Hörnesite is a rare magnesium arsenate that typically forms delicate, white to colorless bladed crystals or radiating sprays. It is most often found in the oxidized zones of arsenic-rich ore deposits, where it occurs as a secondary mineral associated with other rare arsenates.
Is this hörnesite?
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 hörnesite with a known reference. Hörnesite sits at Mohs 1 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Hörnesite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Hörnesite typically shows a pearly luster.
- 4Match the color rangeCompare against the expected color range: white, colorless, pale pink.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: bladed crystals, radial aggregates, crusts.
Often confused with
Hörnesite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Hörnesite leaves white, Vivianite leaves white to light blue; luster reads pearly on Hörnesite and vitreous on Vivianite.
How to tell apart: Newberyite is the harder of the two (Mohs 2.5 vs. 1); luster reads pearly on Hörnesite and vitreous on Newberyite.
Often found alongside hörnesite
Minerals reported to co-occur with hörnesite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mg₃(AsO₄)₂·8H₂O
- Mohs hardness
- 1
- Density
- 2.5 g/cm³
- Streak
- White
- Luster
- Pearly
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Bladed Crystals, Radial Aggregates, Crusts
- Cleavage
- Perfect in One Direction
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Oxidized Zones of Hydrothermal Arsenic-bearing Ore Deposits
- Typical price
- $20-150 thumbnail specimens
Where rockhounds find hörnesite
Classic worldwide localities
- Dognácska, Romania
- Jáchymov, Czech Republic
- Sterling Hill, New Jersey, USA
Field-hunting tip
Look in oxidized zones of hydrothermal arsenic-bearing ore deposits country — that is the host setting where hörnesite typically forms. If you start seeing arsenopyrite, pharmacolite, picropharmacolite in float, you are in the right ground. Field specimens usually show a bladed crystals, radial aggregates, crusts habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find hörnesite on the map
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