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 in the World
Twinning
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