Fig. S1.

Thermodynamic stability diagram showing the Fe²⁺ concentration required to be in equilibrium with various minerals and silica activities ranging from quartz to amorphous silica, as a function of pH. Lines show saturation values at 10 °C and a salinity of 10 g/kg. Minerals are supersaturated above their respective equilibrium lines. All thermodynamic data come from ref. 53 with the exception of Fe saponite (Na_0.35Fe₃Al_0.35Si_3.65O₁₀OH₂) determined in ref. 16. In drawing the stability boundaries for magnetite, redox state is set at the magnetite/hematite boundary, as detailed in the main text. In defining equilibrium lines for ferrian saponite, Al³⁺ is set to saturation with respect to albite, which is reasonable given that the Sheepbed contains ∼20 wt % plagioclase feldspar (9).