Pure goethite particles in the nanometer size range (from
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200 to
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80 nm) with an elongated shape (axial ratio from
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5 to
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8) useful as iron precursors for magnetic recording have been prepared by oxidation of the suspensions resulting from the addition of sodium carbonate to Fe(II) sulfate aqueous solutions under a restrictive set of experimental conditions (Fe(II) concentration, carbonate/Fe(II) mole ratio, temperature, and air flow rate). In all cases, the goethite particles were formed by a dissolution–recrystallization mechanism through an intermediate green-rust phase. The particle size was determined by the carbonate/Fe(II) ratio (which controls the formation pH), the FeSO
4 concentration, and the air flow rate. The smallest particles (length 80 nm) were obtained for a high carbonate/Fe(II) mole ratio (≥3), a low Fe(II) concentration (0.075 mol dm
−3), and an air flow rate of 2 dm
3 min
−1. The goethite particles were also characterized by the electron diffraction and high-resolution TEM finding that they were monocrystalline, having the crystalline
c axis parallel to the longest particle dimension.