Abstract:
Annite is an iron-rich trioctahedral mica close in composition to the hypothetical end-member KFe3[Si3AlO10](OH)2. Annite with an ideal composition is known only as a synthetic compound. Natural annites may approach the theoretical composition. Their Fe2+ content is about 2.0–2.5 per formula unit (28–36 wt % FeO), and Fe3+ is up to 0.5 per formula unit. The synthesis experiments of Eugster showed that the stability of annite is a function of temperature, total pressure, and, especially, oxygen fugacity. The knowledge of the thermodynamic properties of annite and other ferriferous biotites with known Fe2+/Fe3+ and Fe/Al ratios together with phase equilibrium experimental data may help elucidate the conditions for the formation of metamorphic and magmatic rocks containing abundant ferriferous biotites.
