Rhomboclase is a hydrous iron sulfate typically found as an alteration product in the oxidizing zones of sulfide ore deposits. It is best identified by its characteristic tabular, rhombic crystals and its occurrence as secondary crusts or efflorescences in mine workings.
Is this rhomboclase?
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 rhomboclase with a known reference. Rhomboclase sits at Mohs 2, 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: subvitreous to pearly.
- 4Match the color rangeCompare against the expected color range: colorless, bluish, greenish, gray, yellow.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: tabular crystals, crusts, efflorescences.
Often confused with
Rhomboclase vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads subvitreous to pearly on Rhomboclase and vitreous on Coquimbite.
How to tell apart: Streak differs, so Rhomboclase leaves white, Copiapite leaves pale yellow; luster reads subvitreous to pearly on Rhomboclase and pearly on {010} on Copiapite.
Often found alongside rhomboclase
Minerals reported to co-occur with rhomboclase. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- HFe³⁺(SO₄)₂·4H₂O
- Mohs hardness
- 2
- Density
- 2.23 g/cm³
- Streak
- White
- Luster
- Subvitreous to pearly
- Transparency
- Transparent
- Crystal system
- Orthorhombic
- Crystal habit
- Tabular crystals, crusts, efflorescences
- Cleavage
- Perfect on {001}, good on {110}
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- An uncommon secondary mineral formed by alteration of pyrite, especially in an arid climate; typically of post-mining origin
Where rockhounds find rhomboclase
Classic worldwide localities
- Copper Queen mine, Bisbee, Cochise Co., and in the Magma mine, Superior district, Pinal Co., Arizona, USA
- Smolník (Szomolnok), 16 km northeast of Rožňava, Slovakia
- Lesenceistvánd, Hungary
- Nikitovka mercury deposit, Gorlovka, Ukraine
- Esperanza mine, Cerro de Pasco, Peru
- Alcaparrosa, near Cerritos Bayos, southwest of Calama, Antofagasta, Chile
Field-hunting tip
Where it forms: An uncommon secondary mineral formed by alteration of pyrite, especially in an arid climate; typically of post-mining origin. If you start seeing szomolnokite, copiapite, römerite in float, you are in the right ground. Typical habit: tabular crystals, crusts, efflorescences. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify rhomboclase?+
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What rocks look like rhomboclase?+
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Find rhomboclase on the map
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