Voltaite is a rare potassium-iron-aluminum sulfate mineral often forming in acidic mine environments or volcanic areas. It typically presents as dark, resinous octahedral crystals that may dehydrate and crumble if not stored in a humidity-controlled environment.
Is this voltaite?
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 voltaite with a known reference. Voltaite 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: grayish green.
- 3Read the lusterHold the specimen under a strong light. Luster: resinous.
- 4Match the color rangeCompare against the expected color range: black, greenish black, dark green, olive green.
- 5Look at form & habitCrystal system: cubic. Typical habit: crystalline, massive.
Often confused with
Voltaite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Voltaite is noticeably harder (Mohs 3 vs. 1.5); streak differs, so Voltaite leaves grayish green, Pickeringite leaves white; luster reads resinous on Voltaite and vitreous on Pickeringite.
How to tell apart: Voltaite is noticeably harder (Mohs 3 vs. 1.5); streak differs, so Voltaite leaves grayish green, Halotrichite leaves white; luster reads resinous on Voltaite and vitreous on Halotrichite.
How to tell apart: Streak differs, so Voltaite leaves grayish green, Copiapite leaves pale yellow; luster reads resinous on Voltaite and pearly on {010} on Copiapite.
Often found alongside voltaite
Minerals reported to co-occur with voltaite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- K₂Fe₅Fe₃Al(SO₄)₁₂·18H₂O
- Mohs hardness
- 3
- Density
- 2.645 g/cm³
- Streak
- Grayish green
- Luster
- Resinous
- Transparency
- Opaque, translucent in thin splinters
- Crystal system
- Cubic
- Crystal habit
- Crystalline, massive
- Cleavage
- None
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- In volcanic fumaroles, solfataras, and mud volcanos; an alteration product in mineral deposits high in pyrite, especially in arid climates, where it may also be a post-mining product
Where rockhounds find voltaite
Classic worldwide localities
- Bisbee, Cochise Co., at Jerome, Yavapai Co., and in the Magma mine, Superior, Gila Co., USA
- Redington mercury mine, Knoxville, Napa Co., California, USA
- Geysers, Sonoma Co., California, USA
- Sulfur Hole prospect, about 10 km northeast of Yermo, Calico Hills, San Bernardino Co., California, USA
- Solfatara di Pozzuoli, Campi Flegrei, near Naples, and on Vesuvius, Campania, Italy
- Rio Tinto mine, Huelva Province, Spain
Field-hunting tip
Where it forms: In volcanic fumaroles, solfataras, and mud volcanos; an alteration product in mineral deposits high in pyrite, especially in arid climates, where it may also be a post-mining product. If you start seeing halotrichite, pickeringite, alunogen in float, you are in the right ground. Typical habit: crystalline, massive. Train your eye for that shape before scanning the outcrop.
Common questions
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Find voltaite on the map
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