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.
Is this fougèrite?
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 fougèrite with a known reference. Fougèrite sits at Mohs 2 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Fougèrite leaves a pale green streak.
- 3Read the lusterHold the specimen under a strong light. Fougèrite typically shows a earthy luster.
- 4Match the color rangeCompare against the expected color range: green, blue-green, pale green.
- 5Look at form & habitCrystal 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: Streak differs — Fougèrite leaves pale green, Hydrotalcite leaves white; luster reads earthy on Fougèrite and pearly on Hydrotalcite.
How to tell apart: Streak differs — Fougèrite leaves pale green, Pyrostilpnite leaves orange-yellow; luster reads earthy on Fougèrite and adamantine on Pyrostilpnite.
How to tell apart: Greenockite is the harder of the two (Mohs 3-3.5 vs. 2); streak differs — Fougèrite leaves pale green, Greenockite leaves brick-red to orange-yellow; luster reads earthy on Fougèrite and adamantine to resinous on Greenockite.
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
- (Mg,Fe²⁺,Fe³⁺)₆(OH)₁₂(CO₃,OH)₂·3H₂O
- Mohs hardness
- 2
- Density
- 2.2-2.5 g/cm³
- Streak
- Pale Green
- Luster
- Earthy
- Transparency
- Opaque
- Crystal system
- Trigonal
- Crystal habit
- Platy Crystals, Fine-grained Aggregates, Crusts
- Cleavage
- Perfect
- Rarity
- Rare
- Uses
- Collector, Scientific Research
- Host rock
- Hydromorphic Soils, Gley Soils, Anaerobic Sediments
- Typical price
- n/a
Where rockhounds find fougèrite
Classic worldwide localities
- Fougères, France
- various soil horizons worldwide
- sedimentary environments with reducing conditions
Field-hunting tip
Look in hydromorphic soils, gley soils, anaerobic sediments country — that is the host setting where fougèrite typically forms. If you start seeing goethite, magnetite, siderite in float, you are in the right ground. Field specimens usually show a platy crystals, fine-grained aggregates, crusts habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find fougèrite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
