Arseniosiderite is a rare calcium-iron arsenate typically found as spherical or radial fibrous clusters. It is an oxidation product of arsenopyrite and is most commonly identified by its distinct yellowish-brown color and silky, needle-like habit.
Is this arseniosiderite?
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 arseniosiderite with a known reference. Arseniosiderite sits at Mohs 4-4.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Arseniosiderite leaves a yellowish-brown streak.
- 3Read the lusterHold the specimen under a strong light. Arseniosiderite typically shows a silky luster.
- 4Match the color rangeCompare against the expected color range: yellow, brown, reddish-brown.
- 5Look at form & habitCrystal system: tetragonal. Typical habit: fibrous, radial, botryoidal, or globular aggregates.
Often confused with
Arseniosiderite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Arseniosiderite leaves yellowish-brown, Duftite leaves light green; luster reads silky on Arseniosiderite and vitreous on Duftite.
How to tell apart: Streak differs — Arseniosiderite leaves yellowish-brown, Mimetite leaves white; luster reads silky on Arseniosiderite and adamantine on Mimetite.
How to tell apart: Streak differs — Arseniosiderite leaves yellowish-brown, Carminite leaves yellowish-red; luster reads silky on Arseniosiderite and vitreous on Carminite.
Often found alongside arseniosiderite
Minerals reported to co-occur with arseniosiderite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₂Fe³⁺₃(AsO₄)₃O₂·3H₂O
- Mohs hardness
- 4-4.5
- Density
- 3.5-3.9 g/cm³
- Colors
- Streak
- Yellowish-brown
- Luster
- Silky
- Transparency
- Opaque
- Crystal system
- Tetragonal
- Crystal habit
- Fibrous, Radial, Botryoidal, Or Globular Aggregates
- Cleavage
- Perfect
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- Oxidized Hydrothermal Zones of Arsenic-rich Deposits
- Typical price
- $10-100 per specimen depending on size and quality
Where rockhounds find arseniosiderite
Classic worldwide localities
- France
- Germany
- United States
- Czech Republic
- Chile
Field-hunting tip
Look in oxidized hydrothermal zones of arsenic-rich deposits country — that is the host setting where arseniosiderite typically forms. If you start seeing goethite, scorodite, adamite in float, you are in the right ground. Field specimens usually show a fibrous, radial, botryoidal, or globular aggregates habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find arseniosiderite on the map
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