with

Fougèrite vs. its common look-alikes, and how to tell them apart in the field.

How to tell apart: Greenockite is the harder of the two (Mohs 3-3.5 vs. 2).

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Often found alongside fougèrite

Minerals reported to co-occur with fougèrite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: Fe₄Fe₂(OH)₁₂(CO₃)·3H₂O
Mohs hardness: 2
Density: 2.2-2.5 g/cm³
Colors: Bluish gray
Transparency: Translucent
Crystal system: Trigonal
Crystal habit: Platy crystals, fine-grained aggregates, crusts
Rarity: Rare
Uses: Collector, Scientific research
Host rock: Forms by partial oxidation and hydrolysis of aqueous Fe2+

Where rockhounds find fougèrite

Classic worldwide localities

Discovered coloring (green) the soil of the Fougères Forest in the Ille-et-Vilaine region of France (type locality)

Field-hunting tip

Where it forms: Forms by partial oxidation and hydrolysis of aqueous Fe2+. If you start seeing kaolinite, vermiculite, quartz in float, you are in the right ground. Typical habit: platy crystals, fine-grained aggregates, crusts. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify fougèrite?+

Mohs hardness is 2. Common colors include bluish gray.

Where is fougèrite found?+

Notable localities include Discovered coloring (green) the soil of the Fougères Forest in the Ille-et-Vilaine region of France (type locality).

What rocks look like fougèrite?+

Fougèrite is most often confused with Hydrotalcite, Pyrostilpnite, Greenockite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with fougèrite?+

Fougèrite commonly co-occurs with Kaolinite, Vermiculite, Quartz, Illite, Feldspars. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does fougèrite form in?+

Where it forms: Forms by partial oxidation and hydrolysis of aqueous Fe2+. Working float back to the host body is the standard way to chase a fresh occurrence.

What is fougèrite used for?+

Fougèrite is used in collector, scientific research.

Find fougèrite on the map

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