Table 1. Mineral phases identified in incubations with ferrihydrite^a.

		Mineral phase (as % of total Fe in the solids)a
e- donor supplied	LC fit	Ferrihydriteb	Siderite	Vivianite	Disordered Fe(II)c	Green rust	FeS
Acetate	EXAFS	76	-	13	-	10	4
	XANES	75	-	11	-	11	4
Lactate	EXAFS	76	-	6	-	-	17
	XANES	74	-	8	-	-	15
Glucose (white)	EXAFS	13	64d	-	32d	-	-
	XANES	6	66d	-	27d	-	-
Glucose (brown)	EXAFS	84	-	2	4	-	-
	XANES	90	-	8	8	-	-

^a Based on linear combination (LC) fits of the XANES and EXAFS data with up to four of the listed standards. Components were not constrained to add up to 100%. Uncertainties reported by the numerical fitting procedure are <2%. Overall uncertainties are estimated at ±5%. Higher sensitivity to the structural differences between Fe(II) phases was observed in fits of the EXAFS data than in fits of the XANES data.

^b The spectrum used in the LC fit was obtained from the same 2-line ferrihydrite phase that was used in the reactors.

^c This component represents Fe(II) species that do not have a defined structure beyond the first O shell, e.g. disordered precipitates or Fe(II) adsorbed to the mineral or biological surfaces in the system. A spectrum from Fe(II) adsorbed to carboxyl-functionalized beads was used (details in SI section).

^d Crystalline siderite and adsorbed Fe(II) species were used as spectral endmembers to reproduce the data and to quantify the corresponding spectral content. We note that spectra from a fresh amorphous precipitate of Fe(II) in bicarbonate solution could also be fit with a linear combination of crystalline siderite and adsorbed Fe(II) species (not shown). While the data and fits demonstrate the presence of siderite, the noted fraction numbers may not represent the actual content of two distinct species in the sample, but possibly the predominance of an amorphous ferrous carbonate precipitate or a combination of the three species mentioned above. It is not possible to distinguish between these scenarios with a linear combination fit.