Fibrous vonsenite. Monchi Mine, Burguillos del Cerro (Badajoz) Spain — Photo: Bergminer · CC BY-SA 4.0
Vonsenite
Vonsenite is an IMA-recognized mineral species in the borate class, with the chemical formula Fe²⁺₂Fe³⁺O₂(BO₃).
It crystallizes in the orthorhombic crystal system (space group Pbam). It is a member of the Ludwigite structural group.
Key properties: Mohs hardness 5; specific gravity 4.21; Brownish black streak; Metallic to submetallic, silky if fibrous luster; optically biaxial.
IMA symbol Von, grandfathered status — known before the IMA’s founding, named in 1920.
Uncommon in boron-rich skarns.
It is found associated with Magnetite, pyroxene, pyrrhotite.
The mineral was named to honor Magnus Vonsen (1879–1954), amateur mineralogist and mineral collector of Petaluma, California, USA, who discovered the mineral.