Erythrosiderite is a rare potassium iron chloride mineral found primarily in volcanic fumaroles, notably at Mount Vesuvius. It is highly deliquescent, meaning it absorbs moisture from the air and will eventually dissolve or alter if not kept in a sealed, dry container.
Is this erythrosiderite?
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 erythrosiderite with a known reference. Erythrosiderite sits at Mohs 2.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: yellowish-orange.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: red, brown red.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: pseudo octohedral, tabular.
Often confused with
Erythrosiderite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Erythrosiderite leaves yellowish-orange, Eriochalcite leaves white.
How to tell apart: Streak differs, so Erythrosiderite leaves yellowish-orange, Tolbachite leaves yellowish red; luster reads vitreous on Erythrosiderite and pearly on Tolbachite.
Often found alongside erythrosiderite
Minerals reported to co-occur with erythrosiderite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- K₂FeCl₅·H₂O
- Mohs hardness
- 2.5
- Density
- 2.372 g/cm³
- Streak
- Yellowish-orange
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Pseudo octohedral, tabular
- Cleavage
- Perfect on {210} and {011}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- As sublimates around fumaroles (Vesuvius, Italy); rimming rinneite in bedded salt deposits (Stassfurt, Germany; Kansk-Taseev depression, Russia)
Where rockhounds find erythrosiderite
Classic worldwide localities
- Vesuvius, Campania, Italy
- Stassfurt, 34 km south of Magdeburg, Saxony-Anhalt, Germany
- Aislaby, near Whitby, Yorkshire, England
- Kansk-Taseev depression, Siberia, and at the Kliuchevsky volcano, Kamchatka Peninsula, Russia
Field-hunting tip
Where it forms: As sublimates around fumaroles (Vesuvius, Italy); rimming rinneite in bedded salt deposits (Stassfurt, Germany; Kansk-Taseev depression, Russia). If you start seeing kremersite, molysite, hematite in float, you are in the right ground. Typical habit: pseudo octohedral, tabular. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify erythrosiderite?+
Where is erythrosiderite found?+
What rocks look like erythrosiderite?+
What minerals are found with erythrosiderite?+
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What is erythrosiderite used for?+
Find erythrosiderite on the map
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