Phase equilibria of ancient iron-formation at 25°C in the Fe–Si–O–H system using composition of Franklin Seamount vent fluid ([Fetotal] = 3.9 × 10-5m and [Si] = 3.4 × 10-4m). (1) Boundaries based on free energy data from Klein and Bricker. Fe3(OH)8 has converted to Fe3O4 while Fe(OH)3 persists unstably (see text). (2) Boundaries based on Fe3O4 in equilibrium with Fe2O3. Closed square represents location of Franklin Seamount vent fluid from Fig. 7.