Siderotil is a secondary sulfate mineral that typically forms as an efflorescent crust or fibrous coating on rock surfaces in mine workings. It is chemically unstable and dehydration-prone, often requiring careful storage to prevent it from turning into a white powder as it alters to rozenite.
Is this siderotil?
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 siderotil with a known reference. Siderotil sits at Mohs 2.5, 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: white, yellowish, yellowish white, light green, light green white.
- 5Look at form & habitCrystal system: triclinic. Typical habit: fibrous, efflorescent crusts, powdery coatings.
Often confused with
Siderotil vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside siderotil
Minerals reported to co-occur with siderotil. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- FeSO₄·5H₂O
- Mohs hardness
- 2.5
- Density
- 2.1-2.2 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous
- Transparency
- Semitransparent
- Crystal system
- Triclinic
- Crystal habit
- Fibrous, efflorescent crusts, powdery coatings
- Cleavage
- None
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- Forms by dehydration of cuprian melanterite
Where rockhounds find siderotil
Classic worldwide localities
- Yerington district, Lyon Co., at the Watt mine, Reese River district, Lander Co., and from Steamboat Springs, Steamboat Springs district, Washoe Co., Nevada, USA
- Bingham Canyon, Salt Lake Co., Utah, USA
- Mt. Diablo mercury mine, Contra Costa Co., California, USA
- Campbell shaft, Bisbee, Cochise Co., Arizona, USA
- Originally incompletely described from Idrija (Idria), Slovenia, where it may or may not occur
- Capillitas mine, Catamarca Province, Argentina
Field-hunting tip
Where it forms: Forms by dehydration of cuprian melanterite. If you start seeing melanterite in float, you are in the right ground. Typical habit: fibrous, efflorescent crusts, powdery coatings. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify siderotil?+
Where is siderotil found?+
What rocks look like siderotil?+
What minerals are found with siderotil?+
What kind of rock does siderotil form in?+
What is siderotil used for?+
Find siderotil on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
