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. Erythrosiderite leaves a yellowish-orange streak.
- 3Read the lusterHold the specimen under a strong light. Erythrosiderite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: red, dark red, orange-red.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: tabular crystals, crusts, or massive.
Often confused with
Erythrosiderite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Erythrosiderite leaves yellowish-orange, Eriochalcite leaves white.
How to tell apart: Streak differs — Erythrosiderite leaves yellowish-orange, Tolbachite leaves yellowish red; luster reads vitreous on Erythrosiderite and subadamantine 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
- 2.35 g/cm³
- Colors
- Streak
- Yellowish-orange
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Tabular Crystals, Crusts, Or Massive
- Cleavage
- Distinct On {010}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Fumaroles of Volcanic Craters
- Typical price
- $50-300 per specimen
Where rockhounds find erythrosiderite
Classic worldwide localities
- Mount Vesuvius, Italy
Field-hunting tip
Look in fumaroles of volcanic craters country — that is the host setting where erythrosiderite typically forms. If you start seeing halite, sylvite, hematite in float, you are in the right ground. Field specimens usually show a tabular crystals, crusts, or massive habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find erythrosiderite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
