Feodosiyite is a rare copper chloride mineral discovered in the volcanic fumaroles of the Tolbachik volcano in Kamchatka, Russia. It typically forms delicate bright green platy crystals and is highly sensitive to atmospheric moisture, making it a challenging and rare acquisition for specialized mineral collectors.
Is this feodosiyite?
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 feodosiyite with a known reference. Feodosiyite sits at Mohs ~3, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: light green.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: bright green.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: platy crystals.
Often confused with
Feodosiyite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Feodosiyite leaves light green, Atacamite leaves apple-green; luster reads vitreous on Feodosiyite and adamantine to vitreous on Atacamite.
How to tell apart: Streak differs, so Feodosiyite leaves light green, Paratacamite leaves green.
How to tell apart: Streak differs, so Feodosiyite leaves light green, Clinoatacamite leaves greenish white; luster reads vitreous on Feodosiyite and adamantine to vitreous on Clinoatacamite.
Often found alongside feodosiyite
Minerals reported to co-occur with feodosiyite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Cu₁₁Mg₂Cl₁₈(OH)₈·16H₂O
- Mohs hardness
- ~3
- Density
- 2.57 g/cm³
- Colors
- Streak
- Light green
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Platy crystals
- Cleavage
- Imperfect in one direction
- Rarity
- Rare
- Uses
- Collector
- Host rock
- As sublimates on basaltic scoria near a volcanic fumarole vent (~100 °C.)
Where rockhounds find feodosiyite
Classic worldwide localities
- Glavnaya Tenoritovaya (“Major Tenorite”) fumarole, Second scoria cone, Tolbachik volcano, Kamchatka Peninsula, Russia
Field-hunting tip
Where it forms: As sublimates on basaltic scoria near a volcanic fumarole vent (~100 °C.). If you start seeing belloite, avdoninite, sylvite in float, you are in the right ground. Typical habit: platy crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify feodosiyite?+
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Is feodosiyite safe to handle?+
What rocks look like feodosiyite?+
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Find feodosiyite on the map
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