Absence of organic content in BIFs does not exclude a biogenic origin.
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Answering a long debated question combing bond level information with bulk experiments!
The absence of organic compounds from Precambrian iron formations (IF) challenges the hypothesis of their biogenic origin. Here we have address the fate of adsorbed organic compounds during transformation from ferrihydrite to hematite.
We applied dynamic force microscopy to determine the binding energy between hematite and common molecular terminations found in extracellular polymeric substances and biofilms: carboxylic, alcohol and phosphate functional groups (Fig). From our results we can establish that the bond between hematite and alcohol group is approximately 2 times stronger than the bond between hematite-carboxyl and -phosphate groups.
Transformation from the precursor mineral ferrihydrite can take several routes. Some routes go through sequences of transformation that involve dissolution and re-precipitation. During such reactions the adsorbed organic content is likely to desorb. In contrast when ferrihydrite transform to hematite (the most common BIF mineral) the process is through a reorientation of the crystal structure and the mineral are not dissolved. In this type of transformation the adsorbed polymers stand a larger chance of being preserved. Er followed the desorption of glycerol during transformed from ferrihydrite to hematite (glycerol has a high density of alcohol groups).
We find that the transformation process to hematite releases glycerol highlighting that at organic compounds from initial IFs are likely to be desorbed very early during the process of sedimentation.
Combined our results suggest that the absence of organic compounds in IF cannot be an evidence against their biogenic origin.
Schematics of the DFS: self assembled monolayers with carboxyl, hydroxyl and phosphate headgroups covalently bonded to a gold coated AFM tip (thiol functionalization).
In the transformation experiments, ferrihydrite and glycerol were mixed at room temperature and left to equilibrate overnight. One sample was then taken for TGA (equilibrated sample) and the rest was placed in the oven at 90 ºC until the transformation was complete (aged sample).