Pharmacosiderite is a secondary mineral commonly found in the oxidized zones of arsenic-rich deposits. It is best known for its distinctive small, sharp, cubic crystals that often exhibit a beautiful olive-green to yellow color.
Is this pharmacosiderite?
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 pharmacosiderite with a known reference. Pharmacosiderite 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: yellowish green.
- 3Read the lusterHold the specimen under a strong light. Luster: adamantine to greasy.
- 4Match the color rangeCompare against the expected color range: olive green, emerald green, reddish brown, yellowish brown, honey yellow.
- 5Look at form & habitCrystal system: cubic. Typical habit: blocky, crystalline, earthy.
Often confused with
Pharmacosiderite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Scorodite is the harder of the two (Mohs 3.5-4 vs. 2.5); streak differs, so Pharmacosiderite leaves yellowish green, Scorodite leaves greenish white; luster reads adamantine to greasy on Pharmacosiderite and vitreous to subadamantine, subresinous on Scorodite.
How to tell apart: Streak differs, so Pharmacosiderite leaves yellowish green, Pharmacolite leaves white; luster reads adamantine to greasy on Pharmacosiderite and vitreous, pearly on cleavages on Pharmacolite.
Often found alongside pharmacosiderite
Minerals reported to co-occur with pharmacosiderite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KFe₄(AsO₄)₃(OH)₄·6H₂O
- Mohs hardness
- 2.5
- Density
- 2.797 g/cm³
- Streak
- Yellowish green
- Luster
- Adamantine to greasy
- Transparency
- Transparent to translucent
- Crystal system
- Cubic
- Crystal habit
- Blocky, crystalline, earthy
- Cleavage
- On {001}, good to imperfect
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- An oxidation product of arsenic-bearing sulfides
Where rockhounds find pharmacosiderite
Classic worldwide localities
- Mammoth mine, Tintic district, Juab Co., Utah, USA
- Northumberland mine, Nye Co., and as Majuba Hill mine, Antelope district, Pershing Co., Nevada, USA
- Cornwall, from the Carharrack and Tincroft mines and Wheals Gorland and Unity, Gwennap, and a number of other localities, England
- Vaulry, Haute-Vienne, and at the Cap Garonne mine, near le Pradet, Var, France
- Dernbach and Horhausen, Rhineland-Palatinate, Germany
- Clara mine, near Oberwolfach, in the Black Forest, Germany
Field-hunting tip
Where it forms: An oxidation product of arsenic-bearing sulfides. If you start seeing scorodite, beudantite, carminite in float, you are in the right ground. Typical habit: blocky, crystalline, earthy. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify pharmacosiderite?+
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Is pharmacosiderite safe to handle?+
What rocks look like pharmacosiderite?+
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Find pharmacosiderite on the map
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