Table 1 Chemistry used in selected synthesis experiments^a

	100-Fe Control^b	100-Fe³⁺	5-Mg 95-Fe³⁺	15-Mg 85-Fe³⁺	50-Mg 50-Fe³⁺	100-Mg
Chemical Name	Amount Cation (mol)	Amount Cation (mol)	Amount Cation (mol)	Amount Cation (mol)	Amount Cation (mol)	Amount Cation (mol)
Sodium metasilicate—pentahydrate	0.0204	0.0204	0.0204	0.0205	0.0204	0.0205
Iron (II) sulfate—heptahydrate	0.0142	—	—	—	–	–
Sodium dithionite	0.0241	—	—	—	–	–
Iron (III) sulfate—rhomboclase & ferricopiapite	—	0.0138^c	0.0146^c	0.0117^c	0.0069 ^c	–
Magnesium sulfate—epsomite	—	—	0.0007	0.0022	0.0071	0.0142
Fe:Mg ratio	1:0	1:0	19:1	17:3	1:1	0:1

Solution name	Amount (mL)	Amount (mL)	Amount (mL)	Amount (mL)	Amount (mL)	Amount (mL)
Sodium hydroxide (5 M)	19.80	19.80	19.80	19.80	19.80	19.80
H₂O	417.52	419.96	417.11	416.41	417.11	417.87
Sulfuric acid (0.5 M)	41.00	42.00	42.00	42.00	42.00	42.00
Ending pH	12.49	12.69	12.64	12.76	12.79	12.88

All experiments were aged for 1 day at room temperature and then heated for an additional 2 days at 150 °C—for experiments incubated at 100 °C for 60 days see methods
^aFor all experimental conditions see Supplementary Tables 1 and 2
^b100-Fe bearing control condition, which was synthesized from initially ferrous Fe-containing solutions (sodium dithionite was added to maintain reducing conditions for the ferrous Fe) and then oxidized using published methods⁹
^cFe(III) molar concentrations were determined by atomic absorption spectroscopy

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