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. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: yellowish-green, greenish-white.
- 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, so Eldfellite leaves white, Natrojarosite leaves light yellow.
How to tell apart: Streak differs, so Eldfellite leaves white, Copiapite leaves pale yellow; luster reads vitreous on Eldfellite and pearly on {010} 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
- 3.062 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Tabular crystals, crusts, aggregates
- Cleavage
- Perfect on {001, inferred from structure analysis
- Rarity
- Rare
- Uses
- Collector
- Host rock
- As a near surface fumarolic encrustation
Where rockhounds find eldfellite
Classic worldwide localities
- Eldfell volcano, Heimaey Island, Iceland
Field-hunting tip
Where it forms: As a near surface fumarolic encrustation. If you start seeing tamarugite, ralstonite, anhydrite in float, you are in the right ground. Typical habit: tabular crystals, crusts, aggregates. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify eldfellite?+
Where is eldfellite found?+
What rocks look like eldfellite?+
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Find eldfellite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
