Hörnesite - Encyclopedia

    Class : Phosphates, arsenates, vanadates
    Subclass : Hydrated arsenates
    Crystal system : Monoclinic
    Chemistry : Mg3(AsO4)2 8H2O
    Rarity : Rare

Hörnesite (or hoernesite) is a hydrated magnesium arsenate which forms a series with erythrite, in which cobalt replaces magnesium. It is a secondary mineral which forms in multiple contexts : oxidation zones of arsenic polymetallic mineralization, metamorphic limestones released by volcanoes, pegmatites and caves. It is also a mineral found in foundry slag. It was named in honor of Moritz Hörnes, curator of the Imperial Cabinet of Mineralogy in Vienna (Austria). Hörnesite forms small elongated and flattened prismatic crystals generally 1 to 2 mm in size, grouped in radiate groups. It is most of the time white in color, but can take on pinkish hues due to cobalt or green with nickel.

Main photo : Hörnesite from Ightem Mine, Bou Azzer, Morocco © Philippe Font

Hörnesite on conichalcite from Ightem Mine, Bou Azzer, Morocco © Philippe Font
Hörnesite of Mina Casualidad, Almería, Spain © Christian Rewitzer
Hörnesite of Gabe Gottes, Ste-Marie-aux-Mines, Haut-Rhin, France © Jean-Marie Claude
Hörnesite from Svätodusná, Slovakia © Martin Stevko

Hörnesite in the World

The prettiest crystals come from Ightem Mine near Bou Azzer (Morocco) and measure up to 3 mm. Hörnesite is also found in the gold-bearing hydrothermal veins of Sacarimb (Romania) and the uranium-bearing hydrothermal veins of Jachymov (Czech Republic). It is known from the Sterling Hill zinc deposit (New Jersey). It is a mineral also present in the Langban deposit (Sweden), and in the pegmatite of Corrego do Urucum (Minas Gerais, Brazil). Among its most exotic localities, mention the cave of Peach Springs Canyon (Arizona) and the limestone blocks packed in the rhyolite tuffs of Fiano (Italy).

Hörnesite in France

In France, hörnesite is known in the mines of Ste-Marie-aux-Mines (Haut-Rhin), in Matra (Haute-Corse) where it is associated with realgar and orpiment and in the slag of Villanière near from Carcassone (Aude).


No twin known for this mineral species.

Fakes and treatments

No fakes recorded for this mineral species.

Hardness : 1
Density : 2.57 to 2.73
Fracture : Undeterminated
Streak : White

TP : Transparent
RI : 1.563 to 1.596
Birefringence : 0.033
Optical character : Biaxial -+
Pleochroism : None
Fluorescence : None

Solubility : Acids

Magnetism : NoneRadioactivity : None


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