Magnesio-riebeckite
The magnesium end of the riebeckite amphibole series — a sodic amphibole where Mg replaces Fe²⁺.
Intro adapted from Wikipedia (CC BY-SA).
Magnesio-riebeckite is an IMA-recognized mineral species in the silicate class, with the chemical formula Na₂(Mg₃Fe³⁺₂)Si₈O₂₂(OH)₂.
It crystallizes in the monoclinic crystal system. It is a member of the Amphibole structural group.
Key properties: Mohs hardness 6; specific gravity 3.12–3.29; [Vitreous.] luster; optically biaxial.
IMA symbol Mrbk, IMA status: approved, named in 1957.
Found in granulites, ironstones, ferruginous metacherts, greenschist facies schists, and carbonatites.
It is found associated with Stilpnomelane, epidote, quartz, muscovite, winchite.
The name refers to its high magnesium content and similarity to riebeckite.