Phase equilibria of freshly precipitated oxyhydroxides in the Fe–Si–O–H system (1) from Fig. 6 at 25°C, 240 bar, [Fetotal] = 3.9 × 10-5 m and [Si] = 3.4 × 10-4 m which are the physical and geochemical conditions of a vent fluid at Franklin Seamount and at higher temperatures. Calculated and based on the reactions in Table 3 which are drawn from the thermodynamic databases of Sadiq and Lindsay for Fe2+ and Fe3(OH)8, Langmuir36 for Fe(OH)3, and Winters and Buckley for FeSiO3. The stability fields (*) of iron bacteria from Fig. 5 are at 25°C. Dashed lines show change in Fe2+-Fe3(OH)8-Fe(OH)3 boundaries with increase in temperature. Faint line based on the SUPCRT92 database19 for the dissociation of water plotted to show the sensitivity of the Fe(OH)3-Fe3(OH)8 boundary to minute changes in Gibbs free energies. Calculated with aid of SUPCRT92 program. The arrow represents the upward extrapolation to the Fe2+-solid phases boundaries of the measured pH (6.26) of the 20–30°C vent fluid at Franklin Seamount.