Dark microcrystals of the very rare mineral ferro-richterite on a pale colored matrix from Libby, Libby Mining District, Montana, United States of America. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY 4.0
Ferro-richterite
Ferro-richterite is an IMA-recognized mineral species in the silicate class, with the chemical formula Na(NaCa)Fe²⁺₅Si₈O₂₂(OH)₂.
It is a member of the Amphibole structural group.
Key properties: Mohs hardness 5–6; specific gravity 3.46; [White.] streak; [Vitreous.] luster; optically biaxial.
IMA symbol Frct, IMA status: approved, named in 1946.
Found as a rare accessory mineral in sodium-rich syenites and their volcanic equivalents; replacing ferromagnesian minerals in granite.
It is found associated with Feldspar, pyroxenes, zircon, fayalite, ferro-actinolite.
The name refers to the ferrous iron in its composition and similarity to richterite.