Outstanding very thin black single crystal of the rare tetraferriannite. From: Bancroft Area, Ontario, Canada. Tetraferriannite is a member of the phlogopite group. Ex Vandenbroucke Museum collection from Waregem, Belgium — Photo: David Hospital · CC BY-SA 4.0
Tetraferriannite
Tetraferriannite is an IMA-recognized mineral species in the silicate class, with the chemical formula KFe²⁺₃(Si₃Fe³⁺)O₁₀(OH)₂.
It crystallizes in the monoclinic crystal system (space group C2/m). It is a member of the Mica structural group.
Key properties: [Vitreous.] luster.
IMA symbol Tfann, IMA status: approved, named in 1925.
Found in low-grade metamorphosed banded iron formation.
It is found associated with Hematite, magnetite, quartz, ankerite, stilpnomelane, riebeckite.
Prefixes, tetra for the tetrahedral coordination sites in which Fe3+ cations (identified by ferri), reside in a compound analogous to annite.