Jarosite is a hydrous potassium iron sulfate typically found as a secondary mineral in the oxidized zones of ore deposits. It is easily recognized by its distinct mustard-yellow to brownish-yellow color and earthy or crystalline habit in acidic environments.
Is this jarosite?
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 jarosite with a known reference. Jarosite sits at Mohs 2.5-3.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: pale yellow.
- 3Read the lusterHold the specimen under a strong light. Luster: subadamantine to vitreous, resinous on fractures.
- 4Match the color rangeCompare against the expected color range: yellow, brown, yellow brown, light yellow.
- 5Look at form & habitCrystal system: trigonal. Typical habit: crystalline, fibrous, massive.
Often confused with
Jarosite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Goethite is the harder of the two (Mohs 5-5.5 vs. 2.5-3.5); streak differs, so Jarosite leaves pale yellow, Goethite leaves brownish yellow, yellow-orange, ocher-yellow; luster reads subadamantine to vitreous, resinous on fractures on Jarosite and imperfect adamantine metallic to dull earthy; silky when fibrous on Goethite.
How to tell apart: Luster reads subadamantine to vitreous, resinous on fractures on Jarosite and pearly on {010} on Copiapite.
How to tell apart: Streak differs, so Jarosite leaves pale yellow, Limonite leaves yellowish-brown; luster reads subadamantine to vitreous, resinous on fractures on Jarosite and submetallic to earthy on Limonite.
Often found alongside jarosite
Minerals reported to co-occur with jarosite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KFe₃(SO₄)₂(OH)₆
- Mohs hardness
- 2.5-3.5
- Density
- 2.91-3.26 g/cm³
- Streak
- Pale yellow
- Luster
- Subadamantine to vitreous, resinous on fractures
- Transparency
- Transparent to translucent
- Crystal system
- Trigonal
- Crystal habit
- Crystalline, fibrous, massive
- Cleavage
- On {0001}, distinct
- Rarity
- Common
- Uses
- Collector, Geological indicator
- Host rock
- A secondary mineral in oxidized portions of sulfide-bearing rocks, typically altering from pyrite; less common as a low-temperature, primary hydrothermal mineral, including as deposits around hot springs
Where rockhounds find jarosite
5 mapped spotsClassic worldwide localities
- Marysvale, Ohio district, Piute Co., and at the Mammoth mine, Tintic, Juab Co., Utah, USA
- Vulture mine, near Wickenburg, Maricopa Co., at Tombstone, Cochise Co., Arizona, USA
- Meadow Valley mines, Pioche district, Lincoln Co., Nevada, USA
- Barranco del Jaroso, Sierra Almagrera, Almería Province, Spain
- Horní Slavkov (Schlaggenwald), Czech Republic
- Frisches Glück mine, Fürstenberg, near Schwarzenberg, Saxony, Germany
Field-hunting tip
Where it forms: A secondary mineral in oxidized portions of sulfide-bearing rocks, typically altering from pyrite; less common as a low-temperature, primary hydrothermal mineral, including as deposits around hot springs. If you start seeing alunite, pyrite in float, you are in the right ground. Typical habit: crystalline, fibrous, massive. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Utah, New Mexico. Start trip planning there.
Common questions
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Find jarosite on the map
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