Mössbauer Effect Determination of Particle Size in Microcrystalline Iron–Manganese Nodules

Abstract

IRON-MANGANESE nodules from the ocean floor have been extensively studied^1–6. But, because of the fine grain size of the particles of the nodules, structural identification by X-ray and electron diffraction techniques is difficult and the mineralogy of the iron oxide phase has not been well characterized. The observation of the Mössbauer spectrum—in which each nucleus absorbs γ-rays independently—is not limited by particle size in the same way as is the observation of Bragg peaks in diffraction measurements, in which radiation must be scattered coherently from a large number of atoms. The magnetic hyperfine splitting in the Mössbauer spectrum of magnetic materials is affected, however, when the particles are so small that they become superparamagnetic^7–10. We describe here an investigation using the ⁵⁷Fe Mössbauer effect of two iron–manganese nodules in which the iron oxide phase could not be detected by X-ray or electron diffraction.