Abstract—Inorganic magnetite nanocrystals were synthesized in an aqueous medium at 25°C, atmospheric
pressure, ionic strength of 0.1 M, oxygen fugacity close to 0, and under controlled chemical affinity, which
was maintained constant during an experiment and varied between different experiments. The total concentration of iron in the initial solutions, with Fe(III)/Fe(II) ratios of 2, was varied in order to measure the role
of this parameter on the reaction rate, particle morphology, and oxygen isotopic composition. The reaction
rates were followed by a pHstat apparatus. The nature and morphology of particles were studied by
transmission electron microscopy and electron energy loss spectroscopy. Fractionation factors of oxygen
isotopes were determined by mass spectrometry after oxygen extraction from the solid on BrF5 lines. At low
total iron concentrations, goethite and poorly crystalline iron oxides were observed coexisting with magnetite.
At higher concentrations, euhedral single crystals of pure magnetite with an average characteristic size of 10
nm were formed, based on a first-order rate law with respect to total iron concentration. These results confirm
that, under high supersaturation conditions, low-temperature inorganic processes can lead to the formation of
well-crystallized nanometric magnetite crystals with narrow size distribution. The observed oxygen isotope
fractionation factor between magnetite crystals and water was of 0–1‰, similar to the fractionation factor
associated with bacterially produced magnetite. We suggest that the solution chemistry used in this study for
inorganic precipitation is relevant to better understanding of magnetite precipitation in bacterial magnetosomes, which might thus be characterized by high saturation states and pH