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 4.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: ocher-yellow.
- 3Read the lusterHold the specimen under a strong light. Luster: submetallic to silky.
- 4Match the color rangeCompare against the expected color range: yellow, bronze brown, yellow brown, dark brown, black.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: fibrous.
Often confused with
Arseniosiderite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Arseniosiderite leaves ocher-yellow, Duftite leaves pale green to white; luster reads submetallic to silky on Arseniosiderite and dull to vitreous on fractures on Duftite.
How to tell apart: Streak differs, so Arseniosiderite leaves ocher-yellow, Mimetite leaves white; luster reads submetallic to silky on Arseniosiderite and resinous to subadamantine on Mimetite.
How to tell apart: Arseniosiderite is noticeably harder (Mohs 4.5 vs. 3.5); streak differs, so Arseniosiderite leaves ocher-yellow, Carminite leaves reddish yellow; luster reads submetallic to silky on Arseniosiderite and vitreous, pearly on cleavages 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
- 4.5
- Density
- 3.58-3.60 g/cm³
- Streak
- Ocher-yellow
- Luster
- Submetallic to silky
- Transparency
- Opaque, translucent through thin edges
- Crystal system
- Monoclinic
- Crystal habit
- Fibrous
- Cleavage
- On {001}, perfect
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- A rare secondary mineral formed by the oxidation of earlier arsenic-bearing minerals, typically scorodite or arsenopyrite
Where rockhounds find arseniosiderite
Classic worldwide localities
- Sterling Hill, Ogdensburg, Sussex Co., New Jersey, USA
- Mohawk mine, San Bernardino Co., and in the Kalkar quarry, Santa Cruz, Santa Cruz Co., California, USA
- Gold Hill mine, Tooele Co., and the Mammoth and other mines, Tintic district, Juab Co., Utah, USA
- Romanèche, Saône-et-Loire, France
- Hüttenberg, Carinthia, Austria
- Schneeberg, Saxony, and at Wittichen and Neubulach, Black Forest, Germany
Field-hunting tip
Where it forms: A rare secondary mineral formed by the oxidation of earlier arsenic-bearing minerals, typically scorodite or arsenopyrite. If you start seeing beudantite, carminite, scorodite in float, you are in the right ground. Typical habit: fibrous. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify arseniosiderite?+
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Is arseniosiderite safe to handle?+
What rocks look like arseniosiderite?+
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Find arseniosiderite on the map
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