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. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: resinous to waxy, pearly on cleavages.
- 4Match the color rangeCompare against the expected color range: white.
- 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: Luster reads resinous to waxy, pearly on cleavages on Hörnesite and vitreous, pearly on the cleavage, dull when earthy on Vivianite.
How to tell apart: Newberyite is the harder of the two (Mohs 3-3.5 vs. 1); luster reads resinous to waxy, pearly on cleavages 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
- 2.57-2.73 g/cm³
- Colors
- Streak
- White
- Luster
- Resinous to waxy, pearly on cleavages
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Bladed crystals, radial aggregates, crusts
- Cleavage
- Perfect on {010}, poor on {100}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In limestone blocks, thermally metamorphosed, in a volcanic tuff (Fiano, Italy)
Where rockhounds find hörnesite
Classic worldwide localities
- Kalkar quarry, Santa Cruz, Santa Cruz Co., California, USA
- A cave in Peach Springs Canyon, Mohave Co., Arizona, USA
- White Caps mine, Manhattan district, Nye Co., Nevada, USA
- Oraviţa (Oravicza), Băiţa (Rézbánya), and Săcărı̂mb (Nagyág), Romania
- Jáchymov (Joachimsthal), Czech Republic
- Glatschach and Hirt, Carinthia, Austria
Field-hunting tip
Where it forms: In limestone blocks, thermally metamorphosed, in a volcanic tuff (Fiano, Italy). If you start seeing fluoborite, fluorite, hydromagnesite in float, you are in the right ground. Typical habit: bladed crystals, radial aggregates, crusts. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify hörnesite?+
Where is hörnesite found?+
Is hörnesite safe to handle?+
What rocks look like hörnesite?+
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Find hörnesite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
