Eldfellite is a rare sodium iron sulfate that typically forms as a sublimate at active volcanic fumaroles. It appears as delicate pale yellow to colorless crusts or small tabular crystals and is highly sensitive to humidity due to its solubility.
Is this eldfellite?
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 eldfellite with a known reference. Eldfellite sits at Mohs 2-3 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Eldfellite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Eldfellite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: pale yellow, white, colorless.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: tabular crystals, crusts, aggregates.
Often confused with
Eldfellite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Eldfellite leaves white, Natrojarosite leaves yellow; luster reads vitreous on Eldfellite and dull on Natrojarosite.
How to tell apart: Streak differs — Eldfellite leaves white, Copiapite leaves yellow; luster reads vitreous on Eldfellite and pearly on Copiapite.
Often found alongside eldfellite
Minerals reported to co-occur with eldfellite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- NaFe(SO₄)₂
- Mohs hardness
- 2-3
- Density
- 2.42 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Tabular Crystals, Crusts, Aggregates
- Cleavage
- Perfect
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Volcanic Fumaroles
- Typical price
- $50-300 per specimen
Where rockhounds find eldfellite
Classic worldwide localities
- Eldfell volcano (Iceland)
- Mount Vesuvius (Italy)
Field-hunting tip
Look in volcanic fumaroles country — that is the host setting where eldfellite typically forms. If you start seeing thenardite, halite, gypsum in float, you are in the right ground. Field specimens usually show a tabular crystals, crusts, aggregates habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find eldfellite on the map
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