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. Pharmacosiderite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Pharmacosiderite typically shows a adamantine luster.
- 4Match the color rangeCompare against the expected color range: yellow, green, brown, red.
- 5Look at form & habitCrystal system: cubic. Typical habit: pseudocubic crystals, tetrahedral, massive.
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); luster reads adamantine on Pharmacosiderite and vitreous to sub-adamantine on Scorodite.
How to tell apart: Luster reads adamantine on Pharmacosiderite and vitreous 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
- 2.8 g/cm³
- Streak
- White
- Luster
- Adamantine
- Transparency
- Translucent
- Crystal system
- Cubic
- Crystal habit
- Pseudocubic Crystals, Tetrahedral, Massive
- Cleavage
- Poor On {111}
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- Oxidized Zones of Arsenic-rich Hydrothermal Veins
- Typical price
- $10-150 per specimen depending on size and crystal quality
Where rockhounds find pharmacosiderite
Classic worldwide localities
- Cornwall, England
- Schwarzwald, Germany
- Eureka, Utah, USA
- Mapimi, Mexico
- Minas Gerais, Brazil
Field-hunting tip
Look in oxidized zones of arsenic-rich hydrothermal veins country — that is the host setting where pharmacosiderite typically forms. If you start seeing limonite, arsenopyrite, scorodite in float, you are in the right ground. Field specimens usually show a pseudocubic crystals, tetrahedral, massive habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find pharmacosiderite on the map
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