Aspidolite
A sodium mica — effectively the sodium counterpart of magnesium-rich phlogopite.
Intro adapted from Wikipedia (CC BY-SA).
Aspidolite is an IMA-recognized mineral species in the silicate class, with the chemical formula NaMg₃(Si₃Al)O₁₀(OH)₂.
It crystallizes in the triclinic crystal system (space group C2/m). It is a member of the Mica, Clay structural group.
Key properties: Mohs hardness 2–3; specific gravity 2.885; White streak; Pearly luster; optically biaxial.
IMA symbol Asp, IMA status: approved, named in 1869.
Found in thermally metamorphosed rocks (greenschist to amphibolite facies) in the contact aureole of a granite; in metamorphosed evaporite rocks (Algeria); from norite xenoliths (Chile).
It is found associated with Phlogopite, pargasite, magnesiosadanagaite, titanite, calcite, scapolite, apatite, pyrrhotite, chalcopyrite, chlorite, pyrite (Kasuga-mura, Japan); enstatite.
Its name comes from the Greek "aspida" for "a shield" and "lithos" for "stone", alluding to the crystal habit.