Pharmacolite is a secondary arsenic-bearing mineral typically found as delicate acicular crystals or tufted, fibrous crusts. It commonly occurs in the oxidation zones of ore deposits where arsenic minerals have been weathered. Due to its arsenic content, collectors should handle it with care and store it in enclosed containers.
Is this pharmacolite?
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 pharmacolite with a known reference. Pharmacolite sits at Mohs 2-2.5, 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, pearly on cleavages.
- 4Match the color rangeCompare against the expected color range: white, yellow, reddish white, grayish white.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: botryoidal, encrustations, fibrous.
Often confused with
Pharmacolite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads vitreous, pearly on cleavages on Pharmacolite and subvitreous, pearly on {010} cleavages, silky if fibrous on Gypsum.
How to tell apart: Wavellite is the harder of the two (Mohs 3.5-4 vs. 2-2.5).
Often found alongside pharmacolite
Minerals reported to co-occur with pharmacolite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaH(AsO₄)·2H₂O
- Mohs hardness
- 2-2.5
- Density
- 2.53-2.725 g/cm³
- Streak
- White
- Luster
- Vitreous, pearly on cleavages
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Botryoidal, encrustations, fibrous
- Cleavage
- On {010}, perfect
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- An uncommon secondary mineral formed by oxidation of earlier arsenic-bearing minerals
Where rockhounds find pharmacolite
Classic worldwide localities
- Getchell mine, Potosi district, Humboldt Co., and the White Caps mine, Manhattan district, Nye Co., and the Gold Bar mine, Antelope district, Eureka Co., Nevada, USA
- Sterling Hill, Ogdensburg, Sussex Co., New Jersey, USA
- OK mine, San Gabriel Canyon, Los Angeles Co., California, USA
- Wittichen, Black Forest, in the Anton and Sofia mines, Germany
- St. Andreasberg, Harz Mountains, Germany
- Rosenkrantz mine, Schneeberg, Saxony, Germany
Field-hunting tip
Where it forms: An uncommon secondary mineral formed by oxidation of earlier arsenic-bearing minerals. If you start seeing picropharmacolite, hörnesite, haidingerite in float, you are in the right ground. Typical habit: botryoidal, encrustations, fibrous. Train your eye for that shape before scanning the outcrop.
Common questions
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Find pharmacolite on the map
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