Cronstedtite is a rare member of the serpentine group often found in hydrothermal veins as striking triangular plates or pseudo-hexagonal pyramids. Collectors prize it for its unique metallic to vitreous luster and distinct geometry, though it is usually found only in small, delicate specimens.
Is this cronstedtite?
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 cronstedtite with a known reference. Cronstedtite sits at Mohs 3.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: dark olive-green.
- 3Read the lusterHold the specimen under a strong light. Luster: brilliant.
- 4Match the color rangeCompare against the expected color range: black, brownish black, greenish black, dark brown.
- 5Look at form & habitCrystal system: triclinic. Typical habit: amorphous, divergent, pyramidal.
Often confused with
Cronstedtite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Cronstedtite leaves dark olive-green, Chamosite leaves green to gray; luster reads brilliant on Cronstedtite and pearly on Chamosite.
How to tell apart: Cronstedtite is noticeably harder (Mohs 3.5 vs. 2.5); streak differs, so Cronstedtite leaves dark olive-green, Lizardite leaves white; luster reads brilliant on Cronstedtite and waxy on Lizardite.
How to tell apart: Cronstedtite is noticeably harder (Mohs 3.5 vs. 2-2.5); streak differs, so Cronstedtite leaves dark olive-green, Clinochlore leaves greenish white to white; luster reads brilliant on Cronstedtite and pearly, greasy, dull on Clinochlore.
Often found alongside cronstedtite
Minerals reported to co-occur with cronstedtite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Fe²⁺₂Fe³⁺(Si,Fe³⁺)₂O₅(OH)₄
- Mohs hardness
- 3.5
- Density
- 3.34-3.35 g/cm³
- Streak
- Dark olive-green
- Luster
- Brilliant
- Transparency
- Transparent
- Crystal system
- Triclinic
- Crystal habit
- Amorphous, divergent, pyramidal
- Cleavage
- Perfect on {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A low-temperature hydrothermal product in ore veins
Where rockhounds find cronstedtite
Classic worldwide localities
- Long Hill, Fairfield Co., Connecticut, USA
- Cornucopia mine, Nye Co., Nevada, USA
- Přibram, Kutná Hora, Chvaletice, Chyňava
- Herja (Kisbánya), Romania
- Hagental, Harz Mountains, Germany
- Salsigne gold mine, 15 km north of Carcassone, Aude, France
Field-hunting tip
Where it forms: A low-temperature hydrothermal product in ore veins. If you start seeing siderite, pyrite, sphalerite in float, you are in the right ground. Typical habit: amorphous, divergent, pyramidal. Train your eye for that shape before scanning the outcrop.
Common questions
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Find cronstedtite on the map
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