Siderophyllite
A rare iron-rich true mica of nepheline-syenite pegmatites and greisens.
Intro adapted from Wikipedia (CC BY-SA).
Siderophyllite is an IMA-recognized mineral species in the silicate class, with the chemical formula KFe²⁺₂Al(Si₂Al₂)O₁₀(OH)₂.
It crystallizes in the monoclinic crystal system (space group C2). It is a member of the Mica structural group.
Key properties: Mohs hardness 2.5–3; specific gravity 3.1; refractive index 1.611; optically biaxial.
IMA symbol Sid, IMA status: approved, named in 1881.
Found in pegmatites in nepheline syenite; in greisens in granite and aplite.
It is found associated with Microcline, astrophyllite (Pikes Peak, Colorado, USA).
Its name comes from the Greek for iron and leaf, in reference to its composition and habit.